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Microwave Devices Faculty of Engineering EMG2026
MULTIMEDIA UNIVERSITY
FACULTY OF ENGINEERING
LAB SHEET
MICROWAVE DEVICES
EMG 2026
MD 1 - Characteristics of Gunn Diode
MD 2 - Characteristics of Reflex Klystron
Note: Students are advised to read through this lab sheet before doing experiment
GUIDELINE FOR LAB REPORT
The laboratory report should include the following:
1. Objective of the experiment
2. Procedure
3. Measurement results
4. Discussion of the findings
5. Conclusion
The report must be submitted to the Applied EM Lab within 2 weeks from the date of the experiment.
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Microwave Devices Faculty of Engineering EMG2026
OBJECTIVE:
EXPERIMENT 1
CHARACTERISTICS OF GUNN DIODE
1. To identify the principle of operation of Gunn oscillator.
2. To estimate the voltage current characteristic of Gunn diode.
3. To analyse the relationship between voltage across Gunn diode and frequency of the microwave signal
generated.
4. To analyse the relationship between voltage across Gunn diode and output power.
APPARATUS:
Gunn power supply/SWR meter (737021), Gunn oscillator, PIN modulator, Cavity wavemeter, Isolator,
Slotted line probe, Variable attenuator.
INTRODUCTION:
Gunn diode is a type of Transferred Electron Device (TED). Material like GaAs exhibit negative differential
mobility (i.e. decrease in carrier velocity with an increase in electric field) when bias voltage is above a
threshold level. This gives rise to negative resistance characteristic of the Gunn device (i.e. decrease in
current with an increase in voltage, see Figure 1). The shape of the V-I characteristic suggests that the device
may be used as negative resistance amplifier or oscillator.
Unlike other diodes, the Gunn device does not require the presence of a p-n junction. The Gunn diode is a
GaAs material with ohmic contacts on the two ends.
GaAs
L
I (mA)
V th V 1
V
Figure 1: V-I characteristic of Gunn diode.
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Microwave Devices Faculty of Engineering EMG2026
A detailed analysis shows that the transit time T = L/v (where L is length of sample and v is velocity of
charge carriers) of electron through the GaAs sample should be larger than the domain growth time, T D , to
have the negative resistance characteristic.
The domain growth time is given by T
g
= ε
neµ
. (1)
e
where n is charge carrier density, e the charge of an electron, µ e the mobility of electron in the negative slope
region of the V-I characteristic, and ε the permittivity of the GaAs material.
For L/v > T g , the length of sample should be
εv
L > .
neµ
e
A more accurate analysis shows that the required criterion is nL > 10 12 cm -2 .
The threshold electric field for GaAs is 3.2x10 5 V/m. The threshold voltage is therefore
V th
= 0 . 32× L
(2)
where L is in micro-meter. For a sample length of 30 µ m , the threshold voltage will be 9.6 volts.
The frequency of oscillation is related to transit time and hence the length of the device. Shorter sample
length will have a higher operating frequency. Since the threshold electric field is fixed, the operating bias
voltage across the device decreases linearly as the design frequency is increased.
When Gunn diode is placed in a resonant cavity, oscillation occurs at the frequency of the resonant circuit
rather than at the intrinsic frequency (or transit time frequency). The resonant frequency of the resonant
circuit can be several times the intrinsic frequency.
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Microwave Devices Faculty of Engineering EMG2026
PROCEDURE:
Caution: Please DO NOT apply a voltage exceeding 18V across the Gunn diode.
Part A: To determine the V-I characteristic of Gunn diode
Gunn power supply/
SWR meter
To INPUT of
GUNN PIN INPUT
Gunn
diode
PIN
Modulator
Figure 2: Experimental set up.
1. Set up the equipment as shown in Figure 2. The PIN modulator input shall be left open.
2. Before switching on the power supply, adjust the voltage knob to zero (maximum anti-clockwise).
3. Increase the voltage in steps of 0.5 volt by adjusting the voltage knob.
a) Measure the voltage across the Gunn diode and record the reading in Table l.
b) Measure the current (with the same meter by switching to "I" mode) and record it in Table l.
4. When voltage applied to the Gunn diode V d is near the threshold voltage V Th (where the slope changes
from positive to negative), it may be necessary to vary the voltage in steps of 0.25 V or less to accurately
determine V Th .
5. Plot the V-I characteristic on a graph paper.
6. Determine the range of bias voltage over which the V-I characteristic has negative slope.
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Microwave Devices Faculty of Engineering EMG2026
Part B: To study the variation of power output and frequency with respect to bias voltage
1. Set up the equipment as in Figure 2. Additionally, connect the PIN modulation output of the Gunn
power supply to the PIN modulator.
2. Set the applied voltage to the Gunn diode somewhere near the middle of the negative slope region of the
V-I characteristic.
3. Set the slotted-line probe at about 2-mm protrusion depth and adjust the tuner stub/knob for proper
matching of the detector.
4. Set the variable attenuator at 5-mm micrometer setting. (The reading of the SWR meter should be
constant when the probe carriage is moved along the slotted-line waveguide.)
5. Adjust the Gunn diode supply voltage until the SWR meter reading is maximised.
6. Select the appropriate V/dB setting. Adjust the zeroing knob of the SWR meter so that the needle points
at 0 dB. After this, the zeroing knob should be fixed.
7. Increase the Gunn diode voltage V d to 10V.
8. Measure the frequency using the cavity wavemeter.
(Starting with the cavity wavemeter dial set to maximum clockwise, unscrew the dial slowly. At first there
will be little effect on the SWR meter reading until at one point the power will fall sharply. Read the
frequency indicated on the scale.)
You may also use the alternative method given in Appendix A for microwave frequency measurement.
9. Record the applied voltage V d , microwave power output in dB, and oscillation frequency in Table 1.
10. Decrease the applied voltage in steps of 0.5 V and record the corresponding values of power output and
oscillation frequency.
11. Repeat step 10 until V d is lower than the threshold voltage.
12. Plot frequency versus bias voltage.
13. Plot power output versus bias voltage.
14. What is the frequency at which the Gunn oscillator gives maximum power output?
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Microwave Devices Faculty of Engineering EMG2026
Table 1:
Bias Voltage V d Bias Current Power level (dB) Frequency (GHz)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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Microwave Devices Faculty of Engineering EMG2026
MARKING SCHEME MD1
(Student can give a result as long as the result is valid)
Part A: To determine the V-I characteristic of Gunn diode
5. Plot the V-I characteristic on a graph paper.
(10 marks)
6. Determine the range of bias voltage over which the V-I characteristic has negative slope.
(5 marks)
Part B: To study the variation of power output and frequency with respect to bias voltage
9. Record the applied voltage V d , microwave power output in dB, and oscillation frequency in Table 1.
(10 marks)
12. Plot frequency versus bias voltage. (10 marks)
13. Plot power output versus bias voltage.
(10 marks)
14. What is the frequency at which the Gunn oscillator gives maximum power output?
(5 marks)
(Total: 50 marks)
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Microwave Devices Faculty of Engineering EMG2026
OBJECTIVE:
EXPERIMENT 2
CHARACTERISTICS OF REFLEX KLYSTRON
1. To identify the principle of operation of Reflex Klystron.
2. To identify the relationship between repeller voltage and Power output of Reflex Klystron.
3. To analyse the relationship between repeller voltage and frequency of the microwave signal generated by
Reflex Klystron.
4. To analyse the effect of cavity size on frequency of microwave signal generated by Reflex Klystron.
APPARATUS:
Klystron tube, Klystron power supply, Cavity wavemeter, Isolator, Slotted line probe, SWR meter, Matched
load, variable attenuator, and digital multimeter.
INTRODUCTION:
A Reflex Klystron is a microwave oscillator with a single cavity (resonator). The construction of a reflex
klystron is shown in Figure 1(a). The basic configuration in simplified form as shown in Fig 1(b) is
considered here for understanding the operation of Reflex Klystron. The various parts in Figure 1(b) are
(1) Electron Gun; (2) Resonator; (3) Repeller; and (4) Output coupling.
A highly focused beam of electrons passing through the resonator gap will be accelerated or decelerated by
the induced current on the resonator. The alternate action of increasing and decreasing the electron velocity
will modulate the electron beam into varying dense and sparse of electrons referred to as electron bunches.
This electron bunching mechanism is also called intensity modulation or velocity modulation.
The repeller electrode is at a negative potential. It stops the movement of the electrons, turn them around,
and send them back through the resonator gap. As the repelled electrons re-enter the resonator, they give up
their kinetik energy. The field excited in the resonator add in phase with the initial modulating field such that
it reinforce the next wave of electron bunching. As a result, this energy serves as a regenerative feedback to
sustain the oscillation at the resonant frequency of the resonator. This is the case only if the repeller voltage
is set such that the travel time, t o , for the electrons to complete their travel through the gap, turn around, and
back through the gap satisfy the following condition:
t o
⎨
⎧ 3 = n + T
⎩ 4⎭ ⎬⎫
n = 1,2,3,…. (1)
where T is the period of the RF waveform.
If the voltage between the resonator and the repeller is V r , and the distance is d, the deceleration or
retardation experienced by the electron may be expressed as
eVr d
a = /
(2)
m
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Microwave Devices Faculty of Engineering EMG2026
where e is charge of electron and m mass of electron. If an electron leaves the resonator with velocity v o , the
displacement of the electron from the resonator, at any time t, is
1 x
o
− at
2
v t
2
= (3)
This equation can be used to calculate the travel time t o taken by the electron to return to the resonator.
t
r
2v
o
= =
x=
0
a
2v
o
eV
md
r
(4)
Bunching phenomenon in Reflex Klystron
Bunching phenomenon in a reflex Klystron can be visualised by studying the electron trajectories in the
region between the resonator and the repeller. The parabolic trajectories given by equation (3) are shown in
Figure 2.
Let us consider an electron A which passes through the resonator gap with velocity v o when the RF voltage
at the resonator is zero (changing from positive to negative). This electron returns to the resonator at a time t r
later. Another electron B passes through the gap earlier. Because the RF voltage is positive at this instant, the
initial velocity v b for electron B is higher than the initial velocity of electron A (v b > v o ). Thus electron B
would travel further towards the repeller and take a longer time to return back to the resonator. It will then
bunch with electron A. Similarly, an electron C that passes through the gap after A emerges with a lower
initial velocity and therefore takes less time to return to the resonator. Electron C catches up with electron A
and bunch with electrons A and B.
The electron bunches formed as described above would deliver power to the resonator if they pass through
the resonator at an instant when the field in the resonator retards the bunch. Referring to Figure 2, we note
that this happens when t o = (3/4) T.
In general, oscillation will occur when the condition in equation (1) is satisfied. Thus there are several values
of repeller voltage V r that satisfy the oscillation condition. These values correspond to various modes of
operation. A particular mode of operation may be selected by a choice of repeller voltage. Figure 3 shows the
power output versus repeller voltage for various modes of operation (called the reflex Klystron mode curves
or mode characteristics). The mode corresponding to n = l occurs at maximum negative repeller voltage. For
n = 0, the gain mechanism is usually not strong enough to overcome system losses. Higher order modes (n >
1) may present at lower repeller voltages. Frequency versus repeller voltage variations is also shown in
Figure 3.
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Microwave Devices Faculty of Engineering EMG2026
1
2
d
3
4
V a
(b)
V r
(a)
Figure 1: (a) Construction of Reflex Klystron and (b) the simplified diagram.
x o
Repeller
d
Resonator gap
B A C
t o
t
RF voltage
t
3T/4
Figure 2: Bunching phenomenon in Reflex Klystron.
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Microwave Devices Faculty of Engineering EMG2026
RF frequency
V r
n = 1
Output power
n = 3
n = 2
Repeller voltage
V r
Figure 3: Mode characteristics of Reflex Klystron.
Cavity
Wavemeter
SWR meter
Variable
attenuator
Klystron
Source
Isolator
Slotted-line
Probe
short-circuit
plane
Figure 4: Schematic of the experiment set up.
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Microwave Devices Faculty of Engineering EMG2026
PROCEDURE:
CAUTION 1: The RF power levels in the following experiments are not harmful, but a human eye
may be damaged by low level of radiation. Do not look into the waveguide at any time when the
equipment are on.
CAUTION 2: Klystron tube get extremely hot when it is operated and must not be handled by hand.
Measurement of Frequency and power with variation in repeller voltage
1. Set up the equipment as shown in Figure 4. Switch on the power supply and RF output. Wait a few
seconds for the Klystron tube to warm up.
2. Select squarewave modulation. Set the modulation level to maximum. Adjust the repeller voltage to
maximum and then decrease it slowly until the largest deflection (maximum power) is indicated on the SWR
meter.
3. Adjust the modulation frequency so that the SWR meter deflection is further maximised.
4. Set the slotted-line probe at about 2-mm protrusion depth and adjust the tuner stub/knob for proper
matching of the detector.
5. Set the variable attenuator at 5-mm micrometer setting. (The reading of the SWR meter should be
constant when the probe carriage is moved along the slotted-line waveguide.)
6. Adjust the gain knob of the SWR meter so that the needle points at 0 dB. After this, the zeroing knob
should be fixed.
7. Measure the repeller voltage from the connector at the rear panel of the Klystron power supply using a
multimeter and record the reading. The multimeter reading shall be multiplied by 10x to give the repeller
voltage.
8. Measure the frequency using the cavity wavemeter and record the result.
(Starting with the cavity wavemeter dial set to maximum clockwise, unscrew the dial slowly. At first there
will be little effect on the SWR meter reading until at one point the power will fall sharply. Read the
frequency indicated on the scale.)
You may also use the alternative method given in Appendix A for microwave frequency measurement.
9. Vary the repeller voltage.
10. Measure the repeller voltage and record the reading.
11. Measure the frequency using the cavity wave meter and record the reading.
12. Repeat steps 9-11 for the full range of repeller voltage.
13. Plot relative power level in dB versus repeller voltage.
14. Plot frequency versus repeller voltage.
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Microwave Devices Faculty of Engineering EMG2026
Measurement data:
1
Repeller voltage (multimeter
reading × 10V)
Relative output power (dB)
Frequency (GHz)
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
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Microwave Devices Faculty of Engineering EMG2026
Appendix A
Microwave Frequency Measurement Using Slotted-line Probe
1. Adjust the variable attenuator to obtain an SWR greater than 3.
2. Move the probe along the waveguide to find two successive minimum points. Record the positions of
these two points (x 1 and x 2 ) from the Vernier scale of the slotted-line waveguide. Determine the
waveguide wavelength λ g .
Power
λ g = 2d
x1
d
x2
Distance
Fig. A1: Standing wave pattern.
3. Calculate the free-space wavelength λ o using equation A1. The inner dimensions of the waveguide is
given as a=2.2870cm and b=1.0160cm. The cutoff waveleght for TE 10 mode is λ c = 2a.
1 1 1
2
=
2
+
2
(A1)
λ λ λ
o g c
4. Calculate the microwave frequency f o = c/λ o .
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Microwave Devices Faculty of Engineering EMG2026
MARKING SCHEME MD2
(Student can give a result as long as the result is valid)
Measurement of Frequency and power with variation in repeller voltage
1. Measurement data Table. (10 marks)
13. Plot relative power level in dB versus repeller voltage.
(20 marks)
14. Plot frequency versus repeller voltage.
(20 marks)
(Total: 50 marks)
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