T 7.2.1.3 Amplitude Modulation

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TPS 7.2.1.3
Double Sideband AM
Experiment procedure
Assemble the experiment as specified in Fig. 5-3.
Switch on the carrier. Connect the output of the
function generator directly to the modulator input.
Set the function generator to: sine, A M = 2 V and
f M = 2 kHz.
5.1 Investigations on the dynamic
characteristic of the DSB
5.1.1 DSB
Set toggle switch to CARRIER ON setting. Display
the output signal of the modulator M2 on the
oscilloscope (this signal is called the modulation
product) and the modulating signal s M (t) of the
function generator and sketch them. (Modulation
product on channel 2, modulating signal on channel
1 of the oscilloscope). Use Diagram 5.1.1-1.
GATE
1s
10s
0,1s
0,01s
TIME A-B
COUNT A
CHECK
FUNCTION
RATIO A/B
PERIOD A
FREQ A
TTL - IN(A) TTL - IN(A)
ANALOG (A)
TTL - IN(B)
Diagram 5.1.1-1: Dynamic (time) characteristic of the DSB
signal
(1): Modulating signal s M
(t)
(2): Modulation product at the output M2
Shift the AF signal to the upper or lower envelope
curve of the AM signal. Vary the frequency f M
and the amplitude A M of the modulating signal.
What do you observe?
Settings on the Oscilloscope
Input attenuator channel 1
Input attenuator channel 2
2V/DIV
2V/DIV
kHz
V pp
=
MODE
OUT
ATT
dB
20
40
TTL
Time base
Trigger
200 µs/DIV
s M (t)
DC
FUNCTION
Trigger to the modulating signal s M (t). Reduce the
A M signal to approx. 1 V. Determine the modulation
depth m. The following applies for the modulation
depth m:
∆ A D d
m = C −
=
(5.1.1-1)
A D + d
C
+15V
(+5V)
I >
U
U
Fig. 5-3: Experiment set-up for DSB
I >
M1
0V
30