The diagram illustrates a Phase-Locked Loop (PLL) used for demodulating Frequency Modulated (FM) signals. The working of each block is described below:
Input signal: This is the received FM signal, typically denoted as s(t), which carries the frequency variations corresponding to the original message m(t).
PD (Phase Detector): Compares the phase of the input signal with that of the feedback signal generated by the VCO. The output is a signal proportional to the phase difference or error.
F(s) (Loop Filter): Processes the phase error signal from the PD. It smooths the signal to eliminate high-frequency components and to ensure loop stability. This filter also helps determine the **capture range** (the frequency range over which the PLL can acquire lock) and the **lock range** (the frequency range over which the PLL can maintain lock).
VCO (Voltage Controlled Oscillator): Generates a signal whose frequency is controlled by the filtered phase error. The VCO attempts to match the instantaneous frequency of the input FM signal. When the PLL is operating correctly, the VCO's output frequency and phase closely track the input signal's frequency and phase variations – this is known as the **locked state**.
Demodulated FM Output (∼ dĪøi(t)/dt): The output of the loop (or after taking the derivative of the instantaneous phase) approximates the original message signal m(t). In a locked PLL, the voltage controlling the VCO is directly proportional to the instantaneous frequency deviation of the input FM signal, thus providing the demodulated output.