Experiment 2 AM Modulation and Demodulation

whose output is connected to the transmitter antenna. The AM transmitter used in this
experiment is prebuilt on a PCB. The purpose of the experiment is to observe the
main signals and operations carried on them in a system approach.
The mixer in the experimental circuit is the analog multiplier IC (MC1496).
Internally, the IC is based on the Gilbert Cell principle. Figure 2.5 shows a simplified
circuit diagram for operating the IC.
Figure 2.5 AM modulation using the MC1496 IC.
The carrier is fed to pin 10 and the message (audio) to pin 1. The Modulation
Level potentiometer adjusts the modulation index. The output is available at pins 6
and 12. The Carrier Suppress controls carrier suppression. Setting at the either end
position yields full carrier AM, while adjusting to the center yields suppressed carrier
AM.
The complete experiment circuit is in figure 2.9. It consists of:
a) Crystal oscillator. Amplifier A with its associated circuit is used to generate
the carrier, 1MHz (TP4), with the output taken from C6. This is fed to the
carrier input at pin 10 of the analog multiplier IC.
b) Audio generator and amplifier. The audio sine wave signal is generated from
an 8038 waveform generator IC. This signal is fed to a pot to provide control
for the amplitude, i.e. the modulation index, before being amplified by the
amplifier B. The final audio output (TP2) is fed to pins 1 & 4 of the
modulator/multiplier.
c) The balance control, R13 potentiometer, is used to cancel the carrier out.
d) The amplifier C is used to amplify the audio signal for generating the
trapezoidal pattern. This is needed for some oscilloscopes.
e) The output of the mixer IC is taken between pins 12 and 6 into a parallel RLC
tank tuned to 1MHz. Then it is fed to transistor amplifier Q1, which drives the
antenna.
The output of this circuit is in the commercial AM band and can be received
by common AM radio receivers.
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