Lab #6: Amplitude Modulation & Demodulation Physics 426

Lab #6: Amplitude Modulation & Demodulation 

Physics 426 

Name: 

1. 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 1.2- 

MHz 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). 

C:\TEACHING\PHYS426\SPRING00\LAB6.DOC 1



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. 




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. 




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. 




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 what’s now going on in the demodulator circuit, and how does this 

account for the effect that you’ve just witnessed (You may use the back of this sheet.) 




10. Now build the LM386 audio power amplifier circuit shown in Figure 4. Try to do this on the 

same breadboard you’ve 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 you’ve 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).

Lab #6: Amplitude Modulation & Demodulation Physics 426

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