PLL FM demodulator / detector

- details of the concept and circuit for the PLL FM demodulator or detector with principles of PLL FM demodulation.


Phase locked loop, PLL FM demodulator or detector is a form of FM demodulator that has gained widespread acceptance in recent years.

PLL FM detectors can easily be made from the variety of phase locked loop integrated circuits that are available, and as a result, PLL FM demodulators are found in many types of radio equipment ranging from broadcast receivers to high performance communications equipment.

The PLL FM demodulation integrated circuits started to appear when integrated circuit technology developed to the degree to allow RF analogue circuits to be manufactured.

Although high frequencies are not normally needed, for PLL FM demodulators, the circuit must be capable of operating at the intermediate frequency of the receiver, and for receivers using FM this was often 10.7 MHz. Although by today's standards, this is not high, it was necessary for the technology to reach this state before PLL FM demodulators became available.


PLL FM demodulation basics

The way in which a phase locked loop, PLL FM demodulator works is relatively straightforward. It requires no changes to the basic phase locked loop, itself, utilising the basic operation of the loop to provide the required output.


Note on Phase Locked Loops:

Phase locked loops form the basis of many RF systems. They are use the concept of minimising the difference in phase between a reference signal and a local oscillator to replicate the reference signal frequency. Using this concept it is possible to use these loops for many applications from FM demodulators to frequency synthesizers.

Read more about the Phase locked loop PLL


Phase locked loop PLL FM detector / demodulator
Phase locked loop PLL FM demodulator

When used as an FM demodulator, the basic phase locked loop can be used without any changes. With no modulation applied and the carrier in the centre position of the pass-band the voltage on the tune line to the VCO is set to the mid position. However if the carrier deviates in frequency, the loop will try to keep the loop in lock. For this to happen the VCO frequency must follow the incoming signal, and in turn for this to occur the tune line voltage must vary. Monitoring the tune line shows that the variations in voltage correspond to the modulation applied to the signal. By amplifying the variations in voltage on the tune line it is possible to generate the demodulated signal.


PLL FM demodulator performance

The PLL FM demodulator is normally considered a relatively high performance form of FM demodulator or detector. Accordingly they are used in many FM receiver applications.

The PLL FM demodulator has a number of key advantages:

  • Linearity:   The linearity of the PLL FM demodulator is governed by the voltage to frequency characteristic of the VCO within the PLL. As the frequency deviation of the incoming signal normally only swings over a small portion of the PLL bandwidth, and the characteristic of the VCO can be made relatively linear, the distortion levels from phase locked loop demodulators are normally very low. Distortion levels are typically a tenth of a percent.
  • Manufacturing costs:   The PLL FM demodulator lends itself to integrated circuit technology. Only a few external components are required, and in some instances it may not be necessary to use an inductor as part of the resonant circuit for the VCO. These facts make the PLL FM demodulator particularly attractive for modern applications.

PLL FM demodulator design considerations

When designing a PLL system for use as an FM demodulator, one of the key considerations is the loop filter. This must be chosen to be sufficiently wide that it is able to follow the anticipated variations of the frequency modulated signal. Accordingly the loop response time should be short when compared to the anticipated shortest time scale of the variations of the signal being demodulated.

A further design consideration is the linearity of the VCO. This should be designed for the voltage to frequency curve to be as linear as possible over the signal range that will be encountered, i.e. the centre frequency plus and minus the maximum deviation anticipated.

In general the PLL VCO linearity is not a major problem for average systems, but some attention may be required to ensure the linearity is sufficiently good for hi-fi systems.


Summary

The PLL FM demodulator is one of the more widely used forms of FM demodulator or detector these days. Its suitability for being combined into an integrated circuit, and the small number of external components makes PLL FM demodulation ICs an ideal candidate for many circuits these days.

By Ian Poole


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