Lab #6: Amplitude Modulation & Demodulation Physics 426
1.
Name:
Connect your function generator (which must be the SRS for this experiment) directly to one channel of your scope. Set the function generator as follows: frequency = 1.2 MHz DC offset = 0 volts amplitude = 20 V p p (10 V p p on the SRS) modulation mode: AM (INT) modulation shape: sine modulation rate: a few kHz modulation depth: 50% sweep on/off: ON
Set the sweep speed of your scope somewhere in the range from 50 to 250 microseconds/division. Adjust the trigger level until you have a stable display on your scope. What you are seeing now is an amplitude modulated (AM) signal: the amplitude of the 1.2MHz sine wave is being modulated, or changed with time, at a rate of a few kHz. Amplitude modulation is used to encode information in a high-frequency carrier by varying the amplitude of the carrier. The carrier is usually a radio-frequency (RF) signal such as the 1.2-MHz sine wave used here. The rate at which the amplitude of the carrier is changed is called the modulation rate. The modulation rate is typically much lower than the carrier frequency: it is typical to modulate at audio frequencies ( 20 Hz to 20 kHz). In the space below, make a sketch of the amplitude modulated signal you see on your scope.
2. Now apply this AM signal to your Q multiplier (from the previous experiment) through inductive coupling (see Fig. 1).
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Lab #6: Amplitude Modulation & Demodulation Physics 426
L1
L2
Figure 1. RF generator providing input signal to Q multiplier through inductive coupling.
2. Monitor the SRS/PRAGMATIC on one channel of your scope. Look at the output of the Q multiplier on the other channel. In the space below, sketch the Q multiplier output.
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Lab #6: Amplitude Modulation & Demodulation Physics 426
3. See whether you can tune the Q multiplier through the AM carrier frequency. In the space below, comment on what happens to the Q multiplier output when you do this.
4. Change the positive feedback setting of your Q multiplier through its entire range. In the space below, comment on what happens to the Q multiplier output when you do this.
5. In the space on your breadboard just to the right of the Q multiplier, build the AM demodulator (see Fig. 2).
Figure 2. AM demodulator.
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Lab #6: Amplitude Modulation & Demodulation Physics 426
6. On your scope, look simultaneously at the SRS/PRAGMATIC (still in the AM mode) and the AUDIO OUT signal from the demodulator. Try to match the audio with the envelope of the AM signal. In the space below, comment on how well they match. Which edge of the AM envelope (upper or lower edge) does the audio match?
7. Remove L1 and turn the SRS/PRAGMATIC OFF. Using clip leads, connect the upper end of L2 to the antenna distributed throughout the lab. Use a 50-100pF capacitor to couple the antenna to your tuned inductor, as shown in Figure 3. Try to tune your Q multiplier (which is really now an AM receiver) to some AM broadcast station (WTAW 1150 kHz, e.g.). On your scope, observe the AM signal at the source follower output. Try to correlate what you see on the scope with the audio that you hear from the radio receiver in the lab (provided by your instructors). In the space below, comment on how good this correlation seems to be.
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Lab #6: Amplitude Modulation & Demodulation Physics 426
Figure 3. Q-multiplier portion of AM receiver, showing antenna connected to upper end of L2 using 50-100 pF coupling capacitor. (AM receiver circuit also includes demodulator shown in Figure 2.)
8. With your AM receiver tuned to an AM broadcast station (and with the positive feedback adjusted so that your receiver does not oscillate on its own), look at the input to the demodulator and the output from it simultaneously on the scope. Try to match up the demodulator audio output with the envelope of the AM carrier. Which edge of the envelope (upper or lower) does the audio seem to follow?
9. Now turn the diode in your demodulator around so that it now conducts in the opposite direction. Which edge of the envelope of the AM signal are you detecting now? Explain this result in terms of the effect that reversing the diode has on the details of the operation of the demodulator. That is, what is different about whats now going on in the demodulator circuit, and how does this account for the effect that youve just witnessed? (You may use the back of this sheet.)
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Lab #6: Amplitude Modulation & Demodulation Physics 426
10. Now build the LM386 audio power amplifier circuit shown in Figure 4. Try to do this on the same breadboard youve been using (i.e., inside your aluminum BUD chassis). You may not be able to fit another circuit on the front half of this breadboard, but remember that you do have another similar section of breadboard to work with toward the rear of the box.
speaker
Figure 4. LM386 audio power amplifier (gain 200).
11. Try tuning your AM radio receiver to as many different AM stations as you can. When you locate new stations, use a local oscillator (your SRS or PRAGMATIC) to produce beats so that you can determine the carrier frequency of the AM station youve found (be sure to see the demo on this in class). Record all the carrier frequencies of the stations you find (and their call letters, if you can).
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