The diagram shows a Phase-Locked Loop (PLL) based PM demodulator. Here's how each component functions together to retrieve the original message signal:
Input signal \( a(t) \): This is the received PM signal \( S(t) \), typically in the form:
\( S(t) = A_c \cos\left[ 2\pi f_c t + K_p m(t) \right] \)
PD (Phase Detector): Compares the phase of the received PM signal with the phase of the signal from the VCO (Voltage Controlled Oscillator). Outputs a voltage proportional to the phase difference, which directly relates to the modulating signal \( m(t) \). The primary function of the PLL in this context is to continuously track the instantaneous phase variations of the input signal.
F(s): The loop filter smooths the phase detector output, improving the dynamic response and reducing high-frequency noise. For PM demodulation, a high-pass or differentiating filter may not be needed, unlike in FM, because the phase detector directly provides the demodulated signal.
VCO (Voltage Controlled Oscillator): Adjusts its output phase to lock onto the phase of the input signal. It generates a feedback signal used by the PD to keep the loop in lock. The VCO's instantaneous output phase precisely follows the instantaneous phase of the input PM signal when the loop is locked.
Demodulated PM Output \( \sim m(t) \): Since PM involves direct phase variation with \( m(t) \), the output of the PD (after loop stabilization) gives a signal proportional to the original message signal \( m(t) \). Crucially, unlike FM demodulation where a differentiator might be needed at the output, here the phase detector's output directly represents the modulating signal.