31-08-2016, 11:48 AM
1451714276-AnalogDigitalCommunicationsLabManual.pdf (Size: 1.69 MB / Downloads: 7)
Aim: 1. To generate amplitude modulated wave and determine the percentage
modulation.
2. To Demodulate the modulated wave using envelope detector.
Apparatus Required:
1. Amplitude Modulation and Demodulation Trainer
2. Function Generator
3. Oscilloscope
4. Connecting Wires
Theory:
Modulation is defined as the process by which some characteristics of a carrier
signal is varied in accordance with a modulating signal. The base band signal is referred
to as the modulating signal and the output of the modulation process is called as the
modulation signal.
Amplitude modulation is defined as the process in which is the amplitude of the
carrier wave is varied about a means values linearly with the base band signal. The
envelope of the modulating wave has the same shape as the base band signal provided the
following two requirements are satisfied
(1). the carrier frequency fc must be much greater then the highest frequency components
fm of the message signal m (t)
i.e. fc >> fm
(2) The modulation index must be less than unity. if the modulation index is greater than
unity, the carrier wave becomes over modulated
Procedure:
1 Switch on the trainer and check the O/P of carrier generator on oscilloscope.
2. Connect 1KHz with 2 Volts A.F signal at AF I/P to the modulator circuit.
3. Connect the carrier signal at carrier I/P of modulator circuit.
8 | P a g e
P.R.Ratna Raju.K, ECE, MITS Page 8
4. Observe the modulator output signal at AM O/P Spring by making necessary changes
in A.F. signal.
5. Vary the modulating frequency and amplitude and observe the effects on the
modulated waveform.
6. The depth of modulation can be varied using the variable knob (potentiometer)
provided at A.F. input.
7. The percentage of modulation or modulation factor can be calculated using the
following formulas.
8. Find the value of R from fm=1/(2*Pi*R*C) , C=0.1µF
9. Connect the circuit diagram as shown in Fig.2.
10. Feed the AM wave to the demodulator circuit and observe the output
11. Note down frequency and amplitude of the demodulated output waveform.
12. Draw the demodulated wave form., m=1