29 – Jan – 2019 Experiment No – 4
AM Demodulation with Automatic Gain Control
AIM
To demodulate the message content from AM signal. Also detect the automatic gain
control signal from the received AM signal.
THEORY
A simple AM demodulator is a diode envelope detector. It can be implemented by a
simple diode envelope detector to eliminate the negative half of the carrier envelope
followed by a simple RC filter to remove the high frequency carrier. The result will be the
low frequency envelope which is the demodulated message. A point contact diode with low
junction capacitance is used in the circuit as it has to rectify high frequency carrier. It offers
low impedance at high frequency. The RC elements connected after the diode acts as a
filter. It acts as a low pass filter which eliminates high frequency carrier at the same time it
retains the low frequency message signal. Thus, the output of the filter contains the low
frequency modulating signal with a dc offset. The dc offset voltage is proportional to the
strength of the modulated signal received by the receiver in a transmission reception
system, which in turn is proportional to the strength (amplitude) of the carrier. This dc value
may be used for automatic gain control (AGC) of intermediate frequency (IF) amplifier
stages. The Automatic gain control compensates for minor variations in the received RF
signal level. The AGC circuit automatically increases the receiver gain for weak RF input
levels and automatically decreases the receiver gain when strong RF signal is received.
Simple AGC - It is implemented in the form of a circuit which extracts the dc offset voltage
which is present along with the demodulated message. This voltage is fed as degenerative
or negative feedback to the control the gain of super heterodyne receivers.
Delayed AGC - In simple AGC circuits even if the signal level received is low, the AGC circuit
operates and the overall gain of the receiver gets reduced. To avoid this situation, a delayed
AGC circuit is used. In this case AGC bias voltage is not applied to amplifiers, until signal
strength has reached a predetermined level after which AGC bias is applied like simple AGC.
DESIGN
After the positive envelope detector, a properly designed low pass filter is added to
filter out the high frequency carrier and to retain the low frequency modulating signal. This
signal contains a dc level also which can be used for automatic Gain Control (AGC) for the IF
amplifier stages of a superheterodyne receiver.
Let the carrier frequency be fc = 455 kHz and maximum modulating signal frequency
be fm = 10 kHz. In order to design a low pass filter with upper cut-off frequency 10 kHz,
f1 = 1/2πR1C1
10 kHz = 1/2πR1C1
Select C1 = 0.001µF.
Then R1 = 16.1kΩ. Choose R1= 15kΩ or 22kΩ standard resistor values.
Make a π filter (for better performance) using these R1 and C1 values. This completes the
envelope detector part.
AGC Circuit - The AGC low, pass filter Ra and Ca is selected in such a way as to eliminate full
ac from the output and get a pure dc AGC voltage. Hence assuming a cut-off frequency of 10
Hz to eliminate the fluctuations
10Hz = 1/2πR2C2
Assuming C2 = 1µF, we get R2= 22kΩ
The actual modulating signal can be obtained by filtering out the dc components using a
high value capacitance like 10µF.
CIRCUIT DIAGRAM
AM Demodulator with AGC
Delayed AGC
The waveforms taken at different points and different amplitude values. The
first three images correspond to the carrier amplitudes
of 3Vp-p, 4Vp-p and 7Vp-p respectively. The final image
shows the clipped waveform taken after the diode.
OBSERVATION
AGC
Carrier Amplitude (V) Message Amplitude (V) AGC (mV)
2 2.16 123
3 3.36 208
4 4.72 302
5 6.00 397
6 7.36 498
7 8.64 595
8 10.10 688
9 11.40 790
10 12.80 891
DELAYED AGC
Carrier amplitude (in V) Delayed AGC
(in mV)
0.5 1.3
1 1.5
2 4.5
3 23
4 23
5 23
6 24
10 87
11 167
12 225
15 228
16 225
INFERENCE
The average or peak output signal level is used to dynamically adjust the gain of the
amplifiers, enabling the circuit to work satisfactorily with a greater range of input signal levels.
Without AGC the sound emitted from an AM radio receiver would vary to an extreme extent from a
weak to a strong signal; the AGC effectively reduces the volume if the signal is strong and raises it
when it is weaker.
RESULT
The demodulation circuit was designed and implemented with simple and delayed
AGC circuits.
