Microwave Devicesfaculty of Engineering EMG2026

Microwave DevicesFaculty of Engineering EMG2026

MULTIMEDIAUNIVERSITY

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 1 week from the date of the experiment.

Revision Oct 2012

Microwave DevicesFaculty of Engineering EMG2026

EXPERIMENT 1

CHARACTERISTICS OF GUNN DIODE

OBJECTIVE:

1. To examine the principle of operation of Gunn oscillator,

2. To investigate the voltage current characteristic of Gunn diode,

3. To assess the relationship between voltage across Gunn diode and frequency of the microwave signal generated,

4. To evaluate 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 VI 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 pn junction. The Gunn diode is a GaAs material with ohmic contacts on the two ends.

Figure 1: V-I characteristic of Gunn diode.

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, TD, to have the negative resistance characteristic.

The domain growth time is given by .(1)

where n is charge carrier density, e the charge of an electron, e the mobility of electron in the negative slope region of the V-I characteristic, and  the permittivity of the GaAs material.

For L/v > Tg, the length of sample should be .

A more accurate analysis shows that the required criterion is nL > 1012 cm-2.

The threshold electric field for GaAs is 3.2x105 V/m. The threshold voltage is therefore

(2)

where L is in micro-meter. For a sample length of 30, 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.

PROCEDURE:

Part A: To determine the V-I characteristic of Gunn diode

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 Vd is near the threshold voltage VTh (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 VTh.

5. Plot the VI characteristic on a graph paper.

6. Determine the range of bias voltage over which the VI characteristic has negative slope.

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 slottedline 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 Vd 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 Vd, 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 Vd 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?

Table 1:

EXPERIMENT 2

CHARACTERISTICS OF REFLEX KLYSTRON

OBJECTIVE:

1. To investigate the principle of operation of Reflex Klystron,

2. To determine the relationship between repeller voltage and Power output of Reflex Klystron,

3. To develop the relationship between repeller voltage and frequency of the microwave signal generated by Reflex Klystron.

4. Tocomposethe 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, to , for the electrons to complete their travel through the gap, turn around, and back through the gap satisfy the following condition:

n = 1,2,3,….(1)

where T is the period of the RF waveform.

If the voltage between the resonator and the repeller is Vr, and the distance is d, the deceleration or retardation experienced by the electron may be expressed as

(2)

where e is charge of electron and m mass of electron. If an electron leaves the resonator with velocity vo, the displacement of the electron from the resonator, at any time t, is

(3)

This equation can be used to calculate the travel time totaken by the electron to return to the resonator.

(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 vo when the RF voltage at the resonator is zero (changing from positive to negative). This electron returns to the resonator at a time tr later. Another electron B passes through the gap earlier. Because the RF voltage is positive at this instant, the initial velocity vb for electron B is higher than the initial velocity of electron A (vb > vo). 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 to = (3/4) T.

In general, oscillation will occur when the condition in equation (1) is satisfied. Thus there are several values of repeller voltage Vr 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.

Figure 1: (a) Construction of Reflex Klystron and (b) the simplified diagram.

Figure 2: Bunching phenomenon in Reflex Klystron.

Repeller voltage

Figure 3: Mode characteristics of Reflex Klystron.

Figure 4: Schematic of the experiment set up.

PROCEDURE:

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 slottedline 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.

Measurement data:

Appendix A

Microwave Frequency Measurement Using Slotted-line Probe

1. Adjust the variable attenuator to obtain an SWR greater than 3.

1. Move the probe along the waveguide to find two successive minimum points. Record the positions of these two points (x1 and x2) from the Vernier scale of the slotted-line waveguide. Determine the waveguide wavelength g .

Fig. A1: Standing wave pattern.

1. Calculate the free-space wavelength o using equation A1. The inner dimensions of the waveguide is given as a=2.2870cm and b=1.0160cm. The cutoff waveleght for TE10 mode is c = 2a.

(A1)

1. Calculate the microwave frequency fo = c/o.

Revision Oct 2012