EXPERIMENT 3Diode Detector

Objective: You will construct on AM generator and a diode detector then observe its operation under various conditions.

Material Required

1. ET-3100 Experimenter1. DC voltmeter1. Oscilloscope1. XR 2206 (#442-710)1. 1N4149 diode (#56-56)2. 0.001 µ F capacitors2. 0.01 µ F capacitors1. 22 µ F capacitors1. 10 µ F capacitor3. 1 µ F capacitor1. 100 kΩ resistors1. 6.8 kΩ resistors 1. 4.7 kΩ resistors1. 1 kΩ resistors

Introduction

In this experiment, you will construct and test an AM generator using an XR-2206 function generator IC. You will then use a diode detector to demodulate the generator AM output.

Procedure

1. wire the AM generator shown in Figure 4. use the 100 Ω potentiometer on the experimenter as R.

2. recheck your wiring and then turn on the ET – 3100 experimenter. Use your voltmeter to set the positive power supply to +15V.

3. set your oscilloscope for 2 V/cm and 2 ms/cm. adjust the experimenter audio generator for 200 Hz output on the low range connect your oscilloscope to pin 2 of the XR-2206. Use the triggering controls to obtain a stable display or use the experimenter’s square wave output as an external trigger source for the oscilloscope.

4. use potentiometer R adjust the output wave forms percent of modulation. Note the severe distortion that occurs when the wave is over modulated. Now set the modulation to approximately 50%.

5. Turn off your experimenter and construct the circuit of figure.5 connect your oscilloscope to the output of the diode detector circutit. If you have a dual-trace oscilloscope, you can monitor the AM signal on one channel and the detected output on the other.

6. turn on your experimenter. You should see the demodulated signal on yhour oscilloscope. Now use R to vary the percent of modulation. What happens to the detected signal ? ____________________________________________________________________________________________________________________

7. Turn R clockwise until the AM waveform is over modulated. What happens to the detected output? _________________________________________________________________

_________________________________________________________________

Discussion

In step 1, you constructed an AM generator using the XR-2206 function generator IC. In step 4, you saw that R varied the percent of modulation. In step 5, you constructed a diode detector circuit. Then you verified its operation and the fact that as the percent of modulation changes, so too does the detected output amplitude. In step7, you proved that over modulation causes severe distortion of the output wave.

EXPERIMENT 4

Frequency Modulation

Objective: You will construct an FM generator and Observe its output waveform. You will construct a phase-locked Loop demodulator and observe its operation.

Material Required

1: ET-3100 Experimenter1: Voltmeter1: Oscilloscope1: 565 integrated circuit (#442-654)1: XR-2206 CP integrated circuit (#442-710)1: 100 kΩ resistor1: 10 kΩ resistors1: 47 kΩ resistors2: 4.7 kΩ resistors2: 560 Ω resistors1: 150 Ω resistors1: 10 kΩ potentiometer (#10-386)1: 22 µ F Capacitor 1: 10 µ F Capacitor1: 1 µ F Capacitor1: 0.1 µ F Capacitor1: 0.01 µ F Capacitor2: 0.001 µ F Capacitor1: 470 pF capacitor 1: soldering iron and solder

PROCEDURE

1. Turn on the experimenter and adjust the positive power supply to +15 V as measured on the voltmeter. Turn off the experimenter.

2. Construct the circuit shown in Figure 6. use the experimenter’s 100 kΩ potential meter for R. set the experimenter’s generator control fully counterclockwise and the range switch to low. Turn potentiometer R fully counterclockwise.

3. Connect your oscilloscope to pin 2 of the XR-2206 IC (output) and set the oscilloscope time/cm control to 2 µs/cm and the vertical input to 1 V/cm. turn on the experimenter. Your oscilloscope should display a sine wave output. If not check your circuit for wiring errors.

4. Turn potentiometer R to mid-range. Your oscilloscope should show a slightly blurred sine wave such as that shown in figure7. this wave graphically illustrates the frequency deviation of the FM output. It occurs because the oscilloscope triggers each wave at the same point on the display. However, since each cycle has a slightly different frequency, the blurred display results. By varying the setting of potentiometer R you can observe the change in deviation. What quantity is R changing in order to vary the frequency deviation? ___________________

5. Return potentiometer R to its mid range setting. Now slowly in-crease the generator frequency from minimum to maximum. What happens to output frequency deviation? __________________. Is the output of the XR-2206 IC frequency or phase modulation? ______________________________.

6. Turn of your experimenter and read the following discussion.

Discussion In step4, you saw a visual display of the generator’s frequency deviation. R was used to change the amount of deviation. This was possible because R controls the amnplitude of the audio input signal. As the audio amplitude increases so does the frequency deviation. In step5, you increased the audio modulatingfrequency. The frequency deviation should have remained constant. Any slight deviation changes were due to brief generator output amplitude changes. Since the deviation remained constant regardless of the modulatin frequency, the XR-2206 IC generates a true FM output.

Procedure (continued)

7. Cut three 5cm lengths of wire. Remove 5mm of insulation from each end. Now solder a prepared wire to each of the three terminals on the kΩ potentiometer.

8. Add the circuit of figure 8 to the FM generator already on the experimenter. Be sure to double check your wiring.

9. Set deviation control R fully clockwise, for maximum deviation. Also set the generator frequency control fully counterclockwise and the range switch to low.

10. connect your oscilloscope to pin 7 (audio output) of the 565 phase locked loop. Set the time / cm control to 2 ms/cm and the vertical input to 0.5 V/cm. turn on the experimenter and adjust the positive power supply for +12V output and the negative supply for 12V output. At this point, your may or may not have an audio ouput signal displayed on the oscilloscope. You must adjust the 565 PLL to the correct operating frequency. To do this adjust R2 until you obtain a sine wave output on the oscilloscope. At this point, the VCO operating frequency is the same as the input frequency. The sine wave output is the error voltage required to keep the VCO locked onto the input FM signal.

11. Using the generator frequency control and R1 the deviation control verify that the output of the phase locked loop is directly proportional to the modulation signal.

12. Turn off your experimenter and disconnect your circuit.

Discussion

In this part the experiment, you constructed a phase locked loop FM demodulator. In step 10, you adjusted the VCO to the input frequency and from there, the PLL locked onto the incoming signal. In step 11, you verified that it was indeed demodulating the FM wave.