Considerations on Measurement

7/27/2019 Considerations on Measurement

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Considerations onmeasurement

Mr.Song KepingEngineer of Management of station and network Division

State Radio Monitoring Center

[email protected]+8610-68318264

Radio Monitoring and Spectrum Management Training

(China,23-31,May,2005)


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Considerations onmeasurement

Measurement tasks and essential

equipmentTime, frequency and phase domain

FFT analysis

Measurement uncertainty


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Measurement tasks andessential equipment



FFT analysis



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Measurement tasks of monitoring station

-frequency measurements

-field strength and power-flux density measurements-bandwidth measurements

-modulation measurements

-spectrum occupancy measurements

-direction finding



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Functions of equipment

-omnidirectional antennas

-directional antennas-receivers

-direction finders

-frequency measuring equipment

-field-strength meters-bandwidth measurement equipment



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Functions of equipment (2)

-channel occupancy measurement equipment

-frequency spectrum registration equipment-spectrum analysers

-vector signal analysers or modulation analysers

-decoders

-signal generators-recording equipment



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Frequency range of equipment

- 9 kHz-3 GHz

- 9 kHz-30 MHz for HF monitoring stations

- 20 MHz-3 GHz for VHF/UHF monitoring stations



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Further functions of measurement equipment

-field strength measurements along a route;

-TV video signals measurements

-measurement of parameters of digital networks

-monitoring of fixed link emissions

-measurement of satellite signals



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Time, frequency andphase domain



FFT analysis



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Analyse of signals in:-Time domain

-Frequency domain-Phase domain.

Three ways is interchangeable



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11Spec-041

a)

)

c)

T0t f

t ffT fS fT fT + fS

f0 =1

T0

1

Tp

Tpt f

Ap

Periodic rectangular pulses

Amplitude-modulated signal

Sinewave signal

Time domain Frequency domain

Envelope sin (x) =sin (x)

x

An f =Ap

Tp 2

sin n Tp

n Tp

p

Some periodic signals in

time and frequency domain



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Band-limited

random noise

Arbitrary binary

sequence

QPSK signal



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Frequency domain suit to observe:

-channel occupancy

-interference

-harmonic products

-spurious emission

-modulation type (sometimes)



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Constellation diagram

-Vector length represents signal amplitude

-Angle of vector represents phase

-Repetition frequency represents symbol rate

-Analyser should synchronize with the signal



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constellation diagram



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Vector analysers can execute in real time

-If modulation mode and encoding scheme are known

-Display the binary signal sequence

-Display the characters of decoded information



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FFT analysis



FFT analysis



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Fast Fourier Transform (FFT)

-Algorithm for transforming data from the time

domain to the frequency domain

-Used for implement signal analyser

-Executed by microprocessors

FFT analysis


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Factor that complicate the task

-Limited computing power of processor

-Sample and digitize the input before transform

-Not the exact representation because of sample

-Block processing property

FFT analysis


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Sample and digitize

the time domain input

Spec-044

Amplitude

Amplitude

Amplitude

Transform toTime

Frequency

Time

a) Continuousinputsignal

b) Samplesof inputsignal

c) Samplesof the

frequencydomain(calledlines)

FFT analysis


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Spec-045

Time

Frequency

FFT

Time record of N samples

Amplitude

Amplitude

The time record is

transformed as a block into

a block of frequency lines.

FFT analysis


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Procedure of the FFT

-Capturing a band-limited signal (a block)

-A/D convert (sampling)

-Sent to DSP and calculate

-Get frequency domain lines (a block)

-Repeat...

FFT analysis


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Characteristic of FFT analysers

-DSP equipment

-Real-time processing within a certain bandwidth

-Maximum real-time bandwidth depends oncomputing power

-Lack of a real variable frequency span

FFT analysis


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FFT analysers with digitally processed filter

-Make filter shape and bandwidth easier to control

-Possible to reprocess captured signals in option

-Suitable for analysing the time dependent signals

FFT analysis


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FFT analysis vs. other techniques

Measuringprinciple

Advantage Disadvantage

Swept spectrum

analysis

Wide frequency range (RF),

Wide frequency spanHigh dynamic range (e.g. 100 dB)

Delay time for transient signals

Lack of phase information of thesignal

Slow scanning speed for highfrequency resolution

FFT analysis Fast amplitude and phaseinformation

High recording speed for high

frequency resolutionMany analysis capabilities(including transient analysis)

Narrow frequency spanUsually lower dynamic range

(e.g. 60 dB)

Limited time interval

Vector signalanalysis

Benefits of FFT analysis Limited to one signal on the displayMeasurement does not go to 0 Hz

FFT analysis


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FFT analysis



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Take uncertainty into account when measure:

-Frequency

-Signal levels

-Bandwidth

Error:

-Difference between the measured value and the truevalue



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Absolute uncertainty:

-A quantity describes an interval that the true valuewill falls into it in a given probability

-Expressed in the same unit as measured value

Relative uncertainty:

-Quotient of the absolute uncertainty and the bestpossible estimate of the true value



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Influence quantities for frequency measurement

-Reference oscillator

-Measurement procedure-Resolution of reading

-Stability of signal to be measured

-Measurement time in relation of signal



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Influence quantities for field strength and powerflux-density measurement

-Receiver reading

-Attenuation of the connection between antenna andreceiver

-Antenna factor

-Receiver sine-wave voltage accuracy

-Receiver selectivity relative to occupied bandwidth-Receiver noise floor

-Mismatch effects between antenna port and receiver



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Influence quantities for field strength and powerflux-density measurement (2)

-Antenna factor frequency interpolation

-Antenna factor variation with height above groundand other mutual coupling effects

-Antenna directivity

-Antenna cross-polarization response

-Antenna balance

-Shadowing and reflections due to obstacles



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Influence quantities for occupied bandwidthmeasurement

-Measurement principle (FFT or swept spectrum

analysis)-Resolution bandwidth

-Non-linearity of amplitude display

-Time behaviour of signal

-Measurement procedure (e.g. for TDMA signals)

-Receiver/analyser reading


Considerations on Measurement

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