Phase Locked Loop (PLL)

The process to extract the message signal from a frequency modulated wave is known as frequency demodulation. As the information in an FM wave is contained in its instantaneous frequency, the frequency demodulator has the task of changing frequency variations to amplitude variations. Frequency demodulation method is generally categorized into two types: direct method and indirect method. Under direct method category, we will discuss about limiter discriminator method and under indirect method, phase-locked loop (PLL) will be discussed.

Phase-Locked Loop (PLL) as FM Demodulator

A PLL consists of a multiplier, a loop filter, and a VCO connected together to form a feedback loop as shown in Fig. 5.15. Let the input signal be an FM wave as defined by 

We observe that the output of the loop filter with frequency response H(ƒ) is the desired
message signal. Hence the bandwidth of H(ƒ) should be the same as the bandwidth W of the message
signal. Consequently, the noise at the output of the loop filter is also limited to the bandwidth W. On
the other hand, the output from the VCO is a wideband FM signal with an instantaneous frequency that
follows the instantaneous frequency of the received FM signal.