Microwave Engineering Lab Viva

7/31/2019 Microwave Engineering Lab Viva

1/18

MICROWAVE ENGINEERING LAB

VIVA QUESTIONS AND ANSWERS

1. Why cant conventional tubes be used at microwavefrequencies?

Conventional tubes cant be used at microwavefrequencies because of

transit time effect. Lead inductance and inter electrodecapacitance of the devices

will finally limit the output which may even be zero.

2. What is transit time?It is the time taken by the electrons to travel fromcathode to anode.

3. What is the operating principle of reflex klystron?It works on the principle of velocity modul ation and

current modulation.

4. What is velocity and current modulation in a reflexklystron?Variation in velocities of the electrons in the electron

beam is calledvelocity modulation; variation in the current density of the

electronbeam is referred to as current modulation.

5. How does bunching occur in a reflex klystron?A reference electron passing the gap when the gap

voltage is zero travels with no change in velocity. Anelectron leaving the gap earlier during slightly positivevoltage would travel further into repeller space and hencewould take longer time then the reference electron to returnto the gap. An electron leaving the gap later will face


2/18

slightly negative voltage and gets retarded. So it returnsback after a shorter travel in the repeller space. Thus all theelectrons would arrive back to the gap in bunches. Bunchingaround reference electron takes place once per cycle of RFoscillations.

6. Why do different modes of operation exist for a reflexklystron?

There are several combinations of repeller voltageand anode voltage that

provide favorable conditions for bunching. Accordinglythere are several modes

of operation, expressed by N + where N is an integer.

7. What modes are generally used in a reflex klystron?1 and 2 are the most commonly used modes in a

practical reflexklystron.

8. What is the operating frequency and power output of areflex klystron?

Frequency range: 4GHz 200GHz

Output power: maximum 3W in X-band to 10mW at220GHz

9. What is the maximum theoretical efficiency of the reflexklystron?

22.7% for 1 mode.

10. What is mechanical tuning and electronic tuning in a reflexklystron?

Variation in frequency of resonance of cavity byvarying its dimension by

a mechanical method like adjusting screws is calledmechanical tuning. Variation of frequency by adjustingrepeller voltage is called electronic tuning.


3/18


4/18

Transferred electron device and transferred electronoscillator.

20. Draw the equivalent circuit of an X-band GaAs Gunn diode.Equivalent circuit is a parallel combination of

resistance (~100 ) and a capacitance (~.6pF).

21. Mention the characteristics of an X-band GaAs Gunn diode.Typical values are: Bias voltage: 9V

Output power: 300mW Operating current: 950mAEfficiency: 3.5%

22. Can a Gunn diode be used as an amplifier?Yes.

23. Mention some applications of Gunn diode.

It is used as 1) low and medium power oscillators inmicrowave receivers.

2) Pump sources in parametric amplifiers3) Power output oscillators in radars

24. Expand the terms IMPATT, TRAPATT, & PIN referring to thetypes of diodes.

IMPATT Impact Avalanche & Transit Time DiodeTRAPATT Trapped Plasma Avalanche Triggered

Transit DiodePIN Positive Intrinsic Negative

25. What is the use of crystal diode in a microwave benchsetup?

To detect the microwave signal picked up by theprobe and to provide the rectified current.

26. What is a square law device?A device whose output current is proportional to the

square of the input voltage applied.

27. Mention conditions for a crystal diode to function as a

square law device.The rectified current should be less than 20A or input

power should be less than 10mW.


5/18

28. Name the types of crystal diodes used.Point contact type, barrier type & back type are

commonly used types of crystal diodes.

29. Draw the equivalent circuit of a microwave crystal

detector.Equivalent circuit consists of a parallel combination of:1) Lead inductance (L) in series with lead resistance

(R)2) Junction capacitance (C)

30. Define current sensitivity and voltage sensitivity of acrystal detector.

Current sensitivity is the maximum short circuitcurrent for unit input power (short circuit power/input power).Voltage sensitivity is the maximum open circuit voltage for unit

input power (open circuit power/input power).

31. Why is a tuning element provided with a crystal mount?To eliminate reflections from crystal mount.

32. Apart from signal detection what are the other applicationsof crystal diode?

Used as 1) frequency mixers in superhetrodynereceivers

2) Harmonic generators 3) frequency converters

33. Which crystal diode is commonly used for X-bandoperations?

IN 23

34. Give the relation between output current and input voltagefor a crystal diode.

I = KEP where K is a constant due to detector andprobe coupling. P is a constant depending on theindividual crystal & magnitude of operating voltage. P

= 2 for square law operation.

35. What is a wave guide?It is a hollow metallic pipe of either rectangular or

circular cross section, used to guideelectromagnetic waves at microwave frequencies.


6/18

36. Define guide wavelength.It is the distance between two identical points on

standing waves in a waveguide.

37. Define cutoff wavelength for a waveguide.

It is the longest wavelength beyond which there is nopropagation of EM waves within the waveguide.

38. Define dominant mode with respect to a waveguide.Mention dominant mode for rectangular andcircular waveguides.

The mode with the least cutoff frequency is calleddominant mode. For rectangular waveguide thedominant mode is TE10 & for circular waveguide its

TE11.

39. If the broader dimension of a rectangular waveguide is2.2cms, what is the cutoff frequency and wavelengthfor dominant mode?

For rectangular waveguide the dominant mode is TE10.The cutoff wavelength c = 2a = 4.4cms.The cutoff frequency fc = c/ c = 6.8GHz.

40. Write the relation between guide wavelength, cutoffwavelength and free space wavelength.

1/ o2= 1/ c

2

+ 1/ g2

or o = cg/ (g2

+ c2

)1/2

Where o free space wavelength, c - cutoff wavelength, g guide wavelength

41. What are the various power losses in a waveguide?1) Losses in guide walls 2) losses in dielectric (if any

other than air)

42. How are modes excited in a rectangular waveguide?Modes are excited using a probe, a hole or a loop.

Commonly used techniques are electric excitation,magnetic excitation and aperture excitation.

43. What is the preferred frequency range for waveguideoperation? Why?


7/18

Frequency range: 3GHz 100GHz as the dimensionsof the waveguide are convenient in size, easy tofabricate.

44. Is a waveguide equivalent to a high pass filter or a low

pass filter? ExplainWaveguide is equivalent to a high pass filter. It passesfrequencies only above the cutoff frequency fc, butattenuates those below fc.

45. What is TE mode and TM mode?TE mode is the one in which the electric field is

perpendicular the direction of propagation of the EMwave. TM mode is the one in which the magnetic field isperpendicular the direction of propagation of the EM wave.

46. In the notation TEmn and TMmn what do m and n represent?m denotes the half wave periodicity in the x direction

while n represents the half wave periodicity in the ydirection; assuming the wave is traveling in the zdirection.

47. Define wave impedance and characteristic impedance.Wave impedance Zz looking in the z direction is the

ratio of the transverse electric field strength to the

total magnetic field strength. Characteristic impedance Zo ofa line is the impedance offered by the line of infinite length.

48. Write the formula for wave impedance of a TE and TMwave considering propagation in z direction.

TE mode: Zz (TE) = / (1 (f c/f) 2)1/2

TM mode: Zz (TM) = / (1 (f c/f) 2)1/2

Where f = signal frequency and fc = cutoff frequency.

49. What are the applications of waveguide bends, corners andtwists?

Waveguide bends are used to change the direction ofenergy flow at microwave frequencies. Corners are usedto change are used to change the direction ofenergy flow at larger wavelengths. Twists are employed tochange the plane of polarization of the EM wave.


8/18

50. What is a mitered corner?A 900 corner would produce large reflections, so a part

of it is cut which is called mitered corner.

51. Define VSWR.VSWR (voltage standing wave ratio) is the ratio of themaximum field strength to the minimum fieldstrength of standing waves.

52. What is reflection coefficient?It is the ratio of reflected voltage to incident voltage

on standing waves.

53. Write the relation between VSWR and reflection coefficient.S = (1 + K)/ (1 K)

Where S standing wave ratio; K Reflection coefficient.

54. Write the values of VSWR for open circuit, short circuit &matched load.

Open/closed circuit: VSWR = Matched load: VSWR = 1

55. What is the range of VSWR and inverse VSWR?VSWR: 1 - . Inverse VSWR: 0 - 1.

56. What is a matched load?

A load that offers the characteristic impedance to atransmission line is called a matched load.

57. Define attenuation.Attenuation is the reduction in power level, generally

expressed in dB. For any network attenuation = 10log10 (p1/p2) dB where p2 is the output power and p1 isthe input power to the network.

58. Define insertion loss.

Insertion loss is the attenuation introduced due toaddition of any component in any circuit. Ifp1 is the output power without the component and p2 is theoutput power with the component, then the insertion loss in dBis given by 10log10 (p1/p2) dB.


9/18


10/18

65. Mention some applications of directional coupler.1) Power measurements 2) used in reflectometer 3) as

a fixed attenuator 4) to arrange balanced duplexer

66. What is Uniline?A uniline or isolator is a 2 port device that allows freetransmission of power in forward direction and notransmission in the opposite direction.

67. Why is an isolator used in a microwave bench?Isolator is connected between load and source. It

allows free transmission of power from source to load. But ifthere is any mismatch and reflections arise, thesereflections are prevented to travel to the source side.

68. What are the types of ferrite isolators?1) Faraday rotation isolators for power up to a few

hundred watts.2) Resonant absorption isolators for higher power.

69. What is Faraday rotation?Rotation of polarized wave traveling through ferrite.

70. What are the commonly used ferrite materials forisolators?

1) Yitrium iron garnet (YIG)

2) Manganese ferrite 3) Zinc ferrite

71. What is meant by line width of ferrite?It is the range of magnetic field strengths over which

absorption will take place. It is defined between the halfpower points for absorption.

72. What is Curie temperature?It is the temperature at which a magnetic material

looses its magnetic properties.

73. What is a circulator?It is a ferrite multiport (generally 4) device where each

port is connected only to the next clockwise terminal.


11/18

74. What are the main applications of circulators?1) In radar work to isolate transmitter and the receiver

connected to the same antenna 2) To isolate inputand output in parametric amplifier.

75. Mention some types of circulators.Faraday rotation circulator (4 port), Wye circulator ( 3

port)

76. How can you construct a 4 port circulator with magic tees

and a gyrator?To arrange a four port circulator we need twomagic tees and a gyrator such that it provides a phase shift of1800 in the direction of arrow only. A wave incident on port 1splits equally into 2 in phase waves in arms b, d. They reachports a and c of magic tee 1, add up and appear as output atport 2 (adder arms). A wave incident on port 2 splits into two inphase equal amplitude waves at a and c. They arrive at b and dof magic tee 2 with a phase difference of 1800 and emerge outof the subtractor arm (port 3). Similar analysis shows that waveincident on port 3 comes out of port 4 and that incident on port

4 emerges out of port 1 only.

13

4

2


12/18

77. If an isolator is not available explain how a 3 port circulatorcan be used to serve

the function of isolator?

There will free flow of power from source to load (port 1

to 2). Any reflectedenergy due to mismatch from port 2 will be absorbed by

matched termination atport 3.

78. Why is a matched termination needed at port 3 in theabove case?

If matched termination is not connectedreflections from port 1 will be

incident on port 3. Any mismatch at this port will causereflections which will betransmitted to source side. To avoid this, port 3 is

connected with matched load.

a cb

d

Earm

Harm

Earm

H arm

MagicTee

1

MagicTee2

3

21

4

GYRATOR0

0

3 PORTCIRCULATOR

MATCHEDTERMINATION

LOADSOURCE

1 2

3


13/18

79. Is it necessary to connect source to port 1 of a circulatoronly always?

No. If source is connected to port 2 ,then connect loadat port 3 and

matched load to port 1, or connect source to port 3, load to

port 1 and matchedtermination to port 2.

80. What are scattering parameters?They are the relations between reflection coefficients

and transmission coefficients of a microwavenetwork.

81. What is scattering matrix?At microwave frequencies the measurable quantities

are power, reflection coefficient, transmission

coefficient and standing wave ratio. A linear relationbetween input and output of a microwave network in terms

of precisely measurable parameters is calledscattering description (or scattering matrix is amatrix made up of scattering coefficients).

82. Write the S matrix for magic tee.

S = 21

0011

0011

1100

1100

83. Write S matrix for a 4 port ideal circulator.

S =

1000

0010

0001

1000

84. Write the S matrix for an ideal directional coupler.


14/18

S =

00

00

00

00

j

j

j

j

Choosing a reference plane such that is positive real and is positive imaginary.

85. What is magic tee?It is a combination of E plane and H plane tee. It is a 4

port device.

86. What are the properties of magic tee?


15/18

Power fed to arm 3 (H arm) divides equally between arms 1and 2. The electric field in the 2 arms will be in phase. Nocoupling to arm 4. Similarly power fed to arm 4 is notcoupled to arm 3. It is divided equally between arms 1 & 2 butthe electric fields in these differ in phase by 1800. Powersfed in arms 1 & 2 are added in arm 3 (H arm) andsubtracted in arm 4 ( E arm).

87. What are the applications of magic tee?Used as 1) An antenna duplexer, to use as a single

antenna for transmission and reception 2)balanced microwave mixer 3) frequency discriminator 4)impedance measuring bridges.

Adjust the reference impedance till there is no signal in thedetector. Use the equation (Zr 1)/(Zr + 1) = (zx 1)/(Zx +1) to find Zx, because at balance, reflection coefficients ofreference and unknown impedance are the same.

ARM4

ARM

2

E ARM

ARM3

ARM 1

HARM

Z

X

RX

MATCHEDDETECTOR

REFER-ENCE Z

R

SOURCE

MATCHED LOAD

ANTENNA

TX

1

2

3

44

1

2

3


16/18

88. What is pyramidal horn?It is a EH horn where both sides of a waveguide are

flared.

89. Mention different types of horn antennas.1) Sectoral H plane 2) Sectoral E plane 3) Pyramidal

plane 4) conical plane 5) biconical plane

90. What is half power beam width?The angular separation between half power points in

the radiation pattern of an antenna.

91. Define directivity if an antenna.

It is the ratio of the maximum radiation intensity tothe average radiation intensity of an antenna.

92. How are far and near fields identical for a horn antenna?Near field is up to a distance 2D2/ from the antenna.

Any point at a distance greater than this is consideredto be in the far field, where D largest dimension ofhorn mouth

93. What are the other names for the above fields?Near field Fresnel region Far field Fraunhofer

region

94. Define antenna.Antenna is defined as the transition region between

guided and free space wave.

95. What are the devices used for microwave powermeasurements/

1)Bolometer 2)Calorimeter

96. What are the types of Bolometer elements?

Thermisters and Barreters.

97. What is the characteristic of Barreter bolometer?Barrators have positive temperature coefficient of

resistance.


17/18

98. What is the principle of calorimetry?Fluid, generally water absorbs the microwave power,resulting in temperature rise. Noting this temperature rise,power is calculated.

99. Which method is preferred for low power measurements?Bolometer.

100. When is calorimeter used for power measurements?For medium power measurement.

101. Mention the main application of IMPATT diode.Used as transmitter for microwave communication

systems.

102. Mention the application of TRAPATT diode.Used as pulsed transmitter in s-band phased array

radar.

103. What is the S-band frequency range?As per IEEE microwave band designation s-band range: 2- 4

GHz

104. where is BARITT diode used?As local oscillator in radar receiver.

105. npn structure is preferred to pnp in high frequencytransistor. Why?

Mobility of electrons is greater than that of holes

106. What are the commonly used ways of fabricatingmicrowave transistors?

Diffusion and ion implantation.

107. What are the applications of tunnel diode?Used as microwave oscillator and amplifier.


18/18

Microwave Engineering Lab Viva

Documents

Transcript of Microwave Engineering Lab Viva