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Application of PLL : Frequency Demodulation

08:25


FM demodulation or detection can be directly achieved using the PLL circuit. If
the PLL center frequency is selected or designed at the FM carrier frequency, the filtered or output voltage of the circuit  is the desired demodulated voltage, varying in value proportional to the variation of the signal frequency. The PLL circuit thus operates as a complete intermediate-frequency (IF) strip, limiter, and demodulator as used in FM receivers.




One popular PLL ,  The 565 contains a phase detector, amplifier, and voltage-controlled oscillator, which are only partially connected internally. An external resistor and capacitor, R1 and C1, are used to set the free-running or center frequency of the VCO. Another external capacitor, C2, is used to set the low-pass filter passband, and the VCO output must be connected back as input to the phase detector to close the PLL loop. The 565 typically uses two power supplies, V_ and V.
 
The signal at pin 4 is a 136.36-kHz square wave. An input within the lock range of
181.8 kHz will result in the output at pin 7 varying around its dc voltage level set
with input signal at fo.  the output at pin 7 as a function of the input signal frequency. The dc voltage at pin 7 is linearly related to the input signal
frequency within the frequency range fL _ 181.8 kHz around the center frequency
136.36 kHz. The output voltage is the demodulated signal that varies with frequency  within the operating range specified.

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