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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. Revision Sept 2010
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? Revision Sept 2010
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