Project Report

AUTOMATION OF INTERCOMPARISON OF A PAIR OF CESIUM ATOMIC CLOCKS

PROJECT REPORT SUBMITTED BY

SATISH KUMAR

PROJECT GUIDE

Dr. P. Banerjee & Miss Pranalee P. Thorat Time & Frequency Division

National Physical Laboratory, New Delhi

i

DECLARATION

I hereby declare that the project work entitled “Automation of inter-comparison of a pair of Cesium atomic clocks” is a record of my work carried out at

National Physical Laboratory, New Delhi, during the period 6th July, 2009 to 8th

January, 2010, under the guidance of Dr. P. Banerjee, Scientist-“G” and Miss

Pranalee P. Thorat, Scientist-“B”, Time & Frequency Division, NPL, New

Delhi.

Satish Kumar

Roll No: 6110405723 E.C.E. (7th Sem.) Adesh Institute of Engg. & Tech.

Faridkot, Punjab.

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CERTIFICATE

This is to certify that the project work entitled “ Automation of inter-

comparison of a pair of Cesium atomic clocks” has been undertaken at

National Physical Laboratory, New Delhi by Satish Kumar of Adesh Institute of Engg. & Tech. from 6th July, 2009 to 8th January, 2010. The project is a

bonafide work of Satish Kumar and has been carried out under my guidance.

Signature of project guide

Dr. P. Banerjee, Scientist ‘G’, Time & Frequency, NPL, New Delhi.

Signature of project guide

Miss Pranalee P. Thorat, Scientist ‘B’, Time & Frequency, NPL, New Delhi.

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ACKNOWLEDGEMENT

It gives me great pleasure to express my sincere gratitude to Dr.P.Banerjee,

(Scientist ‘G’) and Miss Pranalee P. Thorat (Scientist ‘B’) for their encouraging

nature, valuable guidance and inspiring words and invaluable co-operation

during the period of my work.

I am also thankful to Mrs. Arundhati Chatterjee (Scientist ‘E2’), Mr. Anil

Kumar Suri (Technical Officer) , Mrs. Aritri Nandi and Time & Frequency

Division, NPL , New Delhi for sharing their knowledge and for their support

and encouragement.

I am also grateful to Dr. R.K. Aggarwal, HRD Head, and other members of the

NPL group.

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Table of Contents

Contents Page No.

Declaration i Certificate ii Acknowledgement iii Table of contents iv List of Figures vii List of graphs viii List of tables ix

1. An introduction to National Physical Laboratory

1.1 NPL History 2 1.2 Main activities carried out at NPL 2 1.3 Technologies developed at NPL 5 1.4 Organizational Structure 5 1.5 Research Areas 6

2. Time & Frequency Division 2.1 What is time? 8 2.2 Need for measuring time accurately 10 2.3 Process of measurement of time 13 2.4 Time Interval Counter 16

3. Atomic Clock

3.1 Block diagram of a simple Clock 19 3.2 Accuracy, Precision & Stability of a Clock 20 3.3 Early Clocks 22 3.4 Atomic age of Time Standards 23

3.4.1 Atomic Clock 24 3.4.2 Why use Cesium atoms? 24 3.4.3 Cesium atomic clock 26

3.5 Frequency Stability 29 3.6 Common Methods of measuring Frequency Stability 35

3.6.1 Beat Frequency Method 35 3.6.2 Dual Mixer Time Difference system 36 3.6.3 Loose phase lock loop method 39 3.6.4 Tight phase lock loop method 41 3.6.5 Time difference method 42

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4. Automatic Switching Systems 4.1 Need for automatic switching system 44 4.2 Design requirements of automatic switching system 45 4.3 Internal Architecture of switching system 46 4.4 Circuit diagram of switching system 47

4.4.1 Power supply 48 4.4.2 Microcontroller 52 4.4.3 RS-232 59 4.4.4 Reed relay & Relay driver 63 4.4.5 Light emitting diode. 68

4.5 Front & Rear view of switching system. 70 4.6 Accessing the switching system. 72 4.7 Measurement of circuit delay of the Switching system. 76

5. Automated Data Acquisition System.

5.1 Block diagram of setup for inter-comparison of clocks 89 5.2 Universal Time Interval counter (SRS-620) 90 5.3 Interfacing SRS-620 to PC 91 5.4 Programming the TIC 94 5.5 Serial port programming using C++ 97 5.6 Flow chart 101 5.7 C++ program for automated measurement 102

6. Labs visited at National Physical Laboratory. 6.1 High voltage & High current lab 109

6.2 Energy & Power metering lab 112 6.3 Time & Frequency Research lab 113

7. References 115

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List of Figures

Contents Page No.

1. National Physical Laboratory, New Delhi, India 1

2. Departments of NPL, India 5

3. Two dimensional time depicted in three dimensional space-time 9

4. Start and Stop Pulses of two Clocks 15

5. HP Time Interval Counter 16

6. Counting the time interval 17

7. Basic concept of a clock 19

8. Accuracy & Precision 20

9. High accuracy, but low precision 20

10. High precision, but low accuracy 21

11. Electron Transition 24

12. Cesium atom hyperfine structure 25

13. The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK 26

14. Beam tube of a Cesium atomic clock 27

15. Schematic diagram of a Cesium atomic clock 28

16. A sine function 29

17. Sine wave signal 30

18. Unstable & Stable Frequency 31

19. Beat frequency method- block diagram 35

20. Dual mixer time difference(DMTD) method- block diagram 36

21. Block diagram of loose phase lock loop method 39

22. Block diagram of tight phase lock loop method 41

23. Block diagram of Time interval counter 43

24. Internal architecture of switching system 45

25. Circuit diagram of switching system 47

26. Pin Diagram of ATmega8535 53

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27. Block diagram of ATmega8535 55

28. Asynchronous Data Transfer 60

29. DB9 Connector Configuration 61

30. Pin Diagram of MAX232 62

31. Relay & Reed Switches 63

32. Contacts of a Reed Switch (Side) 64

33. Contacts of a Reed Switch (Top) 64

34. Relay Driver 66

35. Parts of an LED 68

36. Inner workings of an LED 69

37. Packaged Tri-color LED 69

38. Internal Diagram of a Tri-Color LED 70

39. Front View of the Switching System 71

40. Rear View of the Switching System 71

41. Step 2 (Hyper Terminal) 72





46. Experimental Setup to measure cable delay 76

47. Automated Data Acquisition system for time scale generation 87

48. Block diagram of setup for intercomparison of clocks 89

49. Connection between PC and TIC (SRS620) 91

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List of Tables

Contents Page No. 1. Pin Configuration of 8535 51

2. Pin Configuration of MAX232 60

3. cmd Values and Functions 99

4. abyte Values and Meaning 100

5. Junction Design Parameters 111

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List of Graphs

Contents Page Number 1. Stability versus Frequency 19

2. Bridge Rectifier Output 48

3. Filter Output 49

4. Transfer Characteristics 67

5. Ohmic Region 67

6. Variation of average values of S1 78







13. Variation of Jitter for S1 81







20. Start Cable delay 85

21. Stop Cable delay 85

22. Back Cable delay 86

23. Constant voltage steps for a low capacitance junction 111

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1. An Introduction to National Physical Laboratory.

Figure 1: National Physical Laboratory, New Delhi, India

The National Physical Laboratory is the premier research laboratory in India in the field

of physical sciences. It has developed core competencies in standards, apex level calibration, engineering

materials, electronics materials, materials characterization, radio and space physics,

global change and environmental studies, low temperature physics and instrumentation. Established in 1947, it is one of the oldest laboratories of the Council of Scientific and

Industrial Research. Its main activities are:

Research and Development

Consultancy

Sponsored and contract research

Calibration and testing

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1.1 NPL history.

The National Physical Laboratory is one of the earliest national laboratories set up under

the Council of Scientific and Industrial Research. Late Sri Jawaharlal Nehru laid the

foundation stone of NPL on 4th January, 1947. Late Dr. K. S. Krishnan, FRS, was the first

director of the laboratory. The main building of the laboratory was formally opened by

Late Deputy Prime Minister Sardar Vallabh Bhai Patel on 21st January, 1950.

The silver jubilee celebration of the laboratory was inaugurated by Late Prime Minister,

Smt. Indira Gandhi, on 23rd December, 1975.

1.2 Main activities carried out at NPL

1. Development, maintenance and up-gradation of primary / national standards:

Traceability of primary / national standards to international standards

Traceability / development, maintenance and up-gradation of primary /

national standards to international standards

Calibration and consultancy service to calibration laboratories and

industries.

R&D in the development of standards and sponsored research

Awareness service amongst industry and testing laboratories about

measurement uncertainty and organize training for its estimation.

Providing accreditation bodies such as National Accreditation Board

Laboratories (NABL) for calibration & testing.

2. Custodian of Measurement Standards: National Physical Laboratory has the responsibility of realizing the units of

physical measurements based on the International System (SI units) under the

subordinate legislations of Weights & Measures Act 1956 (reissued in 1988 under

the 1976 Act). NPL also has the statutory obligation to realize, establish,

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maintain, reproduce and update the national standards of measurement &

calibration facilities for different parameters.

The laboratory, at present, is maintaining six out of the seven SI base units. These

are meter, kilogram, second, Kelvin, ampere, candela. R&D work is under way

for realizing and establishing the seventh SI base unit of mole (mol). The SI

supplementary units are radian (rad) & steradian (sr).

The derived units for physical measurement that the laboratory currently

maintains are: force, pressure, vacuum, luminous flux, sound pressure, ultrasonic

power & pressure and the units for electrical and electronic parameters viz., dc

voltage, resistance, current and power; ac voltage, current and power; low

frequency voltage, impedance and power; high frequency voltage, power,

impedance, attenuation and noise; microwave power, frequency, impedance,

attenuation and noise.

3. International Traceability: The national standards of physical measurement at NPL are traceable to

international standards. The laboratory periodically carries out inter-comparison

of national standards with the corresponding standards maintained by National

Metrology Institutes (NMIs) of other countries under the consultative committees

of the International Committee of Weights and Measures (CIPM) and the member

nations of Asia Pacific Metrology Program (APMP) The major implication of this

exercise of establishing equivalence of national standards on measurement at NPL

with those of other NMIs is that calibration certificates issued by NPL would have

global acceptability.

4. National Apex Body for Calibration:

The laboratory provides apex level calibration services in the country; offering

National Accreditation Board Laboratories (NABL) for testing and calibration.

The national accreditation body in the country provides

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(i)its qualified assessors as needed for establishing best measurement capability

of the applicant laboratory; in particular its scientific,

(ii) its technical input to enable NABL to decide the suitability of the applicant

laboratory for accreditation, and

(iii) its faculty to train testing laboratories for estimation of uncertainty in their

measurement.

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1.3 Technologies developed at NPL

Some of the important technologies developed by the National Physical Laboratory are

listed below:

• Teleclock Service

• Piezoelectric Accelerometer

• Carbon Composite Half Rings for Orthopaedic Applications

• Temperature Calibration Bath

• Magical Heat PAD/PACK (Reusable)

• Basic Sodar Operating in Monostatic/ Doppler Mode

• Long Afterglow Phosphor

• Force Transducer/ Load Cells

• Blood Glucose Digital Analyzer

1.4 Organizational Structure

Figure 2: Departments at NPL, India

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1.5 Research Areas

• Physico Mechanical Standards

• Electrical & Electronic Standards

• Engineering Materials

• Electronic Materials

• Materials Characterization

• Radio & Atmospheric Sciences

• Superconductivity and Cryogenics

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2. Time & Frequency Division

This section comes under the Electrical & Electronics Standards Branch and maintains

the Indian Standard Time (IST). Time & Frequency section is entrusted with the task of

synchronization and maintenance of international standards of time and dissimination.

The process is quite complicated and involves high degree of technical input. IST is the

Indian Standard Time corresponding to 82.5O E latitude (passing near Allahabad). The

time zone is five and half-hours ahead of the Universal Coordinated Time (UTC), which

was earlier called the Greenwich Meridian Time (GMT).

Atomic clocks across the globe monitor time and synchronize themselves with the help of

global positioning system (GPS) satellites, hovering over the earth. This data is collected

and transmitted via the Internet to the International Bureau of Weights and Measures

(BIPM) in Paris. NPL is equipped with GPS receivers and follows the international

tracking schedule published and updated regularly by BIPM.

NPL has at present 5 atomic clocks namely NPL1, NPL3, NPL5, NPL6, NPL7 for

maintaining the time standard. The accuracy of each of these cesium atomic clocks is of

the order of 10-13. NPL is also involved in the dissemination of IST to the masses,

through the INSAT satellite or the telephone network. The service provided through the

satellite is known as the Standard Time & Frequency Signal (STFS), where any company

desiring the IST has to install necessary apparatus including a dish antenna. Its teleclock

service is available over the standard telephone line and can be accessed by a teleclock

with an inbuilt modem connected to a telephone line.

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2.1 What is time?

Time is a component of the measuring system used to sequence events, to compare the

durations of events and the intervals between them and to quantify the motions of objects.

The American Heritage Dictionary defines time as “A non-spatial continuum in which

events occur in apparently irreversible succession from past through present to future.

An interval separating two points on this continuum, measured essentially by selecting a

regularly recurring event and counting the number of its recurrences during the interval

of duration.”

In physics as well as in other sciences, time is considered to be one of the few

fundamental quantities. Time is used to define other quantities – such as velocity,

acceleration, etc.

From the age of Newton up until Einstein's profound reinterpretation of the physical

concepts associated with time and space, time was considered to be "absolute" and to

flow "equably" (to use the words of Newton) for all observers.

In classical mechanics, Newton's concept of "relative, apparent, and common time" can

be used in the formulation of a prescription for the synchronization of clocks. Events seen

by two different observers in motion relative to each other produce a mathematical

concept of time that works pretty well for describing the everyday phenomenon of most

people's experience.

Einstein resolved the problems with the classical understanding of time by invoking a

method of synchronizing clocks using the constant, finite speed of light as the maximum

signal velocity. This led directly to the result that observers in motion relative to one

another will measure different elapsed times for the same event.

Time has historically been closely related with space, the two together comprising

space-time in Einstein's special relativity and general relativity. According to these

theories, the concept of time depends on the spatial reference frame of the observer. The

past is the set of events that can send light signals to the observer; the future is the set of

events to which the observer can send light signals.

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Time appears to have a direction – the past lies behind, fixed and non-commutable,

while the future lies ahead and is not necessarily fixed. The Second law of

thermodynamics, states that entropy must increase over time, the cosmological arrow of

time points away from the Big Bang, and the radiative arrow of time is caused by light

only traveling forwards in time.

Figure 3: Two-dimensional space depicted in three-dimensional space-time.

One of the most peculiar qualities of time is the fact that it is measured by motion and it

also becomes evident through motion.

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2.2 Need for measuring time accurately

Measurements in frequency and time are of fundamental importance for all experimental

work in science and engineering. For day to day timekeeping our personal watches and

household clocks are accurate enough. For example, a typical quartz clock can keep time

to within a second over a period of 10 days. When it comes to sending data over a phone

line or navigating by satellites, however, more precision is needed.

Telecommunications rely heavily on accurate timing to ensure that the switches routing

digital signals through their networks are synchronized and all run at the same time.

Otherwise, the switches running slow would not be able to cope up with the high traffic

volume and data would be lost. When speaking on the telephone, one might hear a click

or crackle if just one data packet is lost.

For the navigation of ships, aeroplanes, and more recently family care, global position

system (GPS) satellites that orbit the earth broadcast timing signals from their atomic

clock. By looking at the signal from four (or more) satellites, the user’s position can be

determined. The time has to be incredibly accurate as light travels thirty centimeters in 1

nanosecond (or 300 million meters in a second!) so that any small error in the time signal

could put you off course by a very large distance. This system has proved particularly

effective during sea rescue operations and in situations such as Arctic expeditions where

navigating by traditional landmarks and signposts is impossible.

While we are becoming more and more dependent on accurate time it is important to

know from where does this time come from and who looks after the master clock.

The international time standard is maintained by 62 time laboratories around the world

and is based on the average of some 350 atomic clocks. The diversity provides both

safety (a single clock in an earthquake zone would not be a good idea) and accessibility

(each major industrial nation contribute to the time standard, and hence has direct access

to the atomic clocks). In the UK, it is the National Physical Laboratory that maintains the

national time standard. The group of atomic clock at NPL keeps the UK’s time accurate

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to within one second in a million years; which means that the error in a day or a week is

minuscule.

There is a need for making a precise comparison from the various sources of time. When

the sources were observed over a fixed period, it was seen that the variations they showed

were within specifications but these variations were not constant. All the sources showed

different variations. So the problem was to decide which one of the sources should be

considered as reference.

A method of ensembling (average) is adopted which shows that the average of all the

Measurements in frequency and time are of fundamental importance for all experimental

work in science and engineering. For day to day timekeeping our personal watches and

household clocks are accurate enough. For example, a typical quartz clock can keep time

to within a second over a period of 10 days. When it comes to sending data over a phone

line or navigating by satellites, however, more precision is needed.

Telecommunications rely heavily on accurate timing to ensure that the switches routing

digital signals through their networks are synchronized and all run at the same time.

Otherwise, the switches running slow would not be able to cope up with the high traffic

volume and data would be lost. When speaking on the telephone, one might hear a click

or crackle if just one data packet is lost.

For the navigation of ships, aero planes, and more recently family care, global position

system (GPS) satellites that orbit the earth broadcast timing signals from their atomic

clock. By looking at the signal from four (or more) satellites, the user’s position can be

determined. The time has to be incredibly accurate as light travels thirty centimeters in 1

nanosecond (or 300 million meters in a second!) so that any small error in the time signal

could put you off course by a very large distance. This system has proved particularly

effective during sea rescue operations and in situations such as Arctic expeditions where

navigating by traditional landmarks and signposts is impossible.

12

While we are becoming more and more dependent on accurate time, where does this time

come from? Who looks after the master clock?

As a matter of fact, there is no single master clock for the world. Instead, the international

time standard is maintained by 62 time laboratories around the world and is based on the

average of some 350 atomic clocks. The diversity provides both safety (a single clock in

an earthquake zone would not be a good idea) and accessibility (each major industrial

nation contribute to the time standard, and hence has direct access to the atomic clocks).

In the UK, it is the National Physical Laboratory that maintains the national time

standard. The group of atomic clock at NPL keeps the UK’s time accurate to within one

second in a million years; which means that the error in a day or a week is minuscule.

There is a need for making a precise comparison from the various sources of time. When

the sources were observed over a fixed period, it was seen that the variations they showed

were within specifications but these variations were not constant. All the sources showed

different variations. So the problem was to decide which one of the sources should be

considered as reference.

A method of ensembling (average) is adopted which shows that the average of all the

sources is to be considered as the reference. Output from these sources is taken into a

time interval counter and the comparison is made.

sources is to be considered as the reference. Output from these sources is taken into a

time interval counter and the comparison is made.

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2.3 Process of measurement of time

The biggest problem with measurement of time accurately is its dynamic nature. Time

flows and cannot be held constant unlike other measurable quantities like length, mass or

temperature.

Every day we come across a term called “second”. But how long is a second? Is it the

same for all clocks or different for different clocks? A layman would describe one second

as seen by his wrist watch or his clock but for a scientist the definition of a second is of

prime importance.

Until 1956, the second was defined as 1/86400 of the mean solar day. As the

irregularities of the Earth’s rotation had become well known since the early 1930s using

quartz crystal clocks and with advanced astronomical instruments, a definition allowing

variations with time of the basic unit of measurement appeared no longer tolerable.

Thus, in 1956 a new definition was adopted based on ephemeris time One second being

re-defined as a fraction of 1/31556925.9747 of the tropical year. Even though this

definition was infinitely stable by definition it was not adopted for a long time. This is

because ephemeris second was difficult to determine as several years of observation was

required to reduce the probable error to a few parts in 109.

In the mean time during the Second World War, physicists had discovered many atomic

and molecular spectral lines in the centimeter wave band. The use of a particular

resonance as a possibly non-variant frequency led to the birth of a new device, the

“Atomic Clock” which is the modern time scale generators.

Hence, a new definition of a second was adopted in October 1967 during the XIIth

general conference on Weights and Measures and is as follows:

“One second is the time that elapses during 9,192,631,770 cycles of the radiation

produced by the transition between two hyperfine levels of the cesium 133 atom.”

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An interesting thing to be noted in this definition is that the measured quantity is

frequency and not time. While in the old definition, the second was given as a small

fraction of a long period, the new definition gives the second as a large number of very

rapid oscillations. Hence, in the new definition longer time intervals are built up by

successively adding elementary time intervals.

Time is thus displayed using an oscillator – counter – display mechanism. However, there

is a problem with physical oscillators generating stable periodic oscillations. Strict

periodicity implies that each successive cycle is an exact copy of each preceding cycle.

We can start counting at any time i.e. there is no defined origin in the time coordinate.

This problem was overcome by synchronization according to a conventional origin,

which has been agreed on 1 January 1958 0h, 0m, 0s.

Modern clocks give time in terms of their frequency of oscillations. Hence, it is very

important for the frequency of the source in the clock to be very stable so that it gives

accurate time. Five cesium atomic clocks (NPL 1, NPL 2, NPL 3, NPL 6 and NPL 7) are

maintaining time at NPLI. However, it is not possible to maintain any absolute standard

for time. A reference is always required. Hence, NPL 7 is taken as the reference.

Time interval counters are used to measure the error between any two clocks. The signals

that fed into the counter are pulse trains coming from the clocks with a frequency of 1

pulse per second (pps). However, it is not necessary that a set of consecutive pulses from

both the clocks reach the time interval counter at the same time. There will always be

some time difference, which is measured by the time interval counter.

The two inputs to the time interval counter are start and stop. Whenever, a pulse reaches

the start input of the time interval counter, counting starts and it stops upon the reception

of the stop pulse.

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Hence the time interval between the start and stop pulses is measured. The data obtained

from the time interval counter is used to estimate the stability of the clocks using “Allan

variance”.

`

START

STOP Figure 4:Start and Stop pulses of two Clocks The time interval counter shows a new reading after each second. However, it is not

possible to note down the readings manually every second. But it is necessary to take

large number of readings, since more readings give a better estimation of the stability of

the clocks. This calls for a need of a software program so that readings from the serial

port of the time interval counter can be sent to the computer so that evaluation can be

done. NPLI uses this procedure to take readings from the interval counter.

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2.4 Time Interval Counter

Figure 5 Time Interval Counter

An instrument used to measure the time interval between two signals is time interval

counter. A time interval counter (TIC) has inputs for two electrical signals. One signal

starts the counter and the other signal stops it.

TIC’s differs in specification and design, but they all contain several basic parts known as

the time base, the main gate and the counting assembly.

The time base provides evenly spaced pulses used to measured time interval. The time

base is usually an internal quartz oscillator. It must be stable because time base errors will

directly affect the measurements.

The main gate controls the time at which the count begins and ends. Pulses passing

through the gate are routed to the counting assembly, where they are displayed on the

TIC’s front panel or read by computer.

The TIC begins measuring a time when the start signal reaches its trigger level and stops

measuring when the stop signal reaches its trigger level. The time interval between the

start and stop signals is measured by counting cycles from the time base.

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The measurements produced by a TIC are in time units such as microseconds or

nanoseconds.

Figure 6: Counting the time interval

The most important specification of a TIC is resolution. In traditional TIC designs, the

resolution is limited to the period of the TIC’s time base frequency. For example, a TIC

with a 10 MHz time base would be limited to a resolution of 100 ns. This is because

traditional TIC designs count whole time base cycles to measure time interval and cannot

resolve time intervals smaller than the period of one cycle.

To improve this situation, some TIC designers have multiplied the time base frequency to

get more cycles and thus more resolution.

For example, multiplying the time base frequency to 100 MHz makes 10 ns resolution

possible, and 1 ns counters have even been built using a 1GHz time base.

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Presently, there are two Time Interval Counters are available in Time & Frequency

division at NPL, New Delhi. Anyone of them can be used to measure the time difference

between any two of the Cesium clocks. The time difference measured is then used for

generating Time Scale of NPL India.

1 Universal Counter(HP53131/32) 300/500ps

2. Universal Time Interval Counter (SR620) 50ps

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3. Atomic Clock

3.1 A Clock

Figure 7: Basic concept of a clock The basic block diagram of a clock indicates that a clock consists of three components:

1.Frequency Source :

The frequency source used should be stable, precise and accurate for a good clock.

Quartz crystals, microwave oscillators etc. are the examples of some of the frequency

sources that are used. The common quartz wrist watch uses a quartz crystal as a source of

its frequency. The basic formula for calculating the frequency of a quartz tuning fork as a

function of its dimensions (quadratic cross-section) are as follows:

2.Counter –:

The output from the frequency source is next fed to a counter. What the counter actually

does is that it counts the number of pulses from the frequency source. Using some logic

gates and/or functions, the pulses from the frequency source are sequentially counted.

3.Display :

The display is used to show the output of the counter. What the counter counts is

displayed in the display.

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3.2 Accuracy, Precision & Stability of a Clock

Accuracy is the degree of closeness of a measured or calculated quantity to its true value.

Precision is defined as,

1. The ability of a measurement to be consistently reproduced.

2. The number of significant digits to which a value has been reliably measured.

Accuracy is the degree of veracity while precision is the degree of reproducibility.

Figure 8: Accuracy & Precision

The closer a system's measurements to the accepted value, the more accurate the system

is considered to be.

Figure 9: High accuracy, but low precision

The more a system's measurements are reproducible, the more precise the system is

considered to be.

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Figure 10: High precision, but low accuracy

Stability is defined as the tendency to recover from perturbations.

Graph 1: Stability versus Frequency

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3.3 Early clocks The clock is one of the oldest human inventions, meeting the need to

consistently measure intervals of time shorter than the natural units, the day, the lunar

month, and the year. Such measurements require devices. Devices operating on several

different physical processes have been used over the millennia, culminating in the clocks

of today.

Sundials: The sundial, which measures the time of day by the direction of

shadows cast by the sun, was widely used in ancient times. A well-designed sundial can

measure local solar time with reasonable accuracy, and sundials continued to be used to

monitor the performance of clocks until the modern era. However, its practical limitations

- it requires the sun to shine and does not work at all during the night - encouraged the

use of other techniques for measuring time. In India, the King of Jaipur the Pink City, Jai

Singh II constructed many instruments and sundials in the observatories in cities Jaipur,

Varanasi, Ujjain, Mathura between 1724-1730 A.D. He had a good interest in astronomy

and town planning.

Water Clocks: Water clocks were among the earliest timekeepers that didn’t

depend on the observation of celestial bodies. One of the oldest was found in the tomb of

the Egyptian pharaoh Amenhotep I, buried around 1500 BCE. Later named clepsydras

(“water thieves”) by the Greeks who began using them about 325 BCE, these were stone

vessels with sloping sides that allowed water to drip at a nearly constant rate from a small

hole near the bottom. Other clepsydras were cylindrical or bowl-shaped containers

designed to slowly fill with water coming in at a constant rate. Markings on the inside

surfaces measured the passage of “hours” as the water level reached them. These clocks

were used to determine hours at night, but may have been used in daylight as well.

Another version consisted of a metal bowl with a hole in the bottom; when placed in a

container of water the bowl would fill and sink in a certain time. These were still in North

Africa in the 20th century.

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3.4 The Atomic age of Time standards

Scientists had long realized that atoms (and molecules) have resonances; each

chemical element and compound absorbs and emits electromagnetic radiation at its own

characteristic frequencies. These resonances are inherently stable over time and space.

An atom of hydrogen or cesium here today is (so far as we know) exactly like one a

million years ago or in another galaxy. Thus atoms constitute a potential "pendulum"

with a reproducible rate that can form the basis for more accurate clocks.

The development of radar and extremely high frequency radio communications in the

1930s and 1940s made possible the generation of the kind of electromagnetic waves

(microwaves) needed to interact with atoms. Research aimed at developing an atomic

clock focused first on microwave resonances in the ammonia molecule. In 1949, NIST

built the first atomic clock, which was based on ammonia. However, its performance

wasn't much better than the existing standards, and attention shifted almost immediately

to more promising atomic-beam devices based on cesium.

24

3.4.1 Atomic Clock

Atomic clocks make use of the ability of atoms to emit or absorb electromagnetic

waves of a characteristic oscillation frequency f0 during transitions between two energy

states. The value f0 corresponds to the energy difference between the two states, divided

by Planck's constant.

f0 =

hEE

12

-

Figure 11 Electron Transition

Atomic clocks make use of transitions between such energy levels whose natural

lifetime is long and whose energy is not significantly affected by electric fields or

magnetic fields or other perturbations.

3.4.2 Why use Cesium(Cs-133) atoms?

1. Cesium is used because its electronic levels can be excited with radiofrequencies

(microwaves) produced by an electronic circuit and then they de-excited easily. This

process cannot be done in such a simple way with other equivalent atoms.

2. In all frequency standard oscillators, the element used is in vapor form in order to

allow all the atoms to undergo the same (almost exactly) energy transitions.

25

There are several choices of metal, in principle, but they all need to be on the left side

of the periodic table having a single electron in their outermost shell. Cs is the best

and most stable among them for making an atomic clock.

3. Cs has only one stable isotope; meaning that one does not have to bother about

any issues arising from having multiple isotopes around, or having to purify it.

4. Cesium has the advantage of having the largest hyperfine structure, i.e. the energy

difference of the two electron spin states in the presence on the nucleus's magnetic

field is large. This means for a given interrogation time, you will maximize the

number of oscillations between these two states, giving a more precise measurement.

Figure 12: Cesium atom hyperfine structure

5. At an oven temperature of not more than 100 °C, the vapor pressure is high

enough to furnish an intense atomic beam.

26

3.4.3 Cesium Atomic Clock

A cesium atomic clock is a device that uses as a reference the exact frequency of the

microwave spectral line emitted by atoms of the element Cs-133. The integral of

frequency is time. So this frequency of 9,192,631,770 hertz provides the fundamental

unit of time, which may thus be measured by cesium clocks.

Figure 13: The 1955 Cesium Atomic Clock at the National Physical Laboratory,

UK.

Today, cesium clocks measure frequency with an accuracy of from 2 to 3 parts in

1014. This corresponds to a time measurement accuracy of 2 nanoseconds per day or

one second in 1,400,000 years. A cesium clock operates by exposing cesium atoms to

microwaves until they vibrate at their resonant frequencies and then counting the

corresponding cycles as a measure of time. The frequency is that of the energy

absorbed from the incident photons when they excite the outermost electron in a

cesium atom to make a transition from a lower orbit to a higher orbit.

27

Detailed functioning of the Cesium Atomic Clock

Figure 14: Beam tube of a Cesium atomic clock

In a cesium clock, liquid cesium is heated to a gaseous state in an oven. A hole in the

oven allows the atoms to escape at high speed. These particles pass between two

electromagnets whose field causes the atoms to separate into two beams, depending

on which spin energy state they are in. Those in the lower energy state pass through

the ends of a U-shaped cavity in which they are irradiated by microwaves of 3.26-cm

wavelength.

The absorption of these microwaves aids the transitions of many of the atoms from

the lower to the higher energy state. The beam continues through another pair of

electromagnets, whose field again divides up the beam.

Those atoms in the higher energy state strike a hot wire, which ionizes them.

Thereafter, a mass spectrometer selects only the cesium atoms from any impurities

and directs them onto an electron multiplier.

The frequency of the microwaves is adjusted until the electron multiplier output

current is maximized, constituting the measurement of the atoms' resonance

28

frequency. This frequency is electronically divided down and used in a feedback

control circuit (servo-loop) to keep a quartz crystal oscillator locked to a frequency of

5 megahertz (MHz), which is the actual output of the clock, along with a one-pulse-

per-second signal. The entire apparatus is shielded from external magnetic fields.

Figure 15: Schematic diagram of a Cesium atomic clock

29

3.5 Frequency Stability

A sine wave signal generator produces a voltage that changes in time in a sinusoidal way

as shown in figure. The signal is an oscillating signal because the sine wave repeats itself.

A cycle (2 � radians of phase) of the oscillation is produced in one period "T".

Figure 16: A repeating sine function is the basis of an oscillating signal.

It is convenient for us to express angles in radians rather than in units of degrees, and

positive zero-crossings of the voltage will occur every 2 � radians. The frequency "� " is

the number of cycles in one second (Hz), which is the reciprocal of period (seconds per

cycle). The expression describing the voltage "V" out of a sine wave signal generator is

given by

V(t) = V0[1 + a(t)]sin[� (t)]

where V0 is the peak voltage amplitude, and � (t) is the total accumulated phase.

Equivalent expressions are:

V(t)=V0[1+a(t)]sin(2�t / T)

and

V(t)=V0[1+a(t)]sin(2�vt)

30

For the following discussion, we will assume the amplitude noise a(t) is zero. Consider

figure 17. Let's assume that the maximum value of "V" equals 1, hence "V0" = 1. We say

that the voltage "V(t)" is normalized to unity.

Figure 17: For a given phase, �V vs. �t of the sine-wave signal corresponds to a

unique minimum frequency called the instantaneous frequency if �t is

diminishingly small. .

If we are given the frequency of the sine-wave, then no matter how big or small � t may

be, we can determine � V. Let us look at this from another point of view. Suppose we

can measure � V and � t. From this, there is a sine wave at a unique minimum frequency

corresponding to the given � V and � t. For infinitesimally small � t, this frequency is

called the instantaneous frequency at this t. The smaller the interval � t, the better the

approximation of instantaneous frequency at t. In practice, because of finite bandwidths,

we cannot measure the instantaneous frequency.

When we speak of oscillators and the signals they produce, we recognize that an

oscillator has some nominal frequency at which it operates. The "frequency stability" of

an oscillator is a term used to characterize how small the frequency fluctuations are of the

oscillator signal. The IEEE now has a formal definition for "frequency stability". One

31

usually refers to frequency stability when comparing one oscillator with another. As we

shall see later, we can define particular aspects of an oscillator's output then draw

conclusions about its relative frequency stability. People often speak of "frequency

stability" when they mean "frequency instability." Frequency stability is the degree to

which an oscillating signal produces the same value of frequency for any interval, � t,

throughout a specified period of time. An internationally recommended definition of

"frequency instability" is: "The spontaneous and/or environmentally caused frequency

change within a given time interval."

Let's examine the two waveforms shown in figure 18. Frequency stability depends on the

amount of time involved in a measurement. Of the two oscillating signals, it is evident

that "2" is more stable than "1" from time t1 to t3 assuming the horizontal scales are linear

in time. From time t1 to t2, there may be some question as to which of the two signals is

more stable, but it's clear that from time t2 to t3, signal "1" is at a different frequency from

that in interval t1 to t2.

Figure 18: Top: Instantaneous frequency is inconsistent and less stable from t2 to t3.

Bottom: Instantaneous frequency is consistent and more stable throughout.

32

If we want an oscillator to produce a particular frequency � 0, then we're correct in

stating that if the oscillator signal frequency deviates from � 0 over any interval, this is a

result of something which is undesirable. In the design of an oscillator, it is important to

consider the sources of mechanisms which degrade the oscillator's frequency stability.

These undesirable mechanisms cause random (noise) or systematic processes to exist on

top of the sine wave signal of the oscillator. To account for the noise components at the

output of a sine wave signal generator, we can express the output as

V(t)=V0[1+a(t)]sin(2v0t+�(t)) (1.1)

where V0 � nominal peak voltage amplitude, a(t) = deviation of amplitude from nominal,

i.e., � V/V0 � 0 ��nominal fundamental frequency, � (t) = deviation of phase from

nominal.

Ideally "a" and " � " should equal zero for all time. However, in the real world there are

no perfect oscillators. To determine the extent of the noise components "a" and "� ", we

shall turn our attention to measurement techniques.

The typical precision oscillator, of course, presumably has a stable sinusoidal voltage

output with a frequency � and a period of oscillation T, which is the reciprocal of the

frequency (� = 1/T). One goal is to measure the frequency and/or the frequency stability

of the sinusoid. Instability is actually measured, but with little confusion it is often called

stability in the literature. Naturally, fluctuations in frequency correspond to fluctuations

in the period. Almost all frequency measurements, with very few exceptions, are

measurements of phase or of the period fluctuations in an oscillator, not of frequency,

even though the frequency may be the readout. As an example, most frequency counters

sense the zero (or near zero) crossing of the sinusoidal voltage, which is the point at

which the voltage is the most sensitive to phase fluctuations.

One must also realize that any frequency measurement involves two oscillators. In some

instances, one oscillator is in the counter. It is impossible to purely measure only one

oscillator. In some instances one oscillator may be enough better than the other that the

33

fluctuations measured may be considered essentially those of the latter. However, in

general because frequency measurements are always dual, it is useful to define:

as the fractional frequency difference or deviation of oscillator one, � 1, with respect to a

reference oscillator � 0 divided by the nominal frequency � 0. Conceptually, we can also

think of eq. (1.2) as the free running frequency of an individual oscillator, � 1,

differenced with its own nominal value, � 0. Now, y(t) is a dimensionless quantity and

useful in describing oscillator and clock performance; eg., the time deviation, x(t), of an

oscillator over a period of time, t, is simply given by:

(1.3)

We see that the time deviations and the phase deviations are related by a constant,

1/2 � � 0. Since it is impossible to measure instantaneous frequency, any frequency or

fractional frequency measurement always involves some sample time, � t or "� "--some

time window through which the oscillators are observed; whether it's a picosecond, a

second, or a day, there is always some sample time. So when determining a fractional

frequency, y(t), in fact what is happening is that the time deviation is being measured say

starting at some time t and again at a later time, t + � . The difference in these two time

deviations divided by � gives the average fractional frequency over that period � :

Tau, � , may be called the sample time or averaging time; it may be determined, for

example, by the gate time of a counter.

34

What happens in many cases is that one samples a number of cycles of an oscillation

during the preset gate time of a counter; after the gate time has elapsed, the counter

latches the value of the number of cycles so that it can be read out, printed, or stored in

some other way. Then there is a delay time for such processing of the data before the

counter arms and starts again on the next cycle of the oscillation. During the delay time

(or process time), information is lost. We have chosen to call it dead time and in some

instances it becomes a problem. Unfortunately for data processing in typical oscillators

the effects of dead time often hurt most when it is the hardest to avoid. In other words, for

times that are short compared to a second when it is very difficult to avoid dead time, that

is usually where dead time can make a significant difference in the data analysis.

Typically, for many oscillators, if the sample time is long compared to a second, the dead

time makes little difference in the data analysis, unless it is excessive. New equipment or

techniques are now available which contribute zero or negligible dead time.

In reality, of course, the sinusoidal output of an oscillator is not pure, but it contains noise

(frequency) fluctuations as well. This section deals with the measurement of these

fluctuations to determine the quality of a precision signal source.

We will describe five different methods of measuring the frequency fluctuations in

precision oscillators which do not include measuring the frequency directly in a

frequency counter. The direct frequency counter technique is often very limiting because

the number of resolvable digits on the counter are often inadequate for precision

oscillators.

35

3.6 Common Methods of measuring Frequency stability 3.6.1 Beat Frequency method

The first technique is called a heterodyne frequency measuring method or beat frequency

method. The signal from two independent oscillators are fed into the two ports of a

double balanced mixer as illustrated in figure 19.

Figure 19: Measurement of the frequency difference (“beat note”) between

oscillators can increase measurement precision. State-of-the-art oscillators can

readily be measured by this method.

The difference frequency or the beat frequency, ν b, is obtained as the output of a low

pass filter which follows the mixer. This beat frequency is then amplified and fed to a

frequency counter and printer or to some recording device. The fractional frequency is

obtained by dividing ν b by the nominal carrier frequency ν 0. This system has excellent

precision; one can measure essentially all state-of-the-art oscillators.

36

3.6.2 Dual mixer time difference(DMTD) method

This system has proved to be very popular. A block diagram is shown in figure 1.5. To

preface the remarks on the DMTD, it should be mentioned that if the time or the time

fluctuations can be measured directly, an advantage is obtained over just measuring the

frequency. The reason is that one can calculate the frequency from the time without dead

time as well as know the time behavior. The reason, in the past, that frequency has not

been inferred from the time (for sample times of the order of several seconds and shorter)

is that the time difference between a pair of oscillators operating as clocks could not be

measured with sufficient precision. The system described in this section has demonstrated

a precision of 10-13 seconds. Such precision opens the door to making time measurements

as well as frequency and frequency stability measurements for sample times as short as a

few milliseconds and longer, all without dead time.

Figure 20: Measurement of the time difference between two beat notes from two

37

oscillators with a common transfer oscillation can further increase measurement

precision. Instability of transfer oscillator cancels to first order.

In figure 20, oscillator 1 could be considered under test and oscillator 2 could be

considered the reference oscillator. These signals go to the ports of a pair of double

balanced mixers. Another oscillator with separate symmetric buffered outputs is fed to

the remaining other two ports of the pair of double balanced mixers. This common

oscillator's frequency is offset by a desired amount from the other two oscillators. Then

two different beat frequencies come out of the two mixers as shown. These two beat

frequencies will be out of phase by an amount proportional to the time difference

between oscillator 1 and 2--excluding the differential phase shift that may be inserted.

Further, the beat frequencies differ in frequency by an amount equal to the frequency

difference between oscillators 1 and 2.

This measurement technique is very useful where oscillator 1 and oscillator 2 are very

near the same frequency. This is typical for atomic standards (cesium, rubidium, and

hydrogen frequency standards).

Illustrated at the bottom of figure 20 is what might represent the beat frequencies out of

the two mixers. A phase shifter may be inserted as illustrated to adjust the phase so that

the two beat rates are nominally in phase; this adjustment sets up the nice condition that

the noise of the common oscillator tends to cancel (for certain types of noise) when the

time difference is determined. After amplifying these beat signals, the start port of a time

interval counter is triggered with the positive zero crossing of the other beat. Taking the

time difference between the zero crossings of these beat frequencies, one measures the

time difference between oscillator 1 and oscillator 2, but with a precision which has been

amplified by the ratio of the carrier frequency to the beat frequency (over that normally

achievable with this same time interval counter). The time difference x(i) for the

ith measurement between oscillators 1 and 2 is given by eq (1.5).

38

where Δ t(i) is the ith time difference as read on the counter, τ b is the beat period, ν 0 is

the nominal carrier frequency, ф is the phase delay in radians added to the signal of

oscillator 1, and k is an integer to be determined in order to remove the cycle ambiguity.

It is only important to know k if the absolute time difference is desired; for frequency and

frequency stability measurements and for time fluctuation measurements, k may be

assumed zero unless one goes through a cycle during a set of measurements. The

fractional frequency can be derived in the normal way from the time fluctuations.

In eqs (1.5) and (1.6), assumptions are made that the transfer (or common) oscillator is

set at a lower frequency than oscillators 1 and 2, and that the voltage zero crossing of the

beat ν 1 - ν c starts and that ν 2 - ν c stops the time interval counter. The fractional

frequency difference may be averaged over any integer multiple of τ b:

where m is any positive integer. If needed, τ b can be made to be very small by having

very high beat frequencies. The transfer (or common) oscillator may be replaced with a

low phase-noise frequency synthesizer, which derives its basic reference frequency from

oscillator 2. In this set-up the nominal beat frequencies are simply given by the amount

that the output frequency of the synthesizer is offset from ν 2. Sample times as short as a

few milliseconds are easily obtained. Logging the data at such a rate can be a problem

without special equipment.

39

3.6.3 Loose phase lock loop method

This first type of phase lock loop method is illustrated in figure 21. The signal from an

oscillator under test is fed into one port of a mixer. The signal from a reference oscillator

is fed into the other port of this mixer. The signals are in quadrature, that is, they are 90

degrees out of phase so that the average voltage out of the new mixer is nominally zero,

and the instantaneous voltage fluctuations correspond to phase fluctuations rather than to

amplitude fluctuations between the two signals. The mixer is a key element in the system.

The advent of the Schottky barrier diode was a significant breakthrough in making low

noise precision stability measurements. The output of this mixer is fed through a low pass

filter and then amplified in a feedback loop, causing the voltage controlled oscillator

(reference) to be phase locked to the test oscillator. The response time of the loop is

adjusted such that a very loose phase lock (long time constant) condition exists. This is

discussed later in section VIII.

Figure 21: Direct measurement of the phase difference between two oscillators

yields excellent precision. The technique requires electronic frequency control of a

clean reference oscillator to maintain a loose phase lock, hence a zero beat.

The response (or attack) time is the time it takes the servo system to make 70% of its

ultimate correction after being slightly disturbed. The response time is equal to 1/ π wh,

where wh is the servo bandwidth. If the response time of the loop is about a second, then

40

the voltage fluctuations will be proportional to the phase fluctuations for sample time

shorter than one second. Depending on the coefficient of the tuning capacitor and the

quality of the oscillators involved, the amplification used may vary significantly but may

typically range from 40 to 80 dB via a good low noise amplifier. In turn this signal can be

fed to a spectrum analyzer to measure the Fourier components of the phase fluctuations.

It is of particular use for sample times shorter than one second (for Fourier frequencies

greater than 1 Hz) in analyzing the characteristics of an oscillator. It is specifically very

useful if one has discrete side bands such as 60 Hz or detailed structure in the spectrum.

One may also take the output voltage from the above amplifier and feed it to an A/D

converter. This digital output becomes an extremely sensitive measure of the short term

time or phase fluctuations between the two oscillators. Precisions of the order of a

picosecond are easily achievable.

41

3.6.4 Tight phase lock loop method

The second type of phase lock loop method (shown in figure 22) is essentially the same

as the first in figure 21 except that in this case the loop is in a tight phase lock condition;

i.e., the response time of the loop is much shorter than the sample times of interest--

typically a few milliseconds. In such a case, the phase fluctuations are being integrated so

that the voltage output is proportional to the frequency fluctuations between the two

oscillators and is no longer proportional to the phase fluctuations (for sample times

longer than the response time of the loop). A bias box is used to adjust the voltage on the

varicap to a tuning point that is fairly linear and of a reasonable value. The voltage

fluctuations prior to the bias box (biased slightly away from zero) may be fed to a voltage

to frequency converter which in turn is fed to a frequency counter where one may read

out the frequency fluctuations with great amplification of the instabilities between this

pair of oscillators. The frequency counter data are logged with a data logging device. The

coefficient of the varicap and the coefficient of the voltage to frequency converter are

used to determine the fractional frequency fluctuations, yi, between the oscillators, where

i denotes the ith measurement as shown in figure 22. It is not difficult to achieve a

sensitivity of a part in 1014 per Hz resolution of the frequency counter, so one has

excellent precision capabilities with this system.

Figure 22: Tight phase loop lock method

42

3.6.5 Time difference method

The last measurement method we will illustrate is very commonly used, but typically

does not have the measurement precision more readily available in the first four methods

illustrated above. This method is called the time difference method and is shown in figure

23. Because of the wide bandwidth needed to measure fast rise-time pulses, this method

is limited in signal-to-noise ratio. However, some counters are commercially available

allowing one to do signal averaging or to do precision rise-time comparison (precision of

time difference measurements in the range of 10 ns to 10 ps are now available). Such a

method yields a direct measurement of x(t) without any translation, conversion, or

multiplication factors. Caution should be exercised in using this technique even if

adequate measurement precision is available because it is not uncommon to have

significant instabilities in the frequency dividers shown in figure 23--of the order of a few

nanoseconds.

The technology exists to build better frequency dividers than are commonly available, but

manufacturers have not yet availed themselves of state-of-the-art techniques in a cost

beneficial manner. A trick to bypass divider problems is to feed the oscillator signals

directly into the time interval counter and observe the zero voltage crossing into a well

matched impedance. (In fact, in all of the above methods one needs to pay attention to

impedance matching, cable lengths and types, and connectors). The divided signal can be

used to resolve cycle ambiguity of the carrier, otherwise the carrier phase at zero volts

may be used as the time reference. The slope of the signal at zero volts is 2 p V0/t 1,

where t 1 = 1/n 1 (period of oscillation). For V0 = 1 volt and a 5 MHz signal, this slope is

3m volts/ns, which is a very good sensitivity.

43

Figure 23: Measurement of the time difference between two oscillators, usually afterdivision by N to obtain 1 pulse-per-second, yields only moderate measurement performance compared to previous methods. The technique is dependent on several properties of the counter and its trigger circuits.

44

4. Automatic Switching system

4.1 Need for an automatic switching system

At present, NPLI has 5 cesium atomic clocks (NPL 1, NPL 2, NPL 3, NPL 6 and NPL 7).

Data from all the clocks is taken in order to

• Measure their stability by calculating their Allan variance.

• Generation of time scale which can be done in three ways

1. Weighted average

2. Kalman filter

3. Fuzzy logic

Moreover, the data is to be sent to BIPM, Paris which is the central organization for

standards in the world.

Since there can be 7 cesium atomic clocks at NPLI, we can have 42 combinations for

selecting any two clocks and taking the data. This procedure if followed manually is a

very cumbersome process. Hence, this demands for a switching system which can have 7

inputs for the 7 cesium atomic clocks and have 2 outputs corresponding to any pair of

clocks.

45

4.2 Design requirements of automatic switching system

The design of the switching system demands that:

• The time delay of the clock pulses through the switching system should be as low as

possible.

• The delay involved should be constant i.e. it should not change with time. This pre-

requisite eliminates the use of logic gates in the system. This is because the delay

involved with the logic gates is not uniform. It varies with time.

• A mechanism should be present which should indicate that which two clocks have

been selected for start and stop.

• The design requires uninterrupted power supply so that there is no interruption while

readings are being taken.

• The system should be programmable so that the entire process of taking the

measurement is automatic. However, it should also be manually controllable so

that readings can be taken even if the system is not being controlled by a

computer.

46

4.3 Internal architecture of automatic switching system.

Figure 24: Internal architecture of switching system.

• The signals to select a pair of Cesium atomic clocks, either manually or

automatically are fed into the microcontroller. The output of the microcontroller is

then fed to the relay drivers.

• The relay drivers provide the necessary current to drive the relays.

• There are two sets of 7 relays each, for the start pulse and the stop pulse. Apart

from them, the relays also take the output pulse of 1pps from the Cesium atomic

clocks as inputs.

• The combination of the signals from the Cesium atomic clocks and the relay

divers selects the desired relays for the start and stop signals.

• The output of the relays, through LED indicates which Cesium atomic clock has

been selected as the source of the start pulse and which as the source of the stop

pulse.

47

4.4 Circuit diagram of switching system.

Keeping in mind the above design requirements various stages of the automated

switching system design is given below along with the detailed description of each part,

their specification and need for their usage.

• Power Supply

• Microcontroller

• RS-232

• Relays and Relay Drivers

• LED’s

Figure 25: Circuit of the Switching System

Relays

Rectifier

Regulator MAX232

Drivers

Transformer

Microcontroller

Filter

48

4.4.1 Power supply One of the pre-requisites of the switching system is an uninterrupted power supply. There

cannot be a power failure while measurements are being done.

The system requires two voltage levels

• +5V for the TTL driven components like 8535 microcontroller, MAX 232, LED’s,

7805 regulator.

• +24V for biasing the relay drivers.

The different components of the power supply are as follows:

1. Transformer: Standard secondary center tapped 9V-0-9V transformer is used and

both the 9V terminals are taken out to form the 18V a.c.power supply while the 0V

terminal is kept grounded.

2. Bridge Rectifier: The bridge rectifier consisting of four diodes is used to rectify the

voltage so as to make the voltage waveform unidirectional.

Graph 2: Bridge Rectifier Output

49

3. Capacitive Filter: The output of the bridge rectifier contains ripples. Hence, three

capacitors are used to filter out the a.c. component of voltage from the output of the

bridge rectifier. Two of the capacitors are electrolytic having a value of 1000µF while

the other one is a small ceramic capacitor whose value is of the order of nF.

Graph 3: Filter Output

4. Regulator: Since a supply of +5V volt is required for the TTL driven devices, hence

LM 7805 regulator is used for regulating the output from SLB2040 to +5V. It is a

standard IC which is easily available. It has three terminals one for the input, second

for the output and third is ground.

5. Backup: The system has a provision of two backup supplies since the system can

never be turned off. One of the supplies is from the output of the rectifier whereas the

other backup supply, which consists of two supplies, is from the battery room of the

time and frequency section and is of +24V D.C.

6. Protection Devices: Diodes are connected in front of all the power supplies in order

to avoid back charging of the power supplies. The diodes allow current to flow from

the power supplies to the load but not into them since in such a situation the diodes

will become reversed biased and will not conduct any current.

50

D8

UNREGULATED

24Vdc

BATTERY

24Vdc

D9

D6

C6C5

C12C7

0

0

0

0

U1

LM 7805

IN

1

OUT

2

3

GND

C81C82

0

0

CxCyD7

0

SLB2040

J1(3,4)

J1(1,2)

D5D6

D3D2

VSOURCE

FREQ = 50

VAMPL = 220

VOFF = 0

T1

TRANSFORMER CT

15

6

48

0

+24V

-

+

BRIDGE RECTIFIER

2

1

3

4

C8

1000u

C13

1000u

C14

1n

51

• 220V a.c. is fed to the center tapped transformer which steps it down to 18V a.c.

• Output of the centre tapped transformer is fed to the bridge rectifier which makes the

voltage and current unidirectional.

• This voltage has some ripples which are filtered using capacitors.

• This voltage is fed at the intersection point of the two backup supplies through a

diode to prevent back flow of current into the supply. Diodes are also used in front of

the backup supplies as well to prevent overcharging of the backup batteries.

• The +24V supply thus generated is then used to bias the relay driver i.e. MOSFET

2N7000.

• The +24V supply is further fed to a LM 7805 regulator which regulates the voltage to

+5V which is necessary for devices operating on transistor - transistor logic (TTL).

52

4.4.2 Microcontroller

A large number of microcontrollers are available for usage - 8051 and PIC being the most

famous ones. Yet the switching system uses ATmega 8535 microcontroller. This is

because the microcontroller offers many features not available in other microcontrollers:

• Low power consumption - This is essential since the microcontroller is enclosed

in a metallic case and there is no provision of cooling.

• 32 general purpose 8 bit registers.

• Byte oriented two wire serial interfaces.

• Programmable USART (universal synchronous asynchronous receiver

transmitter).

• Its machine cycle is same as its clock cycle whereas standard 8051 has a machine

cycle equal to twelve clock cycles which makes 8535 twelve times faster than 8051.

The microcontroller is used to select the desired pair of atomic clocks.

One port of the microcontroller, consisting of eight pins, is connected to the thumb wheel

pair, TW1 and TW2. The signal from the thumb wheels are fed into the microcontroller

where they are processed and the outputs from the microcontroller are fed to

corresponding relay drivers. The relay drivers then produce a low output current which

ultimately drives the relays to select the desired pair of atomic clocks.

Two pins (14 and 15) of Port D of the microcontroller are connected to the MAX232 IC.

The other side of the MAX232 IC is connected to the DB9 connector. The function of

MAX232 IC is to convert the RS-232 (supported by COM port) to TTL (supported by

microcontroller).

The signal from the computer is fed into the microcontroller where they are processed

and the outputs from the microcontroller are fed to corresponding relay drivers. The relay

53

drivers then produce a low output current which ultimately drives the relays to select the

desired pair of atomic clocks.

The process involving thumb wheels constitute the manual selection mode while the

process involving the computer and MAX232 constitute the automatic selection mode.

Figure 26: Pin Diagram of ATmega8535

54

VCC Digital supply voltage.

GND Ground.

Port A (PA7 - PA0)

Port A serves as the analog inputs

to the A/D Converter. Port A also

serves as an 8-bit bi-directional I/O

port, if the A/D Converter is not

used.

Port B (PB7 - PB0)

Port B is an 8-bit bi-directional I/O

port. Port B also serves the

functions of various special features

of the ATmega8535.

Port C (PC7 - PC0) Port C is an 8-bit bi-directional I/O

port.

Port D (PD7 - PD0)

Port D is an 8-bit bi-directional I/O

port. Port D also serves the

functions of various special features

of the ATmega8535.

RESET Reset input.

XTAL1

Input to the inverting Oscillator

amplifier and input to the internal

clock operating circuit.

XTAL2

Output from the inverting Oscillator

amplifier.

AVCC

Supply voltage pin for Port A and

for the A/D Converter.

AREF

AREF is the analog reference pin

for the A/D Converter.

55

Table 1: Pin Configuration of 8535

Figure 27: Block diagram of ATmega8535

In order to maximize performance and parallelism, the microcontroller uses Harvard

architecture – with separate memories and buses for program and data. Instructions in the

program memory are executed with a single level pipelining. While one instruction is

being executed, the next instruction is pre-fetched from the program memory. This

concept enables instructions to be executed in every clock cycle. The program memory is

In-System Re-Programmable Flash memory.

1. General Purpose Register

There are 32 x 8 general purpose registers with a single clock cycle access time. Six of

the 32 registers can be used as three 16-bit indirect address register pointers.

56

2. Arithmetic Logical Unit (ALU)

The ALU operations are divided into three main categories – arithmetic, logical, and bit-

functions.

3. Status Register

The Status Register contains information about the result of the most recently executed

arithmetic instruction. The Status Register is updated after all ALU operations.

4. Stack Pointer

The Stack is mainly used for storing temporary data, for storing local variables and for

storing return addresses after interrupts and subroutine calls.

5. Flash Program Memory

The ATmega8535 contains 8K bytes on-chip In-System Reprogrammable Flash memory

for program storage.

6. SRAM(Static RAM) Data Memory

The 608 Data Memory locations address the Register File, the I/O Memory, and the

internal data SRAM.

7. EEPROM(Electrically Erasable Programmable ROM) Data Memory

The ATmega8535 contains 512 bytes of data EEPROM memory.

57

Connection b/w Microcontroller & MAX-232

8535

AVCC 30

AREF 32

VCC10GND 31GND11

XTAL112

XTAL213

RESET9

PB01

PB12

PB23

PB34

PB45

PB56

PB67

PB78

PD014

PD115

PD216

PD317

PD418

PD519

PD620

PD721

PA0/ADC0 40

PA1/ADC1 39

PA2/ADC2 38

PA3/ADC3 37

PA4/ADC4 36

PA5/ADC5 35

PA6/ADC6 34

PA7/ADC7 33

PC7/TOSC2 29

PC6/TOSC1 28

PC5 27

PC4 26

PC3 25

PC2 24

PC1/SDA 23

PC0/SCL 22

U4

MAX232

C1+ 1C1- 3C2+ 4C2- 5V+ 2V- 6

R1OUT12 R2OUT9T1IN 11T2IN 10

R1IN 13R2IN 8T1OUT14 T2OUT7

TW1TW2

CONNECTOR DB9

P1

(GND) 5(RI) 9

(DTR) 4(CTS) 8(TxD) 3(RTS) 7(RxD) 2(DSR) 6(DCD) 1

0

58

• For manual mode of operation, the two thumbwheels, TW1 and TW2, are used,

whose signals are fed to the microcontroller.

• The microcontroller processes the signals from TW1 and TW2 and generates output

signals to select the desired pair of relay drivers and eventually the relays.

• When using the PC mode, the desired pair of clocks are selected using a software pro-

gram. The signal is sent to MAX232 through the DB9 connector.

• MAX232 converts the RS-232 logic into TTL and sends the output signal to pins 14

and 15 of the microcontroller.

• The microcontroller processes the signals from MAX232 and generates output signals

to select the desired pair of relay drivers and eventually the relays.

59

4.4.3 RS-232

RS-232 was created to interface between Data Terminal Equipment (DTE) and Data

Communications Equipment (DCE) employing serial binary data interchange. So as

stated the DTE is the terminal or computer and the DCE is the modem or other

communications device.

Most equipment using RS-232 serial ports use a DB-25 type connector. Many PCs today

use DB-9 connectors since all you need in asynchronous mode is 9 signals. Normally the

male connector is on the DTE side and the female connector is on the DCE side even if

this is not always the case.

• Serial Communication

In serial communications data is transferred from sender to receiver one bit at a time

through a single link. The serial port takes 8, 16 or 32 parallel bits from the computer bus

and converts it as an 8, 16 or 32 bit serial stream. The name serial communications comes

from the fact that each bit of information is transferred in series from one location to

another.

In theory a serial link would only need two wires, a signal line and a ground, to move the

serial signal from one location to another. But in practice this doesn't really work for a

long time, some bits might get lost in the signal and thus altering the ending result. If one

bit is missing at the receiving end, all succeeding bits are shifted resulting in incorrect

data when converted back to a parallel signal. Hence, to establish reliable serial

communications these bit errors must be overcome.

• Methods of Serial Transmission

Two common methods are deployed:

1. Asynchronous Mode

2. Synchronous M ode

60

In synchronous data transfer, the receiving system and the transmitting system are

synchronized using a single clock that precisely times the period separating each bit. By

checking the clock the receiving end can determine if a bit is missing or if an extra bit has

been introduced in the stream. One important aspect of this method is that if either end of

the communication loses its clock signal, the communication is terminated.

In asynchronous data transfer, data bits are added at the beginning and end of the data

stream to mark the start and end of data transmission. By introducing a start bit which

indicates the start of a short data stream, the position of each bit can be determined by

timing the bits at regular intervals, by sending start bits in front of each 8 bit streams, the

two systems don't have to be synchronized by a clock signal, the only important issue is

that both systems must be set at the same port speed. When the receiver receives the start

bit it starts a short term timer. By keeping streams short, there's not enough time for the

timer to get out of synchronization.

Figure 28: Asynchronous Data Transfer

61

• DB9 Connector

The different pins and their functions are enumerated below.

Figure 29: DB9 Connector Configuration

62

The MAX232 is an integrated circuit that converts signals from an RS-232 serial port to

signals suitable for use in TTL compatible digital logic circuits. The MAX232 is a dual

driver/receiver.

The drivers provide RS-232 voltage level outputs from a single + 5 V supply.

The receivers reduce RS-232 inputs to standard 5 V TTL levels.

Logic ‘1’ of TTL corresponds to the range of -3V to -24V and logic ‘0’ corresponds to

+3V to +24V.

Figure 30: Pin Diagram of MAX232

VCC +5V supply voltage

GND Ground

C1+,C1-, C2+,C2- Capacitor

T1IN, T2IN Transmit in

T1OUT, T2OUT Transmit out

R1IN, R2IN Receive in

R1OUT, R2OUT Receive out

Vs+ Connected to VCC through a capacitor

Vs- Connected to GND through a capacitor

Table 2: Pin Configuration of MAX232

63

4.4.4 Reed relay & Relay driver

The reed switch is an electrical switch operated by an applied magnetic field. It was

invented at Bell Telephone Laboratories in 1936 by W. B. Ellwood.

The reed switch contains a pair of magnetizable and electrically conductive metal reeds

which have end portions separated by a small gap when the switch is open. The reeds are

hermetically sealed in opposite ends of a tubular glass envelope.

A magnetic field (from an electromagnet or a permanent magnet) will cause the contacts

to pull together, thus completing an electrical circuit. The stiffness of the reeds causes

them to separate, and open the circuit, when the magnetic field ceases.

Reed relays are relatively small in comparison to other electro-magnetic relays. The use

of flexible reeds and self-attraction distinguishes the reed relay from other electro-

magnetic relays.

Figure 31: Reed Relay & Reed Switches

64

The reed relay consists of a pair of contacts on ferrous metal reeds in a hermetically

sealed glass envelope. This tube is surrounded by a magnetic coil which, when activated,

magnetizes the reeds and causing them

to attract each other which closes the contacts. When the coil is de-energized, the spring

tension in the reeds causes them to separate again.

Figure 32: Contacts of a Reed Switch (Side)

Figure33:Contacts of a Reed Switch (Top)

One or more reed switches inside a coil forms a reed relay. Reed relays are used when

operating currents are relatively low, and offer high operating speed, good performance

with very small currents which are not reliably switched by conventional contacts, high

reliability and long life. The inert atmosphere around the reed contacts ensures that

oxidation will not affect the contact resistance.

65

Diagram of Reed Relay & Its Protection Devices

When current flows through the coil of the relay, a magnetic field is developed around it.

As a result the contact switches ‘S1’ and ‘S2’ are attracted towards the coil. Hence, the

switches are closed and provide a path for the current to flow.

The diode D and capacitor C are used as protection devices. When the current through the

relay coil ceases to exist, the magnetic field around the relay coil disappears. As a result

the switches ‘S1’ and ‘S2’ move to their initial state; hence breaking the closed loop. But

the current in the inductor cannot go to zero instantaneously. As the result the current can

flow out through the terminal A and damage the relay driver. To prevent it, a diode has

been used to provide a closed loop path for the current to flow. The capacitor C also helps

in the process of discharging the relay coil.

This type of design has the advantages of high speed operation, long life, and very low

price.

CO

IL

1

2

CD S1 S2

A

66

Relay Driver

There are 14 relay drivers for the 14 relays. MOSFET 2N7000 has been used as a

component of the relay driver. A schematic diagram of the relay driver has been shown

below.

Figure 34: Relay Driver • +24V is required to bias the MOSFET 2N7000, and bring it into the conduction

mode. The +24V is supplied from the power supply.

• +5V is required to turn ON the LED.

• When the MOSFET is biased using +24V and current flows through the relay

coil, a magnetic field is developed around it. As a result, the switches are closed.

• Once the switch S1 is closed, the LED gets a path to +5V and glows to indicate

the start/stop pulse of the clock.

• Simultaneously, the switch S2 also gets closed. Hence, the desired clock is

selected whose signal of 1pps is obtained at the output terminal of the selector switch.

0

+24V

1

2

C D

+5V

2N7000

R

To LED

S1 S2

To start or stop Of TIC

From CLOCK

From Microcontroller

67

The transfer characteristics (ID vs. VGS) and the Ohmic region characteristics (ID vs. VDS)

of MOSFET 2N7000 has been shown below.

Graph 4: Transfer Characteristics

Graph 5: Ohmic Region

68

4.4.5 Light Emitting diode(LEDs)

LEDs are based on the semiconductor diode. When the diode is forward biased, the

electrons are able to recombine with holes and energy is released in the form of light.

This effect is called electroluminescence and the color of the light is determined by the

energy gap of the semiconductor.

• The Physics behind LED

Figure 35: Parts of an LED

Like a normal diode, the LED consists of a chip of semiconducting material impregnated

with impurities to create a p-n junction. As in other diodes, current flows easily from the

p-side to the n-side, but not in the reverse direction. Charge-carriers—electrons and

holes—flow into the junction from electrodes with different voltages. When an electron

meets a hole, it falls into a lower energy level, and releases energy in the form of a

photon.

69

Figure 36: Inner workings of an LED

The wavelength of the light emitted, and therefore its color, depends on the band gap

energy of the materials forming the p-n junction.

The materials used for the LED have a direct band gap with energies corresponding to

near-infrared, visible or near-ultraviolet light.

• Tri-color LED

In tri-color LED more than one LED is housed in one package. Tri-color LEDs may have

two or three leads depending on intentional connection method.

Three lead LEDs have common cathode lead to which both LEDs are connected

internally.

Figure 37: Packaged Tri-color LED

70

In order to turn one of the LEDs or both at the same time connect cathode to ground via

current limiting resistor whose value is equal to one LED required limiting.

The most popular type of tri-color LED has a red and a green LED combined in one

package with three leads. They are called tri-color because mixed red and green light

appears to be yellow and this is produced when both the red and green LEDs are ON.

Figure 38: Internal Diagram of a Tri-Color LED

The diagram shows the construction of a tri-color LED. The centre lead (k) is the

common cathode for both LEDs; the outer leads (a1 and a2) are the anodes to the LEDs

allowing each one to be lit separately, or both together to give the third color.

71

4.5 Front & Rear view of switching system

Front View:

Figure 39: Front View of the Switching System

Figure 40: Rear View of the Switching System

RESET

LED

THUMB WHEEL POWER SUPPLY

SELECTION

START OUTPUT

MANUAL / AUTOMATIC MODE

SELECTION

STOP OUTPUT

CESIUM CLOCK INPUTS 24V BATTERY INPUT

220V A.C. INPUT DB9 CONNECTOR PORT

72

4.6 Accessing the switching system.

The switching system supports standard RS-232 serial port (1200 baud rate, 8 data bits,

No parity, 1 stop bit, no hardware flow). The switching can easily be controlled by a PC

equipped with a RS-232 port. Hyper terminal is windows based software which can be

used to access the Switching system.

Step 1: Open Hyper Terminal from

Start All Programs Accessories Communications HyperTerminal

Step 2: Establish connection.

Figure 41 Step 2

73

Step 3: Setting up usage port which through which the computer is configured to

connect to the selector switch (here it is COM1).

Figure 42: Step 3

Step 4: Next set the baud rate, data bits, parity, stop bits and flow control as per the

configuration of the system (here baud rate is 1200 bps, data bits is 8, parity is none, stop

bits is 1 and flow control is none).

Figure 43 Step 4

74

Step 5: Follow the following path to enable echo of characters.

FileProperties Settings ASCII Setup

Then enable “Echo Typed Characters Locally”.

Click OK and then start sending data through Hyper Terminal.

Figure 44 Step 5

75

Step 6: Now we can select any pair of clock as desired. For eg. If we want to select

clock1 and clock2, then we will send “12” prefixed with character “S”. ‘S’ is just a

signature to indicate that new pair of clocks are being selected.

Figure 45: Step 6

76

4.7 Measurement of Circuit delay of the Switching system

Figure 46: Experimental Setup

There are 5 Cesium atomic clocks at NPLI. The difference between their respective start

and stop pulses is of the order of micro-seconds. Since this difference is very small, the

cable delay also needs to be taken into account for accurate and precise measurement

which is of the order of nanoseconds. The time taken for a signal to travel along a

distance of one meter is 3 nanoseconds. But, this may vary with different signals. Hence

it becomes essential to determine the delay of cables prior to their use and use those

cables only. The device that can be used for measurement of time delay along a cable is

time interval counter. It tells the time delay between receptions of two pulses.

In order to calculate the delay of a cable a signal is applied to the start input of the time

interval counter and the cable whose delay is to be found carries the signal from the start

to the stop of the time interval counter. Hence the time interval counter shows the time

taken for the signal to travel along the cable.

77

In order to measure the delay of the signal through the switching system, a single clock

source is used and is fed to any two inputs of the selector switch. The two inputs are

selected either manually or using a program. The outputs of the switching system are fed

into a time interval counter whose delays are known.

The cable delays and the selector switch delays are important as we are working in the

areas of ultra-high precision and accuracy. Hence, if a delay of the order of few

nanoseconds is compensated for, the result will be even more precise and closer to the

global accepted standard and will help in making a better and precise time scale.

The average value of the circuit delays (taking into account cable delays) and the

variation of the corresponding jitter are graphically shown below.

78

Graph 6: Variation of average values of S1


AVG NET S1

9.0849.2199.0098.9498.9599.084

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S12 to S17

Ave

rage

(ns)

AVG S2 NET

10.614

8.9549.0698.9148.9449.029

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S21 to S27

Aver

age

(ns)

79



AVERAGE NET S3

8.8949.0548.9799.1149.0448.844

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S31 to S37

Aver

age

(ns)

AVERAGE NET S4

8.7048.9498.9898.8648.6398.494

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S41 to S47

Aver

age

(ns)

80



AVERAGE NET S5

8.8699.2598.8748.5798.4398.399

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S51 to S57

Aver

age

(ns)

AVERAGE NET S6

8.8548.7298.4748.2598.2148.064

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S61 to S67

Aver

age

(ns)

81


Graph 13: Variation of Jitter for S1

AVERAGE NET S7

8.6098.2448.0847.9347.8197.789

0

1

2

3

4

5

6

7

8

9

10

11

0 1 2 3 4 5 6 7S71 to S76

Aver

age

(ns)

JITER S1

0.5875340.680626

0.5909240.674364 0.633049

0.635911

00.10.20.30.40.50.60.70.80.9

1

0 1 2 3 4 5 6 7S12 to S27

JITT

ER(n

s)

82



JITTER S2

0.6448580.614257

0.6508770.690969

0.6438980.626683

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S21 to S27

JITT

ER(n

s)

JITTER S3

0.6272950.652024

0.6169810.6386570.6434310.637596

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S31 to S37

JITT

ER(n

s)

83



JITTER S4

0.6119710.650547 0.657746

0.603173 0.6006600.586635

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S41 to S47

JITT

ER(n

s)

JITTER S5

0.6606540.620833 0.6285620.6139940.6355810.616326

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S51 to S57

JITT

ER(n

s)

84



JITTER S6

0.6344360.655096

0.684699

0.6090390.6364390.661880

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S61 to S67

JITT

ER(n

s)

JITTER S7

0.6215160.6082460.6526900.661681

0.6280790.663461

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 1 2 3 4 5 6 7S71 to S76

JITT

ER(n

s)

85

Graph 20 Start Cable delay

Graph 21: Stop Cable delay

start cable delay

0

1

2

3

4

5

6

7

8

9

10

0 10 20 30 40 50 60 70 80 90 100number of samples

valu

e of

sam

pls(

ns)

stop cable delay

0

1

2

3

4

5

6

7

8

9

10


valu

e of

sam

ples

(ns)

86

Graph 22: Back Cable delay

cable delay back

0

1

2

3

4

5

6

7

8

9

10


valu

e of

sam

ples

(ns)

87

5. Automated Data Acquisition System .

Figure 47: Automated Data Acquisition system for time scale generation

NPL1

NPL6

NPL2

NPL7

NPL3

FREQUENCY & PHASE OFFSET GENERATOR

50 ps COUNTER

SRS-620

HP53131 TIC

SELECTOR SWITCH

PC

88

The automated Data acquisition System consists of the following:

1.) 5 Cesium Atomic clocks. :- Cesium atomic clocks are used for generating

time. IST (Indian standard time) is generated using ensemble of these clocks.

They are named as NPL1, NPL2, NPL3, NPL6, and NPL7.

2.) Automatic Selector Switch:- It is a system used for selecting any pair of the

cesium clocks.

3.) PC:- This PC will be used to store the data i.e. the time difference b/w clock,

which will be later on used for time scale generation.

4.) Time Interval Counter: A time interval counter is used for computing the

time difference between two cesium atomic clocks. Presently at NPL, there

are two Time interval counter available

i) Universal Time interval counter ( SR620)

ii) Universal Counter(HP53131/32)

5.) HROG-5:- It is a high resolution phase and frequency offset generator.

89

5.1 Block diagram of setup for intercomparison of clocks

Figure 48: Block diagram.

• One pulse per second (1pps) signal from all seven cesium atomic clocks are fed

into the selector switch whose function is to give a constant delay path to the

clock pulses.

• Selector switch selects any two of the seven clock pulses. It can either be done

manually by using two thumbwheel switches for start and stop or using a

software program.

• The two clock signals viz. start and stop are fed into the time interval counter

whose function is to calculate the delay between start and stop pulses at the two

inputs. The data from the time interval counter can also be received at the

computer terminal by a program.

• The data thus collected from the time interval counter can be used to make time

scales namely paper time scale and physical time scale.

90

5.2 Universal Time interval Counter (SRS-620)

The SR620 Time Interval Counter performs virtually all of the time and frequency

measurements required in a laboratory or ATE environment. The instrument's single-shot

timing resolution and low jitter make it the counter of choice for almost any application.

The SR620 measures time interval, frequency, pulse-width, rise and fall time, period,

phase and events. Time intervals are measured with 25 ps rms resolution, making the

SR620 one of the highest resolution counters available. The instrument's high single-shot

timing resolution, low jitter, and reciprocal counting architecture allow rapid, high

resolution measurements.

Some main features of Time interval counter:

• 25 ps single-shot time resolution

• 1.3 GHz frequency range

• 11-digit frequency resolution (1 s)

• 0.001° phase resolution

• Statistical analysis & Allan variance

• Graphical output to X-Y scopes

• Hardcopy to printers and plotters

• GPIB and RS-232 interfaces

• Optional ovenized timebase

Our main purpose of using the counter is to measure time interval between two cesium

clock with accuracy in nanoseconds. Using SRS-620 model Time intervals from -1ns to

1000 s or +- 1000 s may be measured.

91

5.3 Interfacing SRS-620 to PC

Figure 48: Connection b/w PC and TIC (SRS620)

The SR620 Universal Time Interval Counter may be remotely programmed via either the

RS232 or GPIB (IEEE-488) interfaces. Any computer supporting one of these interfaces

may be used to program the SR620. Both interfaces are active at all times: the SR620 will

send responses to the interface which asked the question. All front and rear panel features

(except power) may be controlled.

92

Communicating with RS232

The SR620 is configured as a DCE (transmit on pin 3, receive on pin 2) and supports

CTS/DTR hardware handshaking. The CTS signal (pin 5) is an output indicating that the

SR620 is ready, while the DTR signal (pin 20) is an input that is used to control the

SR620's transmitting. If desired, the handshake pins may be ignored and a simple 3 wire

interface (pins 2,3 and 7) may be used. The RS232 interface baud rate, number of data

bits, and parity must be set. These may be set in the CTRL submenu of the

CONFIGuration menu. The RS232 delay programs the time interval between the SR620's

transmitted characters if no handshaking is used. The delay is equal to 2ms times the

setting and is usually set to 0 (no delay). However, some slower computers may require a

delay. The RS232 echo should be set OFF if the SR620 is connected to a computer. It

may be ON if connected to a terminal or a terminal emulation program.

The RS-232 baud rate, number of bits per character, and parity bit definition must be set

in the "ctrL" section of the CONFIG menu. The SR620 always sends two stop bits, and

will correctly receive data sent with either one or two stop bits. When connecting to a PC,

use a standard PC serial cable, not a "null-modem" cable. The SR620 is a DCE (Data

Communications Equipment) device, and so should be connected with a "straight" cable

to a DTE device (Data Terminal Equipment). The "minimum" cable will pass pins 2,3

and 7. For hardware handshaking, pins 5 and 20 (CTS and DTR) should be passed.

Occasionally, pins 6 and 8 (DSR and CD) will be needed: these lines are always asserted

by the SR620. There are several software problems which may occur when using the RS-

232 interface:

1) You have sent the wrong command to ask for data from the SR620. Your program

may wait forever for a response which will not come. This may not be your fault: we

have seen Microsoft's Interpreted Basic on an IBM PC occasionally send a curly bracket

(ASCII 253) when it was suppose to have sent a carriage return (ASCII 13).

2) Your computer's baud rate was changed by a previous program and no longer matches

the baud rate set for the SR620. Good programming practice requires that you set the

computer's baud rate at the start of each application program.

93

3) The initial command sent to the SR620 was invalid due to a garbage character left in

the SR620's command queue from power-up, or, the first character in your computers

RS-232 UART is garbage from when the SR620 was turned "ON". It is good practice to

send a few carriage returns to the SR620 to flush its command queue. Also, your program

should read and ignore any characters which may be left in the computer's UART.

4) The SR620 is not sending the correct 'end-of record' marker for your computer. For

Example, it appears that some FORTRANs require two carriage returns for an end-of

record marker. The "ENDT" command may be used to set the end-of-record sequence.

(The end-of-record marker is that sequence which indicates a response is complete. From

a keyboard, a single carriage return is the end-of- record marker.)

5) Answers are coming back from the SR620 to fast, overwriting previous responses

before the computer can get them. To increase the dwell time between characters, use the

"WAIT" command. The dwell time between characters will be 2n ms.

6) The RS-232 echo must be "OFF", otherwise all characters sent to the SR620 will be

echoed back to the source. (See the section on "Configuration Menus" for details on

RS232 configuration.) The computer will most likely confuse echoed commands with the

desired data.

RS232 echo and no echo operation

When the RS232 echo mode is ON the SR620 will echo all characters sent to it , will

send linefeeds in addition to carriage returns, and will return the prompts -> and ?> to

indicate that a command was either processed correctly or contained errors. The RS232

echo mode is good way to become familiar with the commands that the SR620 expects

and the values that it will return. When the unit is controlled by a computer, the echo

feature should be turned OFF.

94

5.4 Programming the Time interval counter

Command Syntax

Communications with the SR620 use ASCII characters. Commands may be in either

UPPER or lower case and may contain any number of embedded space characters. A

command to the SR620 consists of a four character command mnemonic, arguments if

necessary, and a command terminator. The terminator may be either a carriage return

<cr> or linefeed <lf> on RS232, or a linefeed <lf> or EOI on GPIB. No command

processing occurs until a command terminator is received. All commands function

identically on GPIB and RS232. Command mnemonics beginning with an asterisk "*"

are IEEE-488.2 (1987) defined common commands.

These commands also function identically on RS232. Commands may require one or

more parameters. Multiple parameters are separated by commas ",".Multiple commands

may be sent on one command line by separating them by semicolons ";". The difference

between sending several commands on the same line and sending several independent

commands is that when a command line is parsed and executed the entire line is executed

before any other device action proceeds. This allow synchronization to be achieved using

the synchronization commands.

Taking time interval measurement through Time interval Counter

1.) Initializing Time interval counter

By sending the following command we can configure the Counter for Time interval

measurement

“*RST;MODE0;SRCE0;SIZE10;AUTM0”

The meaning of different mnemonics Is as follows:

*RST = It is an interface control command. The *RST common command resets the

SR620 to its default configurations. It is the same as holding down "clr rel" at power on.

All modes are set to their default conditions.

95

MODE0 = It is a measurement control command. Its format is MODE(?)

The MODE command sets the instrument measurement mode according to the following

table: j mode

0 time

1 width

2 rise/fall time

3 frequency

4 period

5 phase

6 count

SRCE0 = The SRCE command sets the source of the measurement. The parameter j = 0

set the source to A, j = 1 sets the source to B, and j = 2 sets the source to REF.

Additionally, in frequency, period, and count modes j = 3 sets the source to ratio (A/B).

In phase mode the source is fixed and may not be set while in rise/fall time REF may not

be selected as the source.

SIZE10 = The SIZE command sets the number of samples in a measurement. The

parameter x may be between 1 and 10^6 in a 1,2,5 sequence. The SIZE? query returns a

floating point number with one significant digit.

AUTM0 = The AUTM command sets the auto measurement mode. The parameter j = 1

sets the AUTO mode "ON" and the SR620 will automatically start a new measurement of

N samples when the old one is complete. The parameter j = 0 sets the AUTO mode

"OFF" and requires an individual command to start each measurement. It is

recommended that auto measurement be OFF if a computer is being used to take data as

this allows synchronizing of the measurements with the returned answers.

2.) Taking reading from TIC

"STRT;*WAI;XAVG?"

By sending the above command line to the TIC we can read the value of the time interval

between any pair of clock.

96

The different mnemonics are explained as follows:

STRT = The STRT command is equivalent to pushing the front panel START button. It

starts the measurement.

*WAI = The *WAI (wait) common command is a synchronization command that holds

off all further command execution until all in progress measurements/scans/prints are

complete. This command ensures that a particular operation is finished before continuing.

An example of the usefulness of this command is ensuring that a measurement is

complete before reading the answer. The command line STRT;*WAI; XAVG? will start

a measurement, wait until it is done, and send back the mean value.

XAVG? = The XAVG? query returns the value of the mean of the last completed

measurement. The number returned is a floating point value with up to 16 digits of

precision. If the REL is set the number returned is the REL'D value.

97

5.5 Serial port programming There are two popular methods of sending data to or from the serial port in Turbo C. One

is using outportb(PORT_ID, DATA) or outport(PORT_ID,DATA) defined in “dos.h”.

Another method is using bioscom() function defined in “bios.h”.

Using outportb() :

The function outportb () sends a data byte to the port ‘PORT_ID’. The function

outport() sends a data word. These functions can be used for any port including serial

port, parallel ports. Similarly to receive data these are used.

• inport reads a word from a hardware port

• inportb reads a byte from a hardware port

• outport outputs a word to a hardware port

• outportb outputs a byte to a hardware port

Declaration:

• int inport(int portid);

• unsigned char inportb(int portid);

• void outport(int portid, int value);

• void outportb(int portid, unsigned char value);

Remarks:

• inport works just like the 80x86 instruction IN. It reads the low byte of a word

from portid, the high byte from portid + 2.

• inportb is a macro that reads a byte

• outport works just like the 80x86 instruction OUT. It writes the low byte of

value to portid, the high byte to portid + 1.

• outportb is a macro that writes value Argument

98

portid:

• Inport- port that inport and inportb read from;

• Outport- port that outport and outportb write to

value:

• Word that outport writes to portid;

• Byte- that outportb writes to portid.

If you call inportb or outportb when dos.h has been included, they are treated as

macros that expand to inline code.

If you don't include dos.h, or if you do include dos.h and #undef the macro(s),

you get the function(s) of the same name.

Return Value:

# inport and inportb return the value read

# outport and outportb do not return

Using bioscom:

The macro bioscom () and function _bios_serialcom() are used in this method in the

serial communication using RS-232 connecter. First we have to set the port with the

settings depending on our need and availability. In this method, same function is used to

make the settings using control word, to send data to the port and check the status of the

port. These actions are distinguished using the first parameter of the function. Along with

that we are sending data and the port to be used to communicate.

99

Here are the deatails of the Turbo C Functions for communication ports.

Declaration:

bioscom(int cmd, char abyte, int port)

_bios_serialcom(int cmd ,int port, char abyte)

bioscom() and _bios_serialcom() uses the bios interrupt 0x14 to perform various

communicate the serial communication over the I/O ports given in port.

cmd: The I/O operation to be performed.

cmd (boiscom) cmd(_bios_serialcom) Action

0 _COM_INIT Initialise the parameters to the port

1 _COM_SEND Send the character to the port

2 _COM_RECEIVE Receive character from the port

3 _COM_STATUS Returns rhe current status of the

communication port

Table No.3

portid: port to which data is to be sent or from which data is to be read.

0:COM1

1:COM2

2: COM3

abyte:

When cmd =2 or 3 (_COM_SEND or _COM_RECEIVE) parameter abyte is ignored.

When cmd = 0 (_COM_INIT), abyte is an OR combination of the following bits (One

from each group):

100

value of abyte Meaning

Action Bioscom _bios_serialcom

0x02

0x03

_COM_CHR7

_COM_CHR8

7 data bits

8 data bits

0x00

0x04

_COM_STOP1

_COM_STOP2

1 stop bit

2 stop bits

0x00

0x08

0X10

_COM_NOPARITY

_COM_ODDPARITY

_COM_EVENPARITY

No parity

Odd parity

Even parity

0x00

0x20

0x40

0x60

0x80

0xA0

0xC0

0xE0

_COM_110

_COM_150

_COM_300

_COM_600

_COM_1200

_COM_2400

_COM_4800

_COM_9600

110 baud

150 baud

300 baud

600 baud

1200 baud

2400 baud

4800 baud

9600 baud

Table no.4

101

For example, if

abyte = 0x8B = (0x80 | 0x08 | 0x00 | 0x03) = (_COM_1200 | _COM_ODDPARITY |

_COM_STOP1 | _COM_CHR8)

the communications port is set to

1200 baud (0x80 = _COM_1200)

Odd parity (0x08 = _COM_ODDPARITY)

1 stop bit (0x00 = _COM_STOP1)

8 data bits (0x03 = _COM_CHR8)

To initialise the port with above settings we have to write,

bioscom(0, 0x8B, 0);

To send a data to COM1, the format of the function will be bioscom(1, data, 0).

Similarly bioscom(1, 0, 0 ) will read a data byte from the port.

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5.6 Flow chart

Select One pair

Take measurement

All pairs meas Completed

Select next pair

S= No of data for each measurement (e,g. 1 or 10) for averaging. S should be a multiple of 10. T= 0h or 4h or 8h or 12h or16h or 20h. TIC initialize= 1. Mode set in time interval mode. 2. Port A in Start Port B in Stop

N Y

N Y

Set “S”

Initialize TIC

Check T

103

5.6 C++ program for automated measurement.

#include <iostream.h> #include <conio.h> #include <time.h> #include <dos.h> #include <bios.h> #include <stdio.h> #include <stdlib.h> /*Header files for accessing different pre-defined functions*/ #include <string.h> #define TRUE 1 #define FALSE 0 #define ON 1 #define OFF 0 #define COM1 0 #define COM4 3 /* Serial Communication Ports defined */ #define SETTINGSS (0x80 | 0x00 | 0x00 | 0x03) /* Port settings for COM4 0x80 -> 1200 BAUD RATE 0X00 -> 1 STOP BIT 0x00 -> NO PARITY 0x03 -> 8 DATA BITS */ #define SETTINGS (0xE0 | 0x00 | 0x00 | 0x03) /* Port Settings for COM1 0xE0 -> 9600 BAUD RATE 0X00 -> 1 STOP BIT 0x00 -> NO PARITY 0x03 -> 8 DATA BITS */ char *datef(); //Function for generating filename long julian(); // Function prototype for calculating the MJD void main() // Beginning of the main function { clrscr();

struct time t; FILE *Dfile; char *fname; //fname-name of file long jd=0; struct date d; getdate(&d); char select[][2]={{'1','2'},{'1','3'},{'1','6'},{'1','7'},{'2','3'},{'2','6'},{'2','7'},

{'3','6'},{'3','7'},{'6','7'}}; int store,intt=0,in,date;

char s,hour,minute,second;

104

char reading; int status,i=0,ch=0,l; int DONE = FALSE,DONE2=OFF; char str[20],*endptr; double value,value1,value2; char arr110[]="*RST;MODE0;SRCE0;SIZE10;AUTM0\n";

/* Commands to set TIC mode. *RST = Clear instrument to default setting MODE0 = Set TIC mode to TIME INTERVAL SRCE0 = Set measurement source to A SIZE10 = Set number of sample to 10 AUTM0 = Auto measurement off. */

char arr120[]="STRT;*WAI;XAVG?\n"; /* Command to Receiving data from TIC

STRT = Start measurement *WAI = Wait unitl measurement is complete XAVG? = Return the average. */

clock_t start, end; struct tm *ltime; /* Time and timezone structure */

time_t system_time,t1; /* The system time in seconds from 1970 */ system_time=time(NULL); ltime=localtime(&system_time); if(ltime==NULL) exit(EXIT_FAILURE);

bioscom(0,SETTINGS,COM1); /* Initializing the TIC with defined settings */ status=bioscom(3,0,COM1); if(status!=0) { printf("Initialising the TIC%s");

while(arr110[i]!='\0') {

bioscom(1,arr110[i],COM1); delay(200);

i++; }

} else cout<<"No status:\n"; delay(1000); fname=datef(); Dfile=fopen(fname,"w+"); fprintf(Dfile,"S.No.\t Date\t MJD\tClockPair\tTime(us)\tHrs.\tMin.\tSec.%s\n\n"); bioscom(0,SETTINGSS,COM4); date=d.da_day;

105

while(1) { gettime(&t);

if((t.ti_hour==00 && t.ti_min==40 && t.ti_sec==00) ||

(t.ti_hour==04 && t.ti_min==42 && t.ti_sec==00) ||

(t.ti_hour==8 && t.ti_min==00 &&t.ti_sec==00) ||



(t.ti_hour==20 && t.ti_min==00 && t.ti_sec==00))

{getdate(&d); if(d.da_day!=date) { fname=datef();

Dfile=fopen(fname,"w+"); fprintf(Dfile,"S.No.\t Date\t MJD\tClockPair\tTime(us)\tHrs.\tMin.\tSec.%s\n\n"); date=d.da_day; } status = bioscom(3, 0, COM4); if(status) //IF STATUS IS CORRECT PROCEED { delay(300);

for(int m=0;m<10;m++) { DONE=FALSE; DONE2=OFF; intt++; cout<<"\nS.No."<<intt;

fprintf(Dfile,"%d\t",intt); jd=julian();

fprintf(Dfile,"%d.%d.%d ",d.da_day,d.da_mon,d.da_year); fprintf(Dfile,"\t%ld\t",jd); julian(); bioscom(1,'S', COM4); //S SENT i.e. THESIGNATURE fprintf(Dfile,"NPL"); for(int n=0;n<2;n++) { bioscom(1,select[m][n], COM4); /* Selection of Clock Pair */ fprintf(Dfile,"%c",select[m][n]); /* Write Clock Pair No. prefixed toreading*/ delay(250); } while(!DONE) { l=0; i=0; while(arr120[i]!='\0')

106

{ bioscom(1,arr120[i],COM1); i++; } while(!DONE2) { reading=bioscom(2,0,COM1); if(reading) { printf("%c",reading); str[l]=reading;l++; //reading=bioscom(2,0,COM1); if(reading=='\r') { //ch--; t1=time(NULL); ltime=localtime(&t1); { value = strtod(str, &endptr); if(value>0.9) { value2=(1-value)*1000000; printf("\nValue2 is:-%.6f",value2); fprintf(Dfile,"\t\t-%.6f",value2);DONE =TRUE;DONE2=ON; } else { value1=1000000*value; printf("\nValue is:%.6f",value1); fprintf(Dfile,"\t\t%.6f",value1);DONE =TRUE;DONE2=ON; } } printf("instant time----->\t%d\t%d\t%dsecs\t",ltime->tm_hour,ltime->tm_min,ltime->tm_sec); fprintf(Dfile,"\t%d\t%d\t%d\n",ltime->tm_hour,ltime->tm_min,ltime->tm_sec); } } }//on } sleep(15);

} } fclose(Dfile);

Dfile=fopen(fname,"a+"); } if(kbhit())

{ if((in = getch())==27) cout<<"\nUser stopped the measurement process. Program will exit:";

107

break; }

} cout<<"\nEnter to exit:";

getch(); } long julian() /* Function for calculating the MJD & write it to afile */ {

int ch,year,day,month,c=1,c1=0; struct date d; unsigned int mjd=54832,mjd1,count=0; getdate(&d); year=d.da_year; day=d.da_day; month=d.da_mon; if(year!=2009) {

for(int j=2009;j<year;j++) { count+=365; if(j%4==0) c1++; }

} if(year%4==0) c+=2; else c+=3; for(int i=1;i<month;i++)

{ if(i==11 || i==4 || i==6 || i==9) c++; count+=31; }

mjd1= count + day + mjd - c + c1; return mjd1;

} char *datef() //function for generating file name { struct date d;

getdate(&d); char *str,*day,*mon,*year; if(d.da_mon==1) mon="ja"; else if(d.da_mon==2) mon="fe"; else if(d.da_mon==3) mon="ma";

108

else if(d.da_mon==4) mon="ap"; else if(d.da_mon==5) mon="ma"; else if(d.da_mon==6) mon="ju"; else if(d.da_mon==7) mon="jy"; else if(d.da_mon==8) mon="au"; else if(d.da_mon==9) mon="se"; else if(d.da_mon==10) mon="oc"; else if(d.da_mon==11) mon="no"; else if(d.da_mon==12) mon="de"; itoa(d.da_day, day, 10); itoa(d.da_year,year,10); strcpy(str,day); strcat(str,mon); strcat(str,year); strcat(str,".txt"); return str;

}

109

6. Labs Visited at National Physical Laboratory 6.1 High Voltage & High Current Lab • A.C. is generated at the generating station.

• High current at low voltage is produced at the generating station. By using a step-up

transformer voltage is increased and current is reduced. As we know that when

current at high voltage is transmitted there is less power loss.

• Power at generating station is to be measured and is also to be measured at the load

center.

• Since the voltages and currents involved are very high, it is not possible to measure

them directly using ammeter and voltmeter.

• Hence voltage and current are stepped down using current transformer and potential

transformer. The parameter of a typical current transformer as shown on its name

plate is as follows:

Power Factor : 0.9

Power Loss : 15 % -18 %

Ratio : 300/5 = 60

Every current transformer will have an output current of 5A. Hence the ratio helps

in determining the input current which is 300A in this case.

• Class

Class of any current transformer tells its accuracy. For example, class 1 current

transformer has an accuracy of 1%.

• Standard current transformer at NPL (Primary Standard) has an accuracy of 10 ppm

(0.001 %).

110

Josephson Voltage Standards

• To maintain a primary voltage standard, Josephson Voltage Standard (JVS)

method is implemented. For maintaining a current standard, the following

equation is used,

I =

RV

For maintaining a resistance standard, Quantum – Hall resistance method is

employed.

• JVS is used to calibrate the secondary standards.

• For maintaining a secondary standard, 4 banks of Zener reference standard is

used, having output voltages of +10V and +1.018V (standard voltages).

• A calibrator is used as the working standard.

The a.c. Josephson Effect is now used by several national laboratories as the reference a

standard for the unit of d.c. voltage.

Large arrays of 2000 or more Josephson junctions have been used in many laboratories to

generate reference voltages on the order of 1 V. These devices can be used to calibrate

standard cells and other reference devices at the 1V level without the use of a voltage

divider. This has resulted in substantial improvements in accuracy and has greatly

simplified the operation of Josephson voltage standards.

Brian Josephson derived an equation for the current that would flow through a tunnel

junction formed by a thin insulating barrier separating two superconductors:

I = IO sin {

∫Vdt

he**4

π

}

111

In this equation, I is the junction current, IO is the critical current (a constant of the

junction), V is the junction voltage and

he

is the ratio of the elementary charge to

Planck’s constant.

When an ac current at frequency f =

hVe **2

is applied to the junction, the junction

oscillation tends to phase-lock to the applied frequency. During this phase lock, the

average junction voltage must equal to

efh

*2*

.

This effect, known as the ac Josephson Effect, is observed as a constant-voltage step at

V =

efh

*2*

in the current-voltage curve of the junction.

Graph 23: Constant voltage steps for a low capacitance junction driven with

microwave current. The important parameters for the Josephson junction are its length, width, critical current

density and the RF-drive frequency.

Junction Material Nb/A12O3/Nb

Length 18 µm

Width 30 µm

Critical Current Density 20 A/cm2

RF-Drive Frequency 75 GHz

Table5: Junction Design Parameters

112

6.2 Energy & Power metering lab.

• The lab deals with testing and calibration of energy meter.

• The procedure used for calibration uses the concept of time and frequency.

• A standard pre-calibrated energy meter and meter to be calibrated is fed with the

same current and voltage.

• The standard meter is set to dissipate 1kwh of energy for a specified number of

pulses (for example, 8000).

• The meter to be tested would also perform the same procedure in a different time

or with a different number of pulses.

• Hence the error is computed.

113

6.3 Time & Frequency Research lab. Cesium Fountain Atomic Clock

The basic principle of the working of the Cesium fountain atomic clock is graphically

shown below.

Figure 47: Stage 1

A gas of cesium atoms is introduced into the clock's vacuum chamber. Six infrared laser

beams then are directed at right angles to each other at the center of the chamber. The

lasers gently push the cesium atoms together into a ball. In the process of creating this

ball, the lasers slow down the movement of the atoms and cool them to temperatures near

absolute zero (0OK or -273OC).

Figure 48: Stage 2

114

The ball is tossed upward by two laser beams through a cavity filled with microwaves.

All of the lasers are then turned off.

Figure 49: Stage 3

The round trip up and down through the microwave cavity lasts for about 1 second.

During the trip, the atomic states of the atoms might or might not be altered as they

interact with the microwave signal.

Figure 50: Stage 4

When their trip is finished, another laser is pointed at the atoms. Those atoms whose

atomic states were altered by the microwave signal emit light. The photons emitted are

measured by a detector. The entire process is repeated until the maximum fluorescence of

the cesium atoms is determined. This point defines the natural resonance frequency of

cesium, which is used to define the second.

115

7. References

1. IEEE transactions on Applied Superconductivity, Vol. 7, No. 2, June 1997

2. IEEE transactions on Instrumentation and Measurement, Vol. 38, No 2, April 1989

3. India Journal of Pure & Applied Physics, Vol. 45, December 2007

4. Wikipedia Online Encyclopedia

5. Google Search Engine

6. National Institute of Science and Technology (NIST) – Website

7. National Physical Laboratory, India (NPLI) – Website

8. National Institute of information & communication technology, Japan- Website

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