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EXPERIMENT E3: RADIO FREQUENCY OSCILLATORS Related course: KEET 4206 (Electronics Communications) OBJECTIVES: To analyse radio frequency oscillators EQUIPMENT: Oscilloscope, signal generator, power supply, voltmeter, frequency counter, 741 op-amps, N Channel JFET - 2N5486, Resistors (R1 = R2 = 10kΩ, R0 = 10kΩ, Rb = 50kΩ potentiometer, R = 50kΩ potentiometer, RG = 100kΩ, capacitors (C1 = C2 = 0.01uF, C = 0.1 uF) PRECAUTIONS: Follow the demonstrator instructions throughout the experiment REFERENCE(S): Refer to the main references of KEET 4206 (Electronics Communications) TESTS: TEST 1: WIEN BRIDGE OSCILLATOR TEST 2: AUTOMATIC GAIN CONTROL TEST 1: WIEN BRIDGE OSCILLATOR

(a) WIEN bridge oscillator (b) Amplifier circuit

Figure 1

1. An oscillator circuit is shown in Figure 1. Connect the circuit. The voltage supply is to be set at 10Vdc.

2. With a Vpp sinusoidal signal source connected to terminal x, observe waveform at terminal y with an oscilloscope.

3. Adjust Rb so that an undistorted waveform is observed at terminal y.

4. Vary the frequency of the signal source and locate the frequency at which the sinusoid at terminal y is exactly in phase with the one at terminal x. Record the exact frequency.

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5. Adjust Rb until the two amplitudes are exactly equal.

6. Remove the signal source and connect terminal x to terminal y with a conductor.

7. The circuit should oscillate, if it does not repeat steps 1 through 6, but adjusting Rb so that voltage at y is 5 or 10% larger than that at x.

8. Record the frequency of oscillation at the op-amp output and sketch the waveforms at V0 and Vp. Record their peak to peak values.

9. Measure the value of Rb without disturbing its setting. TEST 2: AUTOMATIC GAIN CONTROL It can be observed that the output waveform of the WIEN Bridge oscillator is somewhat distorted. This is due to the fact that the circuit poles must be placed just a bit to the right of the imaginary axis for the oscillation to build up. The amplitude then builds up until the nonlinearity of the amplifier reduces the small signal gain sufficiently to move the poles back to the left half plane. This part of the experiment will be concerned with using FET in the voltage controlled resistance region in a circuit which automatically senses the output voltage peak level and adjusts the gain to just precisely place the poles on the imaginary axis.

Figure2: Automatic Gain control Circuit

1. Replace the gain block (amplifier block) from circuit shown in figure t with the circuit shown in Figure 2.

2. Adjust potentiometer R so that the movable contact is close to the bottom. The resistance between the movable contact and ground must be ensured to be low (a few hundreds).

3. Adjust Rb for oscillation with a minimum amount of distortion. Measure its value.

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4. Slowly vary Rg and observe V0 with the oscilloscope. (V0 can be adjusted to any desired value between 2V and approximately 15 V peak to peak. The output should be an almost undistorted sinusoid of constant frequency).

5. Measure the peak to peak value of V0 and Vp. The value of Rx should be measured for 5 values between the two extremes (minimum and maximum values of V0).

6. Replace C with a larger capacitor in the 5 to 10uF range. Repeat steps 1 through 5.

DISCUSSION:

1. Derive the loop gain T(s) for the circuit of Figure 1 in terms of R1, C1, R2, C2 and A.

Evaluate for s = jω and find the frequency ω0 at which L T(jω) = 0o. Then find the required value of voltage gain A. Using the measured values of R1, C1, R2, C2, evaluate these values numerically. Compare the theoretical value of f0 = ω0/2π with the experimental value. Also derive for A. Note that the experimental value for A can be obtained by taking the ratio of measured Vpp to VFpp or from measured l+(Rb/Ra).

2. Derive the peak output amplitude for the WIEN Bridge oscillator with the automatic gain

control as a function of Rg. Assume that the peak detector is ideal, that is VG = V0(peak) is a de voltage (this is equivalent to assuming that RGCG time constant is very large compared to the period of oscillation. Also assume that the FET is ideal and recall that rDS = rDS(on)/[1-(VGS/Vp)]. Draw a graph of Vopp versus Rx for both theoretical and experimental results.

Figure 3: VCR characteristics of N-channel JE

END OF EXPERIMENT