Demodulation of PAM Signals
For pulse amplitude modulated (PAM) signals, the demodulation is done using a Holding circuit. Fig.1 shows the block diagram of a PAM demodulator.
Fig.1 : Block diagram of PAM demodulator
In this method, the received PAM signal is allowed to pass through a Holding circuit and a low pass filter (LPF) as shown in fig.1.
Now, fig.2 illustrates a very simple holding circuit.
Fig.2 : A zero-order holding circuit
Here the switch ‘S’ is closed after the arrival of the pulse and it is opened at the end of the pulse. In this way, the capacitor C is charged to the pulse amplitude value and it holds this value during the interval between the two pulses.
Hence, the sampled values are held as shown in fig.3.
Fig.3 : the output of a Low Pass filter (LPF)
After this the holding circuit output is smoothened in Low Pass filter as shown in fig.3.
It may be observed that some kind of distortion is introduced due to the holding circuit. In fact the circuit of fig.4 is known as zero-order Holding circuit. This zero-order Holding circuit considers only the previous sample to decide the value between the two pulses.
Fig.4 : Output of holding circuit