EXPERIMENT 3: Amplitude Modulation and Envelopes
New Modules: VARIABLE DC, UTILITIES, SPEECH MODULE
Pre-Lab Reading:
1) TIMS-301 Basic Modules User Manual pages: 23-25.
2) Advanced Modules Manual pages: 98-99.
3) Student Text Vol-A1 pages: 47-52, 61-64, 71-74.
In-Lab:
We are going to implement the AM modulation equation
AM (t )  1  m(t )  c(t )
Where
m(t ) = message signal = cos( t )
c (t ) =carrier = cos(t )
This is the block diagram:
(1) Build the patch shown in Figure 1.
1
Figure 1
Set the AUDIO OSCILLATOR frequency to 1 kHz. Switch the Picoscope settings on the toolbar to DC.
(2) Remove the sinewave input of the ADDER which is G gain input. Therefore, CH1A (ADDER output) will
only consist of the VARIABLE DC input. Turn the front panel control on the VARIABLE DC almost fully
anti-clockwise. (exact value is not critical) This will provide a DC voltage of about -2 V. The ADDER will
reverse its polarity. Look at CH1A and adjust g gain of the ADDER to get 1 V DC on CH1A. (Picoscope
settings should be DC otherwise you will not see this signal)
(3) Replace the cords in G gain input of the ADDER. Adjust the G gain till you get the signal in Figure 2 (Save
the figure).
2
V
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2
1
0
-1
-2
-3
-4
-5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
18May2005 14:16
3.5
4.0
4.5
ms
5.0
Figure 2: a(t)
The waveform in Figure 2 is the message signal with an offset DC value of 1 V and peak to peak 2 V
sinewave with a frequency of 1 kHz. This will be applied to the MULTIPLIER X input and the carrier
signal is coming from MASTER SIGNALS and it is peak to peak 4 V and 100 kHz sinewave. The carrier
signal is applied to Y input of the MULTIPLIER to get AM signal as seen in Figure 3. (Save the figure).
V
5
V
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2
1
1
0
0
-1
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18May2005 14:22
3.5
4.0
4.5
-5
5.0 ms
Figure 3: AM(t)
(4) Calculate ‘m’ (depth of modulation) using the equation below.
Figure 4
4) Change the G gain of the ADDER which directly changes the depth of modulation to get the signals shown
in Figure 5. (Save these figures).
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Figure 5
5) For sinusoids it is easy to determine the depth of the modulation using the modulation trapezoid (see pg.
A1-57). Open a new XY scope from the Picosope View menu and adjust the time. You should get the
modulation trapezoid seen in Fig. 6 for m=1. Save your display.
6) Change the adder gain G to get m = 0.5, and you should see a trapezoid shown in Fig. 7. Save your
display.
7) For a little more challenge, now you will use a speech signal as the message and try to determine the depth
of modulation. You can only estimate m in this case. Using the Speech Module, record some speech, and
replay it as the message source instead of the sine wave. Adjust the modulation depth using the adder gain
G, and try to get trapezoids similar to Fig. 6 (for m = 1) and Fig. 7 (for m = 0.5) shown below. Save your
displays.
V
V
18May2005 14:41
Figure 6. Modulation trapezoid for m = 1.
4
V
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V
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0.0
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1.0
1.5
2.0
2.5
3.0
18May2005 14:45
3.5
4.0
4.5
V
V
-5
5.0 ms
18May2005 14:45
Figure 7. Modulation trapezoid for m = 0.5.
8) Add the Envelope Recovery part as seen in Figure 8 to the generator part you have already built in
Figure 1. Envelope recovery is used to demodulate the AM signals.
Figure 8
At CH2B you should recover the message signal, m(t).
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