21-05-2013, 04:01 PM
AMPLITUDE MODULATION & DEMODULATION
Aim:
1. To generate amplitude modulated wave and determine the percentage modulation.
2. To Demodulate the modulated wave using envelope detector.
Theory:
Amplitude Modulation is defined as a process in which the amplitude of the carrier wave c (t) is varied linearly with the instantaneous amplitude of the message signal m(t). The standard form of amplitude modulated (AM) wave is defined by
( ) ( ( )) ( t)
Where Ka is a constant called the amplitude sensitivity of the modulator.
The demodulation circuit is used to recover the message signal from the incoming AM wave at the receiver. An envelope detector is a simple and yet highly effective device that is well suited for the demodulation of AM wave, for which the percentage modulation is less then 100%. Ideally, an envelope detector produces an output signal that follows the envelop of the input signal wave form exactly; hence, the name. Same version of this circuit is used in almost all commercial AM radio receivers.
Procedure:
1. The circuit is connected as per the circuit diagram shown in Fig.1.
2. Switch on =12V Vcc supply
3. Apply sinusoidal signal of 1KHz frequency and amplitude 2 Vp-p as modulating signal, and carrier signal of frequency 11 KHz and amplitude 15 Vp-p.
4. Now slowly increase the amplitude of the modulating signal up to 7V and note down values of Emax and E min.
5. Calculate modulation index using equation
6. Repeat step 5 by varying frequency of the modulating signal.
7. Plot the graphs: Modulation index vs Amplitude & Frequency
8. Find the value of R from taking C=0.01μF
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.