Pulse amplitude modulation may be defined as that type of modulation in which the amplitudes of regularly spaced rectangular pulses vary according to instantaneous value of the modulating or message signal . In fact the pulses in a PAM signal may be of flat top type or natural type or ideal type. Actually all the sampling methods which have been discussed in last sections are basically pulse amplitude modulation methods. Out of these three pulse amplitude modulation methods the flat top PAM is most popular and is widely used.
The reason for using flat top PAM is that during the transmission the noise interferes with the top of the transmitted pulses and this noise can be easily removed if the PAM pulse has flat top. However , in case of natural samples PAM signal the pulse has varying top in accordance with the signal variation. Now when such type of pulse is received at the receiver it is always contaminated by noise. Then it becomes quite difficult to determine the shape of the top of the pulse ans thus amplitude detection of the pulse is not exact. Due to this errors are introduced in the received signal. Therefore, flat top sampled PAM is widely used signal.
Working Principle:-
A sample and hold circuit shown in fig. is used to produce flat top sampled PAM . The working principle of this circuit is quite easy. The sample and Hold circuit consists of two field
effect transistors (FET) switches and a capacitor. The sampling switch is closed for a short duration by a short pulse applied to the gate G1 of the transistor . During this period the capacitor C is quickly charged up to a voltage equal to the instantaneous sample value of the incoming signal x(t) . Now the sampling switch is opened and the capacitor C holds the charge. The discharge switch is then closed by a pulse applied to gate G2 of the other transistor . Due to this the capacitor C is discharged to zero volts. The discharges switch is then opened and thus capacitor has no voltage.
Hence the output of the sample and hold circuit consists of a sequence of flat top samples .
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