21-05-2013, 04:03 PM
FREQUENCY MODULATION AND DEMODULATION
Aim:
1.To generate frequency modulated signal and determine the modulation index and bandwidth for various values of amplitude and frequency of modulating signal,
2. To demodulate a Frequency Modulated signal using FM detector.
Theory:
The process, in which the frequency of the carrier is varied in accordance with the instantaneous amplitude of the; modulating signal, is called „Frequency Modulation‟. The FM signal is expressed as: ( ) ( ( )) Where Ac is amplitude the carrier signal, fc is the carrier frequency fm is the modulation index of the FM wave.
Procedure: Modulation:
1. The circuit is connected as per the circuit diagram shown in Fig.2 (Fig.1 For IC 566)
2. Without giving modulating signal observe the carrier signal at pin no.2 (at pin no.3 for IC 566). Measure amplitude and frequency of the carrier signal. To obtain carrier signal of desired frequency, find value of R from f=1/ (2πRC) taking C=100pF.
3. Apply the sinusoidal modulating signal of frequency 4 KHz and amplitude 3Vp-p at pin no. 7. (pin no. 5 for IC566)
Now slowly increase the amplitude of modulating signal and measure fmin and maximum frequency deviation Δf at each step. Evaluate the modulating index (mf=β) using Δf/fm where Δf=|fc-fmin|. Calculate Band width. BW=2(β+1) fm=2(Δf+ fm)
4. Repeat step 4 by varying frequency of the modulating signal.