AM Detection – A PLL can be used to demodulate AM signals as shown in the Fig. 2.133.
The PLL is locked to the carrier frequency of the incoming AM signal. Once locked the output frequency of VCO is same as the carrier frequency, but it is in unmodulated form. The modulated signal with 90° phase shift and the unmodulated carrier from output of PLL are fed to the multiplier. Since VCO output is always 90° out of phase with the incoming AM signal under the locked condition, both the signals applied to the multiplier are in same phase.
Therefore, the output of the multiplier contains both the sum and the difference signals. The low pass filter connected at the output of the multiplier rejects high frequency components gives demodulated output. As PLL follows the input frequencies with high accuracy, a PLL AM detector exhibits a high degree of selectivity and noise immunity which is not possible with conventional peak detector type AM modulators.