7.4 AN ASTABLE MULTIVIBRATOR AS A FREQUENCY MODULATOR
In communication systems, a modulator is used to transmit information from one end to the other. A modulator is a circuit that changes one of the parameters of the high-frequency signal in accordance with the instantaneous amplitude of the low-frequency signal—that is intelligence. If the frequency of the high-frequency signal, called the carrier, is varied in accordance with the instantaneous amplitude of the low-frequency signal, the process is called frequency modulation.
If the voltage VBB varies in accordance with the amplitude of the modulating signal (say, a sine wave), the frequency of the astable multivibrator varies in accordance with the instantaneous amplitude of the sinusoidal signal as shown in Fig. 7.14. Hence, the circuit may be called a frequency modulator [see Fig. 7.15]. This means that the frequency of the astable multivibrator will vary as per the instantaneous amplitude vs.
FIGURE 7.14 The instantaneous total voltage (vBB) changes as per the sinusoidal signal vs
FIGURE 7.15 An astable multivibrator as a frequency modulator