EC6701 RF and Microwave Engineering VII Semester … RF and Microwave Engineering VII Semester...

EC6701 RF and Microwave Engineering VII Semester

Department of Electronics and Communication Engineering 54

EC6701 - RF And Microwave Engineering

UNIT I

TWO PORT NETWORK THEORY

Low frequency parameters

1. State the limitations in measuring Z, Y and ABCD parameters at microwave frequencies.

[N/D – 11]

The limitations in measuring Z, Y and ABCD parameters at microwave frequencies are,

i. Equipment is not readily available to measure total voltage and current at the ports of the

network.

ii. Short circuit and open circuit are difficult to achieve over a wide range of frequencies.

iii. Presence of active devices such as power transistors and tunnel diodes makes the circuit

unstable.

2. Specify the X-band frequency range and wavelength. [N/D – 07]

The X-band frequency range : 8 – 12.5 GHz

The X-band wavelength : 3.75 cm – 2.4 cm

3. Specify the ABCD relationship of a lossless transmission line. [N/D – 05]

The ABCD relationship of a lossless transmission line is,

V1= AV2 - BI2

I1 = CV2 - DI2

Where,

V1 – voltage at port 1

I1 – current at port 1

V2 – voltage at port 2

I2 – current at port 2

4. Specify the K-band frequency range and wavelength. [M/J – 07]

The K-band frequency range : 18 – 26.5 GHz

The K-band wavelength : 1.67 – 1.13 cm

Formulation of S parameters

5. A 5dB attenuator is specified as having voltage standing wave ratio (VSWR) of 1.2.

Assuming the device is reciprocal, find the S parameters. [N/D – 11]

|S21| = |S12|= 0



|S11| or |S22|= (VSWR-1) / (VSWR+1)

|S11| or |S22|= (1.2-1)/ (1.2+1)

|S11| or |S22|= 0.09

6. What is meant by symmetry of scattering matrix? [M/J – 08]

|S| is a symmetric matrix when the microwave device has the same transmission

characteristics in either direction of a pair of ports.

Sij = Sji

7. Define − Scattering Matrix

Scattering matrix is a square matrix which gives all the combinations of the power

relationships between the various input and output ports of a microwave junction.

8. Why are S parameters used in microwaves?

The S parameters are used in microwaves because of the following characteristics,

i. Increased stability at higher frequencies

ii. Mismatch loss is less

iii. Attenuation loss is less

9. What is insertion loss?

Insertion loss is a measure of the loss of the energy in transmission through a line or

device compared to direct delivery of energy without the line or device.

Properties of S parameters

10. State the properties of S parameter. [N/D – 12]

The properties of S parameter are,

i. |S| is always a square matrix of order (n*n)

ii. |S| is a symmetric matrix

iii. |S| is a unitary matrix

iv. Under perfect matched conditions, the diagonal elements of |S| are zero

Reciprocal and lossless networks

11. State reciprocity theorem.

The theorem states that when some amount of electromotive force (or voltage) is applied

at one point (e.g., in branch k, vk) in a passive linear network, that will produce a current at any

other point (e.g., in branch m, im). The same amount of current (in branch k, vk) is produced

when the same electromotive force (or voltage) is applied in the new location (in branch m, im).

vk/i m = i m/vk



12. Define − Lossless Network

In lossless passive network, the power entering the circuit is always equal to power

leaving network which leads to the conservation of power.

Introduction to component basics

13. Draw the equivalent circuit of a practical capacitor. [N/D – 12]

14. State the different types of high frequency capacitors.

The different types of high frequency capacitors are,

i. Parallel plate capacitor

ii. Leaded capacitor

iii. Perfect capacitor

15. State the different types of high frequency resistors.

The different types of high frequency resistors are,

i. Carbon composite resistors

ii. Metal film resistors

iii. Thin-film chip resistors

16. State the different types of high frequency inductors.

The different types of high frequency inductors are,

i. Simple wire inductor

ii. Coiled wire inductor

17. Define − Straight Wire Inductance

When alternating current is applied in a wire medium, the magnetic field is alternately

expanding and contracting. This produces an induced voltage in the wire that opposes any change

in the current flow. This opposition to change is called ‘Straight Wire Inductance’.



18. Define − Skin Effect

As frequency increases, the electrical signals propagate less inside the conductor.

Because of the current density increases to the perimeter of the wire and causes higher impedance

for the signal. This effect is known as skin effect.

19. Define − Q Factor

It is the measure of ability of an element to store energy and is equal to 2π times the

average energy stored to that of the energy dissipated per cycle.



UNIT II

RF AMPLIFIERS AND MATCHING NETWORKS

Amplifier power relations

1. What is meant by power gain of an amplifier? [N/D – 12]

Transducer power gain is defined as the ratio of power delivered to the load to that of the

power from the source.

Stability considerations

2. Define − Unconditional Stability

Unconditional stability refers to the situation where the amplifier remains stable for any

passive source and load at the selected frequencies and bias conditions.

Gain considerations

3. Define − Unilateral Power Gain

When feedback effect of the amplifier is neglected (i.e. S12 = 0), the amplifier power gain

is known as unilateral power gain.

4. Define − Operating Power Gain

Operating power gain is defined as the power delivered to the load to that of the power

supplied to the amplifier.

5. Define − Available Power Gain

Available power gain is defined as the power available from the microwave network to

that of the power from the source.

Noise figure

6. Write the expression for noise figure of a two port amplifier. [N/D – 11]

The expression for noise figure of a two port amplifier is F = Fmin + (Gn/Rs) |Zs – Zopt|2

Where,

F – Noise figure

Fmin – Minimum noise figure

Gn – Source conductance

Rs – Source resistance

Zs – Source impedance

Zopt – Optimum impedance

7. Define − Noise Figure

Noise figure is defined as the ratio of input SNR to the output SNR.



F = (SNR) O / (SNR) I

Impedance matching networks

8. What is the need for impedance matching network? [N/D – 11]

The need for impedance matching network is,

i. To stabilize the amplifier by keeping the source and load impedances in the appropriate range

ii. To reduce undesired reflections

iii. To improve the power flow capabilities

9. What are the considerations in selecting a matching network? [N/D – 12]

The considerations in selecting a matching network are,

i. Gain and gain flatness

ii. Operating frequency and bandwidth

iii. Output power

iv. Power supply requirements

v. Input and output reflection coefficients

vi. Noise figure

Microstripline matching networks

10. State the various types of waveguide stub.

The various types of waveguide stub are,

i. E stub

ii. H stub

iii. E-H tuner

11. Define – Positive RF Feedback.

Positive feedback is defined as the instability caused due to the increase in the magnitude

of the return voltage in a passive radio frequency waveguide.



UNIT III

PASSIVE AND ACTIVE MICROWAVE DEVICES

Microwave E − plane tee junction

1. Find the resonant frequency of TE 101 mode of an air filled rectangular cavity of

dimensions 5 cm * 4 cm * 2.5cm. [N/D – 11]

Given :

a= 5 cm

b= 4 cm

d= 2.5 cm

m= 1

n= 0

p= 1

fr = ½(µ€)1/2

[(m/a)2 + (n/b)

2 + (p/d)

2 ]

1/2

fr = 6.71 GHz

2. What are scattering coefficients?

The elements representing various ports of a n*n matrix are known as scattering

coefficients or scattering parameters.

Microwave H − plane tee junction

3. Draw the diagram of H-plane Tee junction. [N/D – 12]

Microwave hybrid Tee junction

4. Why is magic tee referred as E-H tee?

Magic tee is constructed by E plane tee and H plane tee perpendicular to each other.

Therefore magic tee is referred as E-H tee.



Microwave directional coupler

5. Draw the structure of two hole directional coupler. [N/D – 11]

6. What is meant by directivity of directional coupler? [M/J – 08]

The directivity of directional coupler is defined as the ratio of forward power Pr to f the

back power Pb and expressed in dB.

7. What are the basic parameters to measure the performance of a directional coupler?

[N/D – 08]

The basic parameters to measure the performance of a directional coupler are,

i. Coupling co-efficient

ii. Directivity

iii. Insertion loss

iv. Isolation

8. What are the basic types of directional coupler? [N/D – 09]

The basic types of directional coupler are,

i. Two hole directional coupler

ii. Four hole directional coupler

iii. Reverse coupling directional coupler

iv. Bethe hole directional coupler

Microwave Rat-race

9. What is meant by hybrid junction?

A hybrid junction is a four-port network in which a signal incident on any one of the port

divides between two output ports with the remaining port being isolated.

10. What is hybrid ring?

Hybrid ring consists of an annular line of proper electrical length to sustain standing

waves, to which four arms are connected at proper intervals by means of series or parallel

junctions.



Microwave Twists

11. What is the need for waveguide twist?

Waveguide twists are used to change the plane of polarization of a propagating wave.

Microwave Ferrite Devices

12. How a Faraday rotation isolator can be constructed by using ferrite rod? (Nov/Dec 2008)

Isolators can be made by inserting ferrite rod along the axis of a rectangular waveguide.

13. Define − Coupling Factor

Coupling factor is a measure of how much of the incident power is being sampled.

14. Define – Isolator

An isolator is a two port non-reciprocal device which produces a minimum attenuation in

the wave in one direction and very high attenuation in the opposite direction.

15. State the need for circulator in microwave applications?

The need for circulator in microwave applications is,

i. Circulator is used as duplexer in radar antenna system

ii. Three port circulator is used in tunnel diode and parametric amplifiers

iii. Circulators are also used in low power applications

16. Define – Non Reciprocal Devices

Non reciprocal devices defined as devices having different forward and reverse

propagating characteristics are known as non-reciprocal devices.

17. State the properties of ferrites.

The properties of ferrites are,

i. Ferrites exhibits strong magnetic ability

ii. Ferrites exhibits high resistivity compared with copper in microwave

iii. Ferrites exhibits non-reciprocal property

Microwave semiconductor devices

18. What are M-type tubes? [M/J – 08]

M type tubes are crossed field devices where the static magnetic field is perpendicular to

the electric field. Here the electrons travel in curved path.

19. State the advantages for microwave IC’s. [M/J – 08]

The advantages for microwave IC’s are,



i. Low package density

ii. Small size and less weight

iii. High reliability

iv. Different transmission structures (micro strip lines, lumped circuit elements, thin film circuits)

are possible

20. What is the other name of O-type tube? [N/D – 07]

The other name for O – tube is linear tube or rectilinear beam tube.

Transferred electron devices

21. State transferred electron effect. [N/D – 12]

When GaAs is biased above a threshold value of the electric field, it exhibits a negative

differential mobility. The electrons in the lower energy band will be transferred into the higher

energy band. This behavior is called transferred electron effect.

22. State Gunn Effect. [M/J – 08]

When the electric field is varied from zero to threshold value, the carrier drift velocity is

increased from zero to maximum. When the electric field is beyond the threshold value of

3000V/cm, the drift velocity is decreased and the diode exhibits negative resistance.

23. What are the materials that exhibit Gunn Effect?

The materials exhibiting Gunn Effect are,

i. Gallium arsenide

ii. Indium phosphide

iii. Cadmium telluride

iv. Indium arsenide

24. What are the modes available in negative resistance devices?

The modes available in negative resistance devices are,

i. Voltage controlled mode

ii. Current controlled mode

Avalanche transit time devices

25. What are the major disadvantages of IMPATT diodes? [N/D – 08]

The major disadvantages of IMPATT diodes are,

i. Avalanche process makes the IMPATT diode noisy

ii. Poor noise figure of 30dB

iii. Low efficiency due to induced electron current



Microwave monolithic integrated circuit

26. State the basic materials required for microwave integrated circuit. [N/D – 07]

The basic materials required for microwave integrated circuit are,

i. Substrate materials

ii. Conductor materials

iii. Dielectric materials

iv. Resistive materials

27. State the different types of lithography. [M/J – 08]

The different types of lithography are,

i. Electron-beam lithography

ii. Ion-beam lithography

iii. Optical lithography

iv. X-ray lithography

28. State the various MMIC fabrication techniques.

The various MMIC fabrication techniques are,

i. Diffusion and ion implementation

ii. Oxidation and film deposition

iii. Epitaxial growth

iv. Lithography

v. Etching and photo resist

vi. deposition



UNIT IV

MICROWAVE GENERATION

High frequency limitations

1. State any four limitations of conventional tubes at high frequencies. [N/D – 11]

The limitations of conventional tubes at high frequencies are,

i. Lead inductance effects

ii. Interelectrode capacitance effects

iii. Transmit angle effects

iv. Gain bandwidth product limitation

Helix travelling wave tube

2. A helix travelling wave tube operates at 4 GHz, under a beam voltage of 10 KV and beams

current of 500mA. If the helix is 25Ω and interaction length is 20cm, find the gain

parameter. [N/D – 11]

Given :

V0 = 10 kV

I0 = 500 mA

Z0 = 25 ohm

F = 4 GHz

L = 20 cm

Gain parameter C = [I0Z0/4V0]1/3

= 0.068

3. What are the high frequency effects in conventional tubes?

The high frequency effects in conventional tubes are

i) Circuit reactance

a)Inter electrode capacitance

b) Lead inductance

ii) Transit time effect

iii) Cathode emission

iv) Plate heat dissipation area

v) Power loss due to skin effect, radiation and dielectric loss.

4. What are the assumptions for calculation of RF power in Reflex Klystron?

i) Cavity grids and repeller are plane parallel and very large in extent.

ii) No RF field is excited in repeller space

iii) Electrons are not intercepted by the cavity anode grid.



iv) No debunching takes place in repeller space.

v) The cavity RF gap voltage amplitude V, is small compared to the dc beam

voltage VO

5. Give the drawbacks of klystron amplifiers.

i) As the oscillator frequency changes then resonator frequency also changes and the

feedback path phase shift must be readjusted for a positive feedback.

ii) The multicavity klystron amplifiers suffer from the noise caused because bunching is

never complete and electrons arrive at random at catcher cavity. Hence it is not used in

receivers.

6. What is the effect of transit time?

There are two effects.

i) At low frequencies, the grid and anode signals are no longer 180O out of phase, thus

causing design problems with feedback in oscillators.

ii) The grid begins to take power from the driving source and the power is absorbed even

when the grid is negatively biased.

7. What are the applications of reflex klystron ?

i) Signal source in MW generator

ii) Local oscillators in receivers

iii) It is used in FM oscillator in low power MW links.

iv) In parametric amplifier as pump source.

8. What is the purpose of slow wave structures used in TWT amplifiers?

i) Slow wave structures are special circuits that are used in microwave tubes to

ii) reduce wave velocity in a certain direction so that the electron beam and the signal

wave

iii) can interact. In TWT, since the beam can be accelerated only to velocities that are

about

iv) a fraction of the velocity of light, slow wave structures are used.

9. How are spurious oscillations generated in TWT amplifier? State the method to suppress it.

i) In a TWT, adjacent turns of the helix are so close to each other and hence

ii) oscillations are likely to occur. To prevent these spurious signals some form of

iii) attenuator is placed near the input end of the tube which absorb the oscillations.

10. State the applications of TWT.

i) Low power, low noise TWT’s used in radar and microwave receivers

ii) Laboratory instruments

iii) Drivers for more powerful tubes

iv) Medium and high power CWTWT’S are used for communication and radar.



11. What do you mean by O-type tubes? Name some O-type tubes.

In O – type tube a magnetic field whose axis coincides with that electron beam is used to hold the

beam together as it travels the length of the tube. It is also called as linear beam tube.

i) Helix Traveling wave tube

ii) Coupled cavity TWT

iii) Forward wave amplifier

iv) Backward wave amplifier

v) Backward wave oscillator

UNIT V

MICROWAVE MEASUREMENTS

Microwave power measurement

1. What is the principle by which high power measurements could be done by calorimetric

method? [M/J – 08]

Principle by calorimetric method are,

i. Direct heating method

ii. Indirect heating method

2. State the demerits of single bridge power meter. [N/D – 08]

The demerits of single bridge power meter are,

i. The change of resistance due to a mismatch at the microwave input ports results in incorrect

reading

ii. The themistor is sensitive to changes in the ambient temperature resulting in false reading

3. State any two sensors used to measure the power. [N/D – 09]

Two sensors used to measure the power are,

i. Barretter

ii. Thermistor

4. What is bolometer? [M/J – 07]

Bolometer is a power sensor whose resistance changes with temperature as it absorbs

microwave power. Examples: Barretter, Thermistor.

Microwave impedance measurement

5. What are the possible errors occur in measurement of standing wave ratio?

[N/D – 12]



The possible errors occur in measurement of standing wave ratio are,

i. Vmax and Vmin may not be measured in the square law region of the crystal detector

ii. Probe thickness and depth may produce reflections in the line

iii. Residual VSWR arises due to mismatch impedance

iv. Harmonics and spurious signals from source cause measurement errors

Microwave SWR measurement

6. Define − Return Loss [N/D – 07]

The return loss is a measure of the power reflected by a line or network or device. Return

loss (dB) = 10 log [input energy to the device / reflected energy at the input of the device]

Return loss (dB) = 10 log [Pi/Pr]

7. Define − Reflection Loss

Reflection loss is a measure of power loss during transmission due to the reflection of the

signal as a result of impedance mismatch.

8. Define − Insertion Loss

Insertion loss is a measure of loss of energy in transmission through a line or device

compared to direct delivery of energy without the line or device.

9. What is a VSWR meter?

VSWR meter is a highly sensitive, high gain, low noise voltage amplifier tuned normally

at fixed frequency of 1 kHz at which microwave signals are modulated. This meter indicates

calibrated VSWR reading for any loads.

10. What is calorimeter?

Calorimeter is a convenient device for measuring the high power at microwave

frequencies which involves conversion of microwave energy in to heat, absorbing the heat in a

fluid and determine the temperature.

11. What are tunable detectors?

The tunable detectors are used to demodulate the signal and couple the required output to

high frequency scope analyzer. The low frequency demodulated output is detected using non

reciprocal detector diode mounted in the microwave transmission line.

12. What is calorimetric direct heating method?

In calorimetric direct heating method, the rate of production of heat can be measured by

observing the rise in temperature of the dissipating medium.

13. What is calorimetric indirect heating method?

In calorimetric indirect heating method, heat is transferred to another medium before

measurement.

EC6701 RF and Microwave Engineering VII Semester … RF and Microwave Engineering VII Semester...

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Transcript of EC6701 RF and Microwave Engineering VII Semester … RF and Microwave Engineering VII Semester...