PLL FM demodulator: phase locked loop FM detector

Frequency Modulation Tutorial Includes:
Frequency modulation, FM     Modulation index & deviation ratio     FM sidebands, bandwidth     FM demodulation     FM slope detector     FM ratio detector     Foster Seeley detector     PLL FM demodulator     Quadrature demodulator     MSK     GMSK    

Modulation formats:     Modulation types & techniques     Amplitude modulation     Phase modulation     Quadrature amplitude modulation    

Phase locked loop, PLL FM detectors can easily be made using a phase locked loop as there is a great the variety of phase locked loop integrated circuits.

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 technology started to be used when integrated circuits took over for many radio functions. The PLL could easily be integrated into the radio IC by simply adding a little extra circuitry to the IC.

This added very little cost and only required a few external components - normally just resistors and capacitors which are cheap.

The PLL technology eliminates the costly RF transformers needed for circuits like the ratio FM detector and the Foster Seeley circuit.

Typically a phase locked loop FM demodulator does not require the use of an inductor, let alone a transformer which is even more costly to manufacture.

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.

To look at the operation of the PLL FM demodulator take the condition where no modulation is applied and the carrier is 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.

Although no basic changes to the phase locked loop are required for it to be able to demodulate FM, a buffer amplifier is typically provided from the tune line to prevent the tune line being loaded by other sections of the receiver.

The buffer amplifier provides a lower output impedance and as a result, this prevents loading from the audio amplifier from upsetting the loop in any way.

There are many different ICs that enable FM to be demodulated. One of the most popular has been the 565 that has been around for many years in a variety of forms. Even though the circuit is quite old, it performs well, and often little will be gained by going to other chips.

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:   One of the advantages of the PLL FM demodulator is its high degree of linearity. This is governed by the voltage to frequency characteristic of the VCO within the phase locked loop.
  
  Normally the phase locked loop will be able to operate over a wide bandwidth - normally this is much wider than the bandwidth of the FM signal or even the IF stages of the FM receiver. As the frequency deviation of the incoming FM signal covers only a small portion of the PLL bandwidth the overall conversion is very linear.
  
  The VCO voltage to frequency curve is the main determining factor and this can be made to be very linear for the range needed for FM demodulation. Distortion levels for PLL FM demodulators are normally very low and are typically of the order of a tenth of a percent. This makes the PLL FM demodulator a very good option for high fidelity tuners as well as for many other applications including radio communications, etc.
• Insensitive to amplitude noise:   In general the phase locked loop FM demodulator is very insensitive to amplitude noise. As the phase locked loop will track the frequency of the incoming signal, it provides a relatively high degree of AM noise immunity. Obviously it can help if the IF amplifier of the radio is run into saturation such that the signal level is limited and noise is removed, but even on its own the PLL FM demodulator provides good noise immunity.
• Ease of incorporation into ICs:   Phase locked loops are very easy to implement in an integrated circuit. PLLs have long been available as ICs and this has meant that the technology is easy to implement.
  
  Also the PLL FM demodulator blocks are available for IC designers, and therefore many radio IF amplifier ICs have demodulators for AM and FM built in. Often the FM demodulator can be a phase locked loop demodulator.
• Manufacturing costs:   As the phase locked loop FM demodulator lends itself to integrated circuit technology, only a few external components are required to complete the FM demodulator. One particular advantage is that often no inductor is required for the VCO circuit.
  
  As inductors are relatively expensive components, this can considerably reduce overall component costs and make this approach very attractive for large scale manufacture. These facts make the PLL FM demodulator particularly attractive for modern applications.

Although phase locked loop FM demodulators have been very widely used for their many advantages, the IQ based demodulators are probably now in more common use. They are very easy to incorporate into an IC and require even fewer components.

That said, PLL base demodulators are still quite common and they perform very well.

PLL FM demodulator design considerations

When designing a phase locked loop 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.

Phase locked loop FM demodulators are used in many radio receivers both domestic and professional for the demodulation of FM signals. The PLL FM demodulator provides a very attractive option in many instances, offering exceedingly low levels of distortion, and the ability to be incorporated into integrated circuit technology.

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