EC 73 - MICROWAVE ENGINEERING
UNIT I
1. Define Microwave.
Microwaves are generally described as electromagnetic waves with
frequencies that range from approximately 1GHz to 300 GHz. Therefore,
microwave signals, because of their inherently high frequencies, have relatively
short wavelengths, hence the name “micro” waves.
2. Define a microwave junction.
The point of interconnection of two or more microwave devices is called
microwave junction.
3. Define scattering matrix.
In a microwave junction there is intersection of three or more
Components. There will be an output port; in addition there may be reflection
from the junction of other ports. Totally there may be many combinations, these
are represented easily using a matrix called S matrix.
4. What are the properties of s-matrix?
1. It possess symmetric property Sij =Sji
2. It possess unitary property
[S][S]*= [I]
3. Under perfect matched conditions, tfe diagonal elements of [S] are zero.
5. Write the unitary property for a lossless junction.
For any lossless network the sum of the products of each term of any one
row or of any column of the S-matrix multiplied by its complex conjugate is unity.
6. Define tee-junction.
In microwave circuits a waveguide or co-axial line with three independent
ports is commonly referred to as a tee-junction.
7. What is E-plane Tee?
An E-plane tee is a waveguide tee in which the axis of its side arm is
parallel to the E-field of the main guide.
8. What is H-plane Tee?
An H-plane tee is a waveguide tee in which the axis of its side arm is
shunting the E-field or parallel to the H-field of the main guide.
9. Define difference arm.
In E-plane tee, the power out of port 3 is proportional to the difference
between instantaneous powers entering from port 1 and port 2. Therefore, this
third port is called as difference arm.
10. Define difference arm.
In H-plane tee, if two input waves are fed into port 1 and port 2 of the
collinear arm, the output wave at port 3 will be in phase and additive. Because of
this, this third port is called as sum arm.
11. What do you mean by hybrid junction?
A hybrid junction is a four port network in which a signal incident on any
one of the ports divides between two output ports with the remaining port being
isolated.
12. Why bends are used?
Bends are used to alter the direction of propagation in a waveguide
system.
13. 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.
14. Define isolator.
An isolator is a two port non-reciprocal device which produces a minimum
attenuation to wave in one direction and very high attenuation in opposite
direction.
15. Why isolators are called uniline?
An ideal isolator completely absorbs the power for propagation in one
direction and provides lossless transmission in the opposite direction. Thus
isolators are called uniline.
16. Give the applications of directional coupler
1. Unidirectional power measurement
2. SWR measurement
3. Unidirectional wave launching
4. Reflectometer
5. Balanced duplexer
17. Give a note on directional couplers.
Directional couplers are transmission line devices that couple together two
circuits in one direction, while providing a great degree of isolation in the opposite
direction.
18. What are the different types of Directional coupler?
1. Two hole directional coupler
2. Be the hole directional coupler
3. Four hole directional coupler
19. Define Coupling factor.
The coupling factor of a directional coupler is defined as the ratio of the
incident power Pi to the forward power Pf measured in dB.
20. Define directivity of directional coupler.
The directivity of a directional coupler is defined as the ratio of the forward
power Pf to the back power Pb measured in dB.
21. What is Faraday’s rotation law?
If a circularly polarized wave is made to pass through a ferrite rod which
has been influenced by an axial magnetic field B, then the axis of polarization
gets tilted in clockwise direction and amount of tilt depends upon the strength of
magnetic field and geometry of the ferrite.
22. List two microwave devices using Faraday rotation principles
Isolator, Circulator
23. Give some coupling parameters of directional coupler?
Coupling coefficient, Directivity, Insertion loss, Isolation
24. What are ferrites and give its properties?
Ferrites are ceramic like materials. These are made by sintering a mixture
of metallic oxides.
Properties:
1. Specific resistivities may be used as much as 1014 greater than that of
metals.
2. Dielectric constants around 10to 15 or greater.
3. Relative permeability is 1000.
25. Give some examples of ferrite devices?
Isolator, Circulator, Phase shifters, Modulators, Power limiters.
UNIT II
26. What are the advantages of microwave transistors?
Microwave transistors are miniaturized designs to reduce device and
package parasitic capacitances and inductances and to overcome the finite
transit time of the charge carriers in the semiconductor materials.
27. What is bipolar transistor?
Bipolar is three semiconductor region structures where charge carriers of
both negative and positive polarities are involved in transistor operation.
28. Write the applications of bipolar transistors.
Bipolar transistors are suitable for oscillator and power amplifier
applications in addition to small signal amplifiers.
29. What is MESFET?
If the field effect transistor is constructed with metal semiconductor
schottky barrier diode, the device is called metal-semiconductor field effect
transistor.
30. What is negative resistance?
Negative resistance is defined as that property of a device which causes
the current through it to be 180 degree out of phase with the voltage across it.
31. Define Gunn Effect.
Guneffect was first observed by GUNN in n_type GaAs bulk diode.according
to GUNN,above some critical voltage corresponding to an electric field of
2000-4000v/cm,the current in every specimen became a fluctuating fuction of
time.The frequency of oscillation was determined mainly by the specimen and
not by the external circuit.
32. What are the various modes of operation of Gunn diode?
1. Gunn oscillation mode.
2. Stable amplification mode.
3. LSA oscillation mode.
4. Bias circuit oscillation mode.
33. Mention the name of domain modes available in Gunn oscillation mode.
1. Transit-time domain mode.
2. Delayed domain mode.
3. Quenched domain mode.
34. What are the applications of GaAs MESFET?
1, Used in microwave integrated circuits for high power, low noise
applications.
2. Used in broadband amplifier application.
35. What is negative resistance in Gunn diode?
The carrier drift velocity increases linearly from 0 to maximum,
when the electric field is increased from 0 to threshold value in
Gunn diodes. When the electric field is beyond the threshold value of
3000v/cm the drift velocity is decreased and the diode exhibit
negative resistance.
36. What is Transferred electron effect?
Some materials like GaAs exhibit negative differential
mobility, when biased above a threshold value of the electric
field. This behaviour is called transferred electron effect.
37. What are time parameters for TED’S
1. Domain growth time constant
2. Dielectric relaxation time
3. Transit time.
38. Define Avalanche transit time devices.
Avalanche transit time devices are p-n junction diode with the highly
doped p and n regions. They could produce a negative resistance at microwave
frequencies by using a carrier impact ionization avalanche breakdown and
carriers drift in the high field intensity region under reverse biased condition.
39. What are modes available in avalanche device?
There are three modes avalanche device
1. IMPATT- Impact Ionization Avalanche Transit Time Device.
2. TRAPATT- Trapped Plasma Avalanche Triggered Transit Device and
3. BARITT- Barrier Injected Transit Time Device.
40. What are the factors reducing efficiency of IMPATT diode?
1. Space charge effect
2. Reverse saturation current effect
3. High frequency skin effect
4. Ionization saturation effect.
41. List the type of circuit used for IMPATT diode circuits.
1. Broadly tunable circuits
2. Low ‘Q’circuits
3. High ‘Q’circuits
42. What are the applications of low Q-oscillators and amplifier circuits?
1. Final output stage of FM telecommunication transmitter
2. Up converter pump
3. CW Doppler radar transmitter
43. What are the Key phenomenon taking places in TRAPATT diode?
The Key phenomena are
1. Carrier generation by impact ionization producing a current pulse of
phase delay of 90 degree.
2. An additional phase shift introduced by the drift of carriers.
44. What is the operating frequency of TRAPATT devices?
TRAPATT devices operate at frequencies from 400 MHz to about 12GHz.
45. Explain plasma formation in TRAPATT diode.
During the operation of the diode a high field avalanche zone propagates
through the depletion region and fills the layer with a dense plasma of electrons
and holes which get trapped in the low field region behind the zone.
46. What are the applications of TRAPATT devices?
The applications are
1. Phased-array Radar systems
2. Intermediate frequency transmitters.
3. Proxity fuse sources
4. Radio altimeters
5. Microwave landing systems.
47. What is the main advantage of TRAPATT over IMPATT?
TRAPATT diode has much greater efficiency than IMPATT.
48. Write the properties of parametric up converter.
1. The output frequency is equal to the sum of the signal frequency and
the pump frequency.
2. There is no power flow in the parametric device at frequencies other
than the signal, pump and output frequencies.
49. What is the parametric device?
A parametric device is one that uses a non-linear reactance or time
varying reactance. The word parametric is derived from the parametric excitation,
which is a reactive parameter, can be used to produce capacitance or inductive
excitation.
50. Give the applications of parametric amplifier.
1. Space communication systems.
2. Radio telescopes and
3. Tropo-receivers.
UNIT III
51. What is transit time?
The time taken by an electron to travel from the cathode to the anode
plate of an electron tube is known as transit time.
52. Write the classification of microwave tubes.
They are classified into two types.
1. O-type microwave tube or linear beam
2. M- type microwave tube
53. 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
54. Define velocity modulation.
The variation in electron velocity in the drift space is known as velocity
modulation.
55. What do you mean by klystron?
A klystron is a vacuum tube that can be used either as a generator or as
an amplifier of power at microwave frequencies operated by the principles of
velocity and current modulation.
56. Mention the application of two-cavity klystron.
1. Used in Troposphere scatter transmitters.
2. Satellite communication ground stations.
3. Used in UHF TV transmitters.
4. Radar transmitters.
57. Define Transit time in Reflex klystron.
The time taken by the electron to travel into the repeller space and back to
the gap.
58. 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.
59. What are the assumptions for calculation of RF power in Reflex
Klystron?
i) Cavity grids and repeller is 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 is small compared to the dc beam
voltage.
60. Give the drawbacks of klystron amplifiers.
1. As the oscillator frequency changes then resonator frequency also
changes and the feedback path phase shift must be readjusted for a positive
feedback.
2. 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.
61. What is the effect of transit time?
There are two effects.
) At low frequencies, the grid and anode signals are no longer 180 O out of
phase, thus causing design problems with feedback in oscillators.
2) The grid begins to take power from the driving source and the power is
absorbed even when the grid is negatively biased.
62. What are the applications of reflex klystron?
1) Signal source in MW generator
2) Local oscillators in receivers
3) It is used in FM oscillator in low power MW links.
4) In parametric amplifier as pump source.
63. Give the performance Specification of Reflex klystron?
Frequency range: 2- 200 GHz
Band width: + 30 MHz
Power output: 10 mW – 2.5W
Efficiency: 20 to 30%
64. What is TWTA?
A Traveling Wave Tube Amplifier (TWTA) circuit uses a helix slow wave
non-resonant microwave guiding structure. It is a broadband device.
65. What is the purpose of slow wave structures used in TWT amplifiers?
Slow wave structures are special circuits that are used in microwave tubes
to reduce wave velocity in a certain direction so that the electron beam and the
signal wave can interact. In TWT, since the beam can be accelerated only to
velocities that are about a fraction of the velocity of light, slow wave structures
are used.
66. How are spurious oscillations generated in TWT amplifier? State the
method to suppress it.
In a TWT, adjacent turns of the helix are so close to each other and hence
oscillations are likely to occur. To prevent these spurious signals some form of
attenuator is placed near the input end of the tube which absorb the oscillations.
67. State the applications of TWT.
1) Low power, low noise TWT’s used in radar and microwave receivers
2) Laboratory instruments
3) Drivers for more powerful tubes
4) Medium and high power CWTWT’S are used for communication and
radar.
68. What are the advantages of TWT?
1. Bandwidth is large.
2. High reliability
3. High gain
4. Constant Performance in space
5. Higher duty cycle
69. What are the applications of klystron amplifier?
(1) UHF TV Transmitters
(2) Long ranger radar
(3) Linear particle accelerator
(4) Troposcatter links
(5) Earth station transmitter.
70. Name four types of slow wave structures.
1. Helical line.
2. Folded back line.
3. Zigzag line and
4. Interdigital line.
71. Why magnetron is called as cross filed device?
In magnetron, the dc magnetic field and dc electric field are perpendicular
to each other and hence magnetron is called as a cross filed device.
72. What are the types of magnetron?
1. Split anode magnetron
2. Cyclotron-frequency magnetron
3. Traveling wave magnetron
73. State the applications of magnetrons.
1. Radar transmitters
2. Industrial heating
3. Microwave ovens.
74. What is frequency pulling and frequency pushing in magnetrons?
Frequency pulling is caused by changes in the load impedance reflected
into the cavity resonators.
Frequency pushing is due to the change in anode voltage which alters
the orbital velocity of electron clouds.
75. State the characteristics of magnetron and of 2-cavity klystron
amplifier.
Magnetron:
Operating frequencies - 70 GH z
Output power - 40 MW
Efficiency - 40 to 70%
2-cavity klystron:
Efficiency - 40%
Power output -> 500 KW
Pulsed power-_> 30 MW
Power gain- about 30 dB.
UNIT IV
76. What is strip line?
The strip line consists of a central conductor called strip and two ground
plates. The dominant mode in strip line is TEM.
77. Define microstrip line.
The microstrip line consists of a conductor strip and a ground plane. The
electromagnetic wave propagates in quasi TEM mode.
78. Mention the types of losses in microstrip line.
1. Dielectric loss
2. Ohmic loss and
3. Radiation loss
79. What is parallel strip line?
The parallel strip line consists of two perfectly parallel strips separated by
a perfect dielectric slab of uniform thickness.
80. What do you mean by coplanar strip line?
The coplanar strip line consists of two conducting strips on one substrate
surface with one strip grounded.
81. Define partially shielded strip line.
A partially shielded strip line has its strip conductor embedded in a
dielectric medium, and its top and bottom ground planes have no connection.
82. Why conventional open wire lines are not suitable for microwave
transmission?
The conventional open wire transmission lines are not suitable for
microwave transmission due to high radiation losses that are associated when
the wavelength becomes smaller than the physical lengths of the conventional
lines at high frequency.
83. Define planar transmission.
A planar transmission line is a transmission line with conducting metal
strips that lie entirely in parallel lines.
84. Write the advantages of microstrip lines.
1. The microstrip lines have a power handling capacity of a few watts
which is quite adequate for most microwave circuits.
2. With the advantage of low loss, high electric constant materials,
microstrip has become popular, particularly in the fabrication of microwave
integrated circuits.
3. Microstrip lines are used to interconnect high-speed logic circuits in
digital circuits.
85. Define attenuation constant.
The sum of dielectric and ohmic losses may be expressed as losses per
unit length in terms of an attenuation constant.
The attenuation constant α = αd+αc
Where αd is the dielectric attenuation constant and
αc is the ohmic attenuation constant.
86. What are the advantages of coplanar strip line over conventional
parallel strip line?
1. The two strips of coplanar strip lines are on the same substrate for
convenient connections.
2. It eliminates the difficulties involved in connecting the active and
passive circuit components in shunt from the conducting strip to the ground plane
on the same side of the substrate.
3. Reliability is increased than conventional strip line.
87. Write the classification of electronic circuits.
Electronic circuits are broadly classified into three categories based on the
circuit technology.
1. Discrete circuit
2. Integrated circuit and
3. Monolithic Microwave Integrated Circuit(MMIC)
88. What do you mean by discrete circuit?
The circuit elements are separately manufactured and then interconnected
by conducting wires is now referred to as discrete circuit.
89. Define IC.
The IC consists of a single crystal chip of semiconductor typically 50 x 50
mils in cross section containing both active and passive elements and their
interconnection.
90. What are the advantages of MMICs over discrete circuits?
MMICs offer the following advantages over discrete circuits.
1. Small size and weight.
2. High reliability.
3. Improved reproducibility.
4. Improved performance and
5. Eventual cost reduction when produced in large quantities.
91. Mention the materials used in MMICs.
The basic materials for monolithic microwave integrated circuits are
broadly divided into four categories.
1. Substrate materials.
2. Conductor materials.
3. Dielectric materialsand
4. Resistive materials.
92. What is called as film integrated circuit?
An MMIC whose elements are formed on an insulating substrate, such as
glass or ceramic, is called a film integrated circuit.
93. What is the need for dielectric materials?
Dielectric materials are used in monolithic microwave integrated circuits
for blockers, capacitors and some couple-line structures.
94. What is the need of resistive materials?
Resistive materials are used in monolithic microwave integrated circuits
for bias networks, terminations and attenuators.
95. Write some of the properties of resistive materials.
1. Good stability.
2. Low temperature coefficient of resistance.
3. Adequate dissipation capability.
96. Why monolithic technology is not well suitable for microwave
integrated circuits?
Monolithic technology is not well suited for microwave integrated circuits
because the processing difficulties, low yields and poor performance have
seriously limited their applications.
97. What is the need of diffusion and ion implantation?
Diffusion and ion implantation are the two processes used in controlling
amounts of dopants in semiconductor fabrications.
98. Write the advantages of ion-implantation method?
1. Precise control of the total amount of dopants.
2. The improvement of reproducibility and
3. Reduced processing temperature.
99. What is lithography?
Lithography is the process of transferring patterns of geometric shapes on
a mask to a thin layer of radiation sensitive material, which is known as resist, for
covering the surface of a semiconductor wafer.
100. Name the different types of lithography.
1. Electron beam lithography.
2. Ion-beam lithography.
3. Optical lithography and
4. X-ray lithography.
UNIT V
101. What do you mean by slotted line?
Slotted line is a fundamental tool for microwave measurements. Slotted
line consists of a section of waveguide or co-axial line with a longitudinal slot.
This slot is roughly 1 mm wide and allows an electric field probe to enter the
waveguide for measurement of the relative magnitude of field at the location of
the probe.
102. Define reflection co-efficient.
The ratio of electric field strength of reflected and incident wave is called
reflection co-efficient.
103. What is voltage standing wave ratio?
Voltage standing wave ratio is defined as the ratio of maximum voltage to
the minimum voltage.
104. Define return loss.
The return loss is a measure of the power reflected by a line or network or
device.
105. Mention the drawback in return loss measurements.
1. The unstability of the signal source causes a change of signal power
level during the measurement of input and reflected signal levels at different
instants of time.
2. Non-ideal directional couplers and detectors are also cause error.
106. Define power.
Power is defined as the quantity of energy dissipated or stored per unit
time.
107. What are the methods to detect microwave power?
1. Bolometer and
2. Calorimeter method.
108. What is Bolometer?
It is a power sensor whose resistance change with changed
temperature as it absorb the microwave power. It is a short thin
metallic wire sensor with positive temperature coefficient of resistance.
109. What is calorimeter?
It is convenient device setup for measuring the high power at microwave
which involves conversion of microwave energy in to heat, absorbing the heat in
a fluid and determine the temp.
110. Mention the sensors used for microwave power measurements.
The sensors used for microwave power measurements are the Schottky
barrier diode, Bolometer and the Thermocouples whose resistance changes with
the applied power.
111. What is a VSWR meter?
VSWR meter is a highly sensitive, high gain, high theta, low noise
voltage amplifier tuned normally at fixed frequency of 1KHZ of
which microwave signals modulated. This meter indicates calibrated
VSWR reading for any loads.
112. What is calorimetric direct heating method?
In the calorimetric direct heating method, the rate of production of heat
can be measured by observing the rise in the temperature of the dissipating
medium.
113. What is calorimetric direct heating method?
In the calorimetric indirect heating method, heat is transferred to another
medium before measurement.
114. List the different types of Impedance measurement methods?
1. Slotted line method
2. Reflectometer method
115. What do you meant by reflection loss?
The reflection loss is a measure of power loss during transmission due to
the reflection of the signal as a result of impedance mismatch.
116. Define insertion loss?
The insertion loss is a measure of the loss of energy in transmission
through a line or device compared to direct delivery of energy without the line or
device.
117. What are the contributions of insertion loss?
The insertion loss is contributed by
1. Mismatch loss at the input
2. Attenuation loss through the device.
3. Mismatch loss at the output
118. How do you measure microwave frequency?
1. Wavemeter method
2. Slotted line method
3. Downconversion method
119. What is a wave meter?
It is a device used for frequency measurement in microwave. It has
cylindrical cavity with a variable short circuit termination .It changes the resonant
frequency of cavity by changing cavity length.
120. Define scattering parameters.
Scattering parameters are defined as the ratio of the outgoing waves to
the incident waves. The incident and reflected amplitudes of microwaves at any
port are used to characterize a microwave circuit.
121. Define dielectric constant?
It is defined by the ratio of permittivity of medium to permittivity of
free space.
122. What do you meant by isolation?
The isolation between E and H arms are defined as the ratio of the power
supplied by the generator connected to the E-arm (port 4) to the power detected
at H-arm (port 3) when side arms 1 and 2 are terminated in matched load.
123. List the methods for measuring dielectric constants?
1. Waveguide method
2. Cavity pertubation method
124. What are classifications of power measurements?
The classifications of power measurements are
1. Low power (less than 10 mW)
2. Medium power (from 10 mW to 10 W) and
3. High power (>10 W)
125. Distinguish between low frequency measurements and microwave
measurements.
Low frequency
measurements
At low frequency it is
convenient to measure
voltage and current and use
them to calculate power.
At low frequency, circuits use
lumped elements.
Microwave measurements
At microwave frequencies the
amplitudes of the voltages and
current on a transmission line
are the functions of a distance
and are not easily measurable.
At microwave frequencies, the
circuit elements are distributed.