22-05-2012, 10:20 AM
MICROWAVE ENGINEERING LAB-QUESTIONS AND ANSWERS
QUESTIONS AND ANSWERS.pdf (Size: 84.81 KB / Downloads: 183)
1. Why can’t conventional tubes be used at microwave
frequencies?
Conventional tubes can’t be used at microwave
frequencies because of
transit time effect. Lead inductance and inter electrode
capacitance 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 from
cathode 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 reflex
klystron?
Variation in velocities of the electrons in the electron
beam is called
velocity modulation; variation in the current density of
the electron
beam 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. An
electron leaving the gap earlier during slightly positive
voltage would travel further into repeller space and hence
would take longer time then the reference electron to
return to the gap. An electron leaving the gap later will
face slightly negative voltage and gets retarded. So it
returns back after a shorter travel in the repeller space.
Thus all the electrons would arrive back to the gap in
bunches. Bunching around reference electron takes place
once per cycle of RF oscillations.
6. Why do different modes of operation exist for a reflex
klystron?
There are several combinations of repeller voltage
and anode voltage that
provide favorable conditions for bunching. Accordingly
there are several modes
of operation, expressed by N + ¾ where N is an integer.
7. What modes are generally used in a reflex klystron?
1 ¾ and 2 ¾ are the most commonly used modes in
a practical reflex
klystron.
8. What is the operating frequency and power output of a
reflex klystron?
Frequency range: 4GHz – 200GHz
Output power: maximum 3W in X-band to 10mW at
220GHz
9. What is the maximum theoretical efficiency of the reflex
klystron?
22.7% for 1 ¾ mode.
10. What is mechanical tuning and electronic tuning in a
reflex klystron?
Variation in frequency of resonance of cavity by
varying its dimension by
a mechanical method like adjusting screws is called
mechanical tuning. Variation of frequency by adjusting
repeller voltage is called electronic tuning.
11. What is the range of frequency variation of reflex klystron
in the X-band by the above two methods?
Mechanical tuning: +20MHz. Electronic tuning:
+8MHz
12. What are applications of reflex klystron?
They are used as 1) signal source in microwave
generators. 2) Local oscillators in receivers. 3) Pump
oscillators in parametric amplifiers.
13. Leaving reflex klystron, what other devices are used as
microwave signal sources?
1) Magnetron oscillator 2) Backward wave oscillator
3) Gunn oscillator 4) IMPATT diode oscillator 5)
TRAPPAT diode oscillator
14. Is Gunn diode a p-n junction?
No. It isn’t.
15. What is Gunn Effect?
When the voltage gradient across a slice of GaAs
exceeds about 3300V/cm, negative
resistance will develop and oscillations occur if the slice is
connected to a suitable tank circuit.
16. What are the materials used in the construction of Gunn
diode?
GaAs (Gallium Arsenide), InP (Indium Phosphate)
doped in Si.
17. What is negative resistance?
A device is said to have negative resistance
characteristics if, during its operation an increase in the
voltage across it leads to decrease in its current.
18. Can any negative resistance device be used for
amplification?
Yes.
19. What are the other names of Gunn diode and Gunn
oscillator? Transferred electron device and transferred electron
oscillator.
20. Draw the equivalent circuit of an X-band GaAs Gunn
diode.
Equivalent circuit is a parallel combination of
resistance (~100Ω) 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: 950mA
Efficiency: 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 in
microwave receivers.
2) Pump sources in parametric amplifiers
3) Power output oscillators in radars
24. Expand the terms IMPATT, TRAPATT, & PIN referring to
the types of diodes.
IMPATT – Impact Avalanche & Transit Time Diode
TRAPATT – Trapped Plasma Avalanche Triggered
Transit Diode
PIN – Positive Intrinsic Negative
25. What is the use of crystal diode in a microwave bench
setup?
To detect the microwave signal picked up by the
probe 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 20μA or
input power should be less than 10mW.