Microwave
Unit - I
1. Why h, y or z parameters cannot be used at microwave frequencies.
Suitable equipments are not available at microwave frequencies for the measurements of voltages and current at the ports of the networks
Short circuit and open circuit conditions are difficult at these frequencies.
Active devcies like power transistors, tunnel diodes etc don’t have suability under open /short circuit conditions.
2. What is the frequency range of C and S BANDS?
C band- 4 to 8 GHz
S band 2 to 4 GHz
3. Name any four applications of microwave frequencies
Long distance communications such as repeaters
aircraft altimeters
vulcanization in rubber industries
polymerization in the chemical industries
4. Voltmeters and ammeters do not exist at microwave frequencies why?
They are not stable at microwave frequencies
5. Explain the need for S parameters
At microwave frequncies, ammeters and voltmeters do not exist and it is not possible to use h, y or z parameters. Hence the linear relationship between the incident transmitted waves and reflected waves are measured in the form of s-parameters in order to study the behaiveiour of s-matrix.
6. List the properties of s matrix
Unityproperty, Symmetry, zero, phase shift property
7. For a two port junction define S11, S12, S21 and S22
S11=Reflection coefficient measured at port1 when V1 exists and port2 is matched.
S12=Reflection coefficient measured at port1 when V2 exists and port1 is matched.
S21=Reflection coefficient measured at port2 when V1 exists and port2 is matched.
S22=Reflection coefficient measured at port2 when V2 exists and port1 is matched.
8. In what way the low frequency concepts are useful at microwave frequencies.
In order to carry out a simplified analysis for waveguide theory as is done in the circuit theory.
9. Write a scattering matrix for a 2port and explain the parameters involved
S= S11 S12
S21 S23
Where
S11=Reflection coefficient measured at port1 when V1 exists and port2 is matched.
S12=Reflection coefficient measured at port1 when V2 exists and port1 is matched.
S21=Reflection coefficient measured at port2 when V1 exists and port2 is matched.
S22=Reflection coefficient measured at port2 when V2 exists and port1 is matched.
10. What is the condition under which S parameters are equal to their corresponding traanposes, S=ST?
When a junction is said to be a reciprocal junction the s-matrix satisfies the symmetry property and the S-matrix is equal to its corresponding transposes, S=ST .
11. What are called S PARAMETERS?
The linear relationship described between the incident /transmitted waves and the reflected waves are called S-pramterrs.It is a measure of the reflection/transmission coefficient at any port of the microwave network.
12. Draw a 2 port network with ABCD parameters.
| 2 port network |
____ ______
____ _____
V1=AV2-BI2
I1=CV2-DI2
13. What is the X-band frequency range?
8.2 GHz to 12.4 GHz
14. Write down the parameter equations in matrix form for a 2post network
Z= Z11 Z12
Z21 Z22
Z11=V1/I1 at i2=0; input impedance (driving point impedance at port 1) when port2 is open circuited.
Z12=V1/I2 at i1=0; reverse transfer impedance with port1 open circuited.
Z21=V2/I1 at i2=0; reverse transfer impedance with port1 open circuited.
Z22=v2/i2 at i1=0; output impedance (driving point impedance at port 2) when port1 is open circuited.
Part B
1.Briefly explain Waveguide Tees.
2.Write short notes on rat-race circuits.
3.With neat diagram explain the working principle of Directional couplers.
4.Write short notes on Circulator & Isolator.
5.Write short notes on Corners, Bends & Twist.
6.Derive the S-matrix for 2 Hole Directional coupler.
7. Derive the S-matrix for Circulator.
8.Derive the S-matrix for Isolator
9.Write short notes on E-plane & H-plane Tee.
10. Why we need S-matrix for Microwave analysis, write the S-matrix for 2 port network.
UNIT - II - (PART A ANSWERS)
- Compare & contrast waveguides & transmission lines.
| Wave Guides | Transmission lines |
| Used at microwave frequencies & less lossy at high frequencies | Used at conventional lower frequencies & more lossy |
| Conduction through the dielectric filling the waveguide | Conduction through walls |
| Behaviour & properties in terms of electrical & magnetic fields | Behaviour & properties in terms of voltages & currents |
| Power handling capability very high | Power handling capability less |
- Explain the significance of ferrite devices in microwave circuits. (Apr’2K, Oct’2K)
Ferrite devices changes the polarization of the waves that passes through them. This is useful for the construction of unidirectional devices such as isolators & circulators
3. Explain the function of Gyrator. ( Oct’97,98,99)
A gyrator is a device that produces 180 degrees phase shift of waves progressing
in forward direction, but no phase shift in the reverse directon.
- Give two applications of gyrator.
Can be used as a non reciprocal phase shifter.
Can be used as in transmitters as local oscillators.
- Explain a typical application of a circulator. ( Oct’2K)
Multiple isolation in radars, parametric amplifiers.
- List the different types of directional couplers.
Bethe hole DC, crossed guide DC, coupled line couplers, branch line couplers, and Lange DC are the different types of directional couplers.
- Explain the purpose of tuning plunger in a variable type termination.
It varies the location of the terminal plane which in turn changes the wave pattern and hence VSWR.
- What is the working principle of a microwave phase shifters. ( Oct’2K)
A change in the path length or the phase constant produces a shift in the phase in phase shifters.
- Bring out the differences between a directional coupler and a magic Tee.
In a directional coupler the output is coupled into only one port and all other ports are matched, whereas in a magic Tee the output is split between any two ports while all the other ports matched.
Directional coupler is a unidirectional device whereas a magic Tee produces phase variation in the wave in all ports irrespective of the direction n which it travels.
Magic Tee produces combined output of two sources connected at a time, whereas a DC handles a single source at a time.
- Define the two performance factors of directional couplers. ( Nov’96, Apr’98)
The two performance factors of DC are the Coupling Factor and Directivity. Coupling Factor defines the ratio of the amount of power coupled in coupled port to that of power at input port in decibels. Directivity is defined as the ratio of powers at the isolated port and the incident ports at decibels.
- What is meant by matched termination?(Oct’97).How can it be achieved.(Oct’2K)
Matched termination provides a termination of a network with its characteristic impedance, i.e. VSWR = 1. This can be achieved by totally avoiding the reflections and all the power that is fed to the output port s absorbed at the output port itself and not reflected.
- Differentiate fixed & variable attenuators. ( Oct’97 )
Fixed attenuators provide fixed amount of attenuation (3,10 or 20 dB), whereas variable attenuators provide variable amount of attenuation ranging from
(0 to ∞ dB).
- What is meant by Isolator? ( Apr’ 98 )
Isolators are unidirectional device which isolates the source and load by providing no amount of attenuation in the forward direction and infinite amount of attenuation in the reverse direction.
- Define Insertion Loss & isolation of a microwave device. ( Oct’ 2000)
Insertion loss is the forward transmission loss ( ≈ dB ) in decibels measured when the isolator is connected in the forward direction, i.e. from source towards load. When the isolator is connected in the reverse direction, the reverse attenuation will be high ( ∞ dB ) as measured in the difference in the power outputs of the input and output ports – isolation loss.
16.Write the equation of VSWR. (Oct’ 98, Apr’ 2K)
VSWR = V max / V min
- List the various WG bends in microwave ( Ocy’99 )
E plane bends and H plane bends are the various WG bends in microwave.
- What is meant by circulator? ( Apr’ 2K )
A circulator is a 3 – port isolator. The power from one port is coupled to the immedialtely next port in a sequential fashion. For ex. Input power at 1 will give almost all of this power at port 2, while port 3 is matched. 1→ 2, 2 → 3, 3 → 1 is only possible and not in the reverse manner.
Part B
1. Explain with neat diagram Microwave Bipolar transistors.
2. Explain Gunn Effect & RWH Theory of Gunn Diode.
3. Explain various modes of operation in Gunn Diode.
4. Write short notes on IMPATT Diode.
5. Write short notes on TRAPATT Diode.
6. Write short notes on Parametric up converter & down converter.
7. Write short notes on Parametric Amplifier.
Unit - III
- What is meant by Applegate diagram?
Applegate diagram is the distance-time diagram used to plot the velocity of the electrons in klystrons.
- List the different types of Magnetrons
- Negative Resistance magnetrons
- Cyclotron frequency magnetron
- Traveling wave magnetrons
- Which tube is used for high power microwave generation.
Magnetron is used for high power microwave generation
- Explain Hull Cut-off condition
Hull cut-off condition gives the cut-off magnetic field in a magnetron such that the electron grazes the anode and returns back to the cathode.
- Explain phase focusing effect in magnetrons
The alternating field applied in magnetron changes its phase in such a way that the electrons spend more time in the interaction space and form bunches with the favored electrons. This effect is known as Phase focusing effect.
- What are slow wave structures?
Slow wave structures are used to reduce the phase velocity of the wave traveling in waveguides.
- How a Helix structure is useful for SWS?
A helical structure is basically a slow wave structure which is used to reduce the phase velocity of the traveling wave. This enables a continuous interaction between RF field and the electron beam which in turn amplifies the input wave exponentially.
- What is meant by ‘Strapping’ in magnetrons?
In order to prevent mode jumping in magnetrons, two rings of heavy gauge wire is used to connect alternate anode poles. This is called as strapping.
- What is the use of admittance spirals in klystrons?
Admittance spirals are the loci of admittances of Reflex klystron plotted in rectangular coordinates. The necessary condition for sustained oscillation can be marked in this diagram in order to differentiate the oscillation and non-oscillation regions. This diagram also shows that the power output in Reflex klystron is discontinuous because the admittance plot spirals out both in the oscillation and non-oscillation regions.
- What is meant by pushing and pulling in magnetrons?
The change in resonant frequency of the magnetron due to changes in the anode voltage is called frequency pushing whereas the change in resonant frequency of the magnetron due to changes in load impedance is called frequency pulling.
- List the advantages of Reflex klystron over multi-cavity klystrons.
Reflex klystrons can be used as an oscillator without any complex feedback circuitry as required in multi-cavity klystrons. As it is a narrow bandwidth device it can be tuned to operate at a single desired frequency in resonant circuits.
- Define drift space of a klystron tube.
The drift space is the space between the buncher cavity and the catcher cavity in klystron tubes.
- Explain back-heating effect in microwave tubes
The electrons fall back toward the cathode itself due to transit time effect and heats up the cathode increasingly. This is called back heating effect.
- Why should the repeller voltage be more negative than the beam voltage in a single cavity klystrons?
If the repeller voltage becomes less negative or even positive with respect to beam voltage then the electrons hit the repeller with a greater force and the repeller draws more current which causes permanent damage to it. Hence repeller should be maintained at a more negative potential than the beam voltage in reflex klystrons.
- Why Π mode is preferred for magnetrons?
Π mode is preferred in magnetrons since it provides self-consistent oscillation and increased power output when compared to any other modes.
- Explain the need for attenuators in TWT
Attenuators are used to attenuate the unwanted signal traveling towards the input end due to reflections arising from impedance mismatch.
- What is meant by velocity modulation?
The change in the velocity of the electrons under the influence of an alternating field is termed as velocity modulation
- The spacing between intermediate cavities in a multi cavity klystron decreases. Why?
To achieve maximum degree of bunching the distance between any two cavities must be an optimum one. This distance Lopt inversely proportional to V1, the amplitude of RF voltage setup in the subsequent cavities. Since electrons are once agin velocity modulated in every cavity, V1 keeps growing and Lopt diminishes. Hence the intercavity distance decreases in a multi cavity klystron.
- What is the maximum electronic efficiency of a Reflex klystron?
22.7 % is the maximum efficiency of a Reflex klystron.
- Bring out the differences between electronic tuning and mechanical tuning in a single cavity klystron.
In electronic tuning the repeller voltage is varied to obtain changes in resonant frequency whereas in mechanical tuning a mechanical screw adjustment produces a change in the physical size of the cavity to produce changes in resonant frequency.
- Which is the best suitable time for the electrons to come back to the cavity in the Reflex klystron?
The best suitable time for the electrons to come back to the cavity in a Reflex klystron is n+3/4 cycles with respect to the reference electron starting from cavity where n is a positive integer. N=1 gives more power output.
- Define transit time in a Reflex klystron.
The time taken by electron to travel into the repeller space and come back toward the cavity is called the transit time in Reflex klystron.
- Bring out the differences between the TWT & Klystron
| TWT | Klystron |
|
|
- State the output power & frequency range of oscillation of a reflex klystron.
In a RK the output power ranges from 10 mW to 2.5 wW and output frequency ranges from 4 GHz to 200 GHz
- In Klystron, at what value of X(bunching parameter) the fundamental current component has its maximum value
X = 1.841
- What is meant by bunching?
The electrons traveling with different velocities join together at their transit towards the output end. This collection of different velocity modulated electrons is called bunching
- What is the basic difference between klystron and Reflex klystron?
Klystron can be readily used as an amplifier whereas RK can be used as an oscillator.
- List the characteristics of TWT
- posses a reduced phase velocity of the waves through a SWS
- has a high bandwidth and excellent gain
PART B
1. Explain Velocity Modulation & Bunching process of Klystron.
2. Derive output power & Efficiency of Klystron.
3. Explain Velocity Modulation & Bunching process of Klystron Amplifier.
4. Derive output power & Efficiency of Klystron Amplifier.
5. Explain Helix TWT Amplifier in detail.
6. Briefly explain Magnetron Oscillators.
7. Write short notes on different types of Magnetron Oscillators.
UNIT IV
- What is the value of ‘Saturated drift velocity’ of electrons in a semiconductor?
Saturated drift velocity of electrons in a semiconductor is 6 * 10^6 m/s.
- What is the maximum value of E-field that can be sustained in a semiconductor?
And without having dielectric breakdown for germanium and silicon.
Maximum value of E field that can be sustained in Ge is 10^5 v/cm and 2*10^5 v/cm.
- What are the high frequency limitations faced by the semiconductors of
microwave frequency ?
High frequency limitations are
a) Transit time effect leads to difficulty in specifying the performance of the
Transistors. || and || value become complex
b) Interelectrode capacitance depends on width of depletion layers of the
Junction which is inturn bias dependent. Lead inductances effect are lesser
Than in tubes.
- What are the variouis microwave transistors structures?
Inter ditated, matrix and overlay are the various microwave transistor devices.
- What is meant by TEDs?
TEDs are transferred electron devices where there is a transfer of electron from a
lower valley of higher mobility to a n upper valley of lower mobility in the conduction band on application of kinetic energy greater than thermal energy but less than forbidden energy gap.
- What are the various materials used for Gunn diodes?
GaAs, InP, CdTe, InAs are materials used in Gunn diode.
- Why are electrons very hot in TEDs?
It is operated well above the thermal energy of electrons hence the electrons are very hot.
- List difference between tranistor and TEDs
Transistor TED
1. Depends on junction property Depends on bulk property
2. Use elemental semiconductor Use compound semiconductor
3. Electrons are warm Electrons are very hot.
- What is the difference between negative resistance tunnel diode and negative
Resistance TEDs?
In negative resistance tunnel diodes tunneling of electrons takes place through allowable empty energy states whereas in negative resistance TEDs, electrons enters into a negative differential mobility state which produce negative resistance effect.
- Explain the effect of empty energy band in TEDs.
As the imparted energy increases above the thermal energy, the electrons crosses the narrow forbidden energy gap and enters into the empty energy band whre there is negative differential mobility of electrons.
- Why low frequency oscillations cannot be obtained from TEDs.
Due to bulk property of the material as the voltage is increased above the threshold voltage due to negative resistance effect, it produces high frequency oscillations due to its bulk property. So it cannot produce low frequency oscillation.
- Explain the plasma formation and plasma extraction in TRAPATT diodes.
As avalanche takes places a plasma of electrons and holes is generated placing a large potential across the junction which opposes the applied voltage,. The total voltage is hereby reduced and current pulse is trapped behind it. When this pulse travels across n drift region and reaches the end plasma extraction takes place and one again voltage increases.
- TRAPATT mode can operate at comparatively low frequenicies. Why?
Because the drift velocity in a TRAPATT dioe is much less than in a IMPATT diode the operating frequencies are lower.
- What are the four different modes of operation of GUNN diode.
Gunn oscillation mode, stable amplification mode, LSA oscillation mode and bias current osillation mode.
- What are the 3 possible domain modes for Gunn oscillation modes in a gunn
diode ?
Transit time domain mode, Quenched and delayed domain mode and LSA
oscillation domain mode.
- Expand the terms IMPATT and TRAPATT.
IMPATT - Impact Avalanche and Transit Time diode
TRAPATT – Trapped plasma Avalanche Triggered Transit mode.
- What is meant by LSA oscillation mode in Gunn diode?
In LSA oscillation mode, stable and sustained oscillations are produced. The transit time frequency is an integral multiple of the domain growth time constant.
- What is meant by IMPATT and TRAPATT mode.
In IMPATT mode avalanche of charge carriers are produced and the applied voltage and pulsed current output are 180 degree out of phase whereas in TRAPATT mode due to plasma formation a current pulse is trapped and plama gets slowly extracted.
- What is Gunn effect/
Gunn effect was first observed by GUNN in n-type GaAs bulk diodes. According to GUNN above some critical voltage corresponding to an electric field of 2000-4000 v/cm the current in every specimen became a fluctuating function of time. The frequency of oscillation was determined mainly by the specimen and not by the external circuit.
- What is avalanche transit time effect?
Due to excess of charge carriers produced there is a delay in charge carriers moving towards the other terminal of the diode. This effect is called avalanche time effect.
PART B
1. Briefly Explain Gunn Effect & modes of operation of the Gunn Diode.
2. Explain Physical description, Avalanche Multiplication & Carrier current & External Current of Read Diode.
3. Explain Physical structure, negative resistance, power output & efficiency of IMPATT Diode.
4. Explain Physical structure, negative resistance, power output & efficiency of TRAPATT Diode.
5. Explain Physical structure, negative resistance, power output & efficiency of BARITT Diode.
6. Explain Manley –Rowe Power relation & Parametric Amplifier.
7. Explain Microstrip lines in detail.
8. Explain Different types of Microstrip lines.
9. Explain the procedure for Monolithic Microwave integrated circuits.
UNIT V
1. What devices are used in tunable detectors ?
Specially designed point contact diodes or semiconductor schottky
barrier diodes.
2. For which parameter measurement is the slotted line section used?
a. VSWR
b. wavelength
c. impedance
d. reflection co-eff & return loss.
3. what is a VSWR meter ?
A VSWR meter is a sensitive high gain , high Q ,low noise voltage
amplifier tuned at a fixed frequency of 1 khz to which the microwave signal
is down modulated.
4. what is a spectrum analyser?
A spectrum analyser is a broad band super heterodyne receiver which
plots the amplitude vs freq of the received signal.
5. list the basic design considerations of a spectrum analyser?
· Freq sweep rate
· Freq sweep rangr
· B.W of IF amplifier
· Center freq of IF amplifier.
6. What is the disadvantage of using a slotted line section?
The amplitude & phase measurements are limited to a single freq.
7. What is a network analyser?
A n/w snslyser measuers both the amplitude & phase of a signal over
a wide freq range within a reasonable time.
8. What sensors are used for power measurements?
· Schottky barrier diode
· Bolometer
· Thermocouple
9. Explain the principle used in low power measurements .
Power meter consisits of a power sensor, which converts the microwave power to heat energy , the corresponding temp rise provides a change in the electrical parameters resulting in an o/p current in low freq circuitry which indicates the power.
10. Explain the principle used in high power measurements.
High power measurement uses uw calorimeters in which temp rise of the load provides a direct measure of the power absorbed by the load.
11. What are the 2 most common types of bolometer?
· Barretter
· Thermistor
12. What are the 2 types of calorimeters used?
· Static calorimeter
· Circulating calorimeter
13. What are the 2 calorimeteric methods ?
· Direct heating
· Indirect heating
14. Define insertion loss & return loss.
Insertion loss = 10 log p0/p1.
Return loss = 10 log pr/pi = 20 log I T’I where T’ =reflection co-eff.
15. Describe the principle of wavemeter freq measurement.
A wavemeter is a cylindrical cavuty with a variable short ckt termination which changes the resonance freq of the cavity by changing the cavity length.
PART B
1. Explain Slotted line VSWR Measurement.
2. Explain VSWR through return loss Measurement.
3. Explain Power Measurement.
4. Explain Impedance Measurement.
5. Explain Insertion loss & Attenuation Measurement.
6. Explain Measurement of Scattering parameters.
7. Explain Dielectric constant Measurement of a solid using Waveguides.
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