- In an FM signal, the instantaneous frequency varies in accordance with the modulating signal. For a sinusoidal modulating signal, the frequency deviation in an FM signal is sinusoidal, and it is proportional to the modulating, amplitude. Recall that the changes in the instantaneous frequency of the carrier signal occur with respect to the previously attained value of the carrier frequency. This clearly suggests that a PLL can be used to demodulate an FM signal.
- Suppose the center frequency of the FM signal is fc, and it lies within the hold-in range of PLL the VCO is locked to fc, by applying an demodulated carrier at the input of the phase detector. When VCO is locked to fc, the error signal is zero, and therefore, the control signal that changes the VCO frequency is also equal to zero. If an FM signal is applied to the phase detector, there will be a difference in the phases of the VCO output and the input FM signal. The control signal is produced in proportion to the phase difference at an instance of time. This control voltage will modify the VCO frequency, which is again compared with the incoming frequency. In this way, the current incoming frequency is compared with the previously attained value of the VCO frequency, which is the previously attained frequency of the FM Signal.
- The VCO, therefore, tries to track the instantaneous frequency of the applied FM signal. The control signal is produced in proportion to the difference between the VCO frequency and the instantaneous frequency of FM signal. In other words, the control signal so produced is proportional to the frequency deviation in the FM signal. Since the frequency deviation si proportional to the modulating signal, the control signal appearing at the output of LPF is the modulating signal. Therefore, the FM signal is demodulated by PLL.
- A PLL is also available as an integrated circuit IC. IC 565 PLL can be used for FM detection.
- Figure (a) shows the circuit diagram of an FM detector using 565 PLL.
- Figure (a) shows the circuit external to the IC 565 PLL for FM detection. The circuit shown in this figure is a general circuit. The choice of the timing components, resistor R0 and capacitor C0 decides the various parameters and the free-running frequency of PLL. Accordingly, the values of other components are also chosen.
- In Figure (a), only a few components are externally connected to IC. The power supply, Vcc, is connected between pin numbers 1 and 10, with +Vcc applied at pin number 10. The timing resistor Ro is connected to pin number 8, and the timing capacitor, Co is connected to pin number 9. The VCO output, which is available at pin number 4 is applied to phase-detector input at pin number 5. Pin number 4 is shorted with pin number 5 as no external component is required in this case.
- The input FM signal, Vi, is applied to pin number 2 through the coupling capacitor Cc. A part of this signal is also applied to pin number 3 through the potential divider network, consisting of R2, R3, and R4. The dc power supply is also provided to the input pins 2 and 3 through R1 from +Vcc supply. The capacitor C2 is used to filter out an AC ripple, if present in the DC supply.
- The demodulated FM signal is nothing but the control signal, which is available at pin number 7. Therefore, the signal available at pin number 7 is the required modulating signal.
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