FM Detector Circuits: Discriminator, Quadrature and PLL

When you listen to a repeater on 2 metres, a circuit in your handheld turns tiny changes in the frequency of a carrier into the sound of a human voice. That circuit is the FM detector, and it works on a principle that is quite unlike the detector of an AM or SSB receiver. Instead of following the strength of the signal, it follows how fast the signal’s frequency is moving. The details differ widely, from the old Foster-Seeley discriminator with its centre-tapped transformer to the one-chip quadrature detector in a modern handheld and the numerical demodulator in a software defined radio, yet the aim is always the same: voltage out proportional to frequency deviation in. This guide takes the common types in turn and explains why each one exists, how they cope with noise and where you meet them in amateur equipment.

What an FM Detector Must Do

In frequency modulation the information sits in the instantaneous frequency of the carrier. A voice peak makes the carrier move, for example, 3 kilohertz above its nominal frequency, and a voice trough makes it move 3 kilohertz below it, as described in the guide to the CTCSS tones and the deviation used on repeaters. The amplitude of the carrier stays constant, or should. The detector therefore has to perform a conversion from frequency to voltage, ideally linear over the whole deviation range, and ideally unaffected by any change of amplitude caused by noise, fading or interference.

Two other duties follow. The detector sits after the intermediate frequency filter, so it must work at the IF, commonly 455 kilohertz, 10.7 megahertz or a few tens of megahertz in a double conversion receiver. And its output must be cleaned up by a de-emphasis network and, in a handheld or a mobile radio, passed through a squelch circuit that listens to the noise above the voice band to decide whether a signal is present at all.

The Limiter: The Quiet Hero

Before the detector comes a limiter, a stage or several stages of strongly saturating amplifiers. Its job is to remove every variation of amplitude from the signal, so that only the frequency information remains. This is where FM gets its famous resistance to impulse noise and to fading: the noise that adds to the amplitude is clipped away, and what remains of it is a smaller disturbance in the phase. A limiter also produces the capture effect, in which a stronger signal suppresses a weaker one on the same channel almost completely, instead of the two being heard together as in AM.

The price is that an FM receiver is silent about the signal strength unless extra circuitry measures the level before the limiter. The S-meter of an FM radio is typically driven from the gain-controlled IF amplifiers or from a special logarithmic detector. Another consequence is the threshold effect: when the signal falls to a level near the noise, the limiter starts to follow the noise and the audio becomes a loud, harsh hiss, not a gradual fading as in SSB. Weak signal work on VHF therefore tends to use SSB, CW or digital modes, as described in the article on the 2 metre band.

Slope Detection: The Simplest Trick

If you tune an AM receiver to the side of an FM signal, so that the carrier falls on the slope of the IF filter, the amplitude at the output rises and falls with the frequency, and the AM detector produces audio. This is slope detection. It is how many hams first heard FM on an old communications receiver, and it works for broadcast, but it has a high distortion, a poor use of the signal and no limiting, so it responds to noise and to amplitude changes. It is the idea behind the better circuits, which use a tuned circuit with a response that changes with frequency to turn the frequency swings into amplitude swings, and then an ordinary diode detector.

The Foster-Seeley and Ratio Detectors

The Foster-Seeley discriminator uses a transformer with a centre tap, two diodes and a phase relationship between the primary and secondary voltages. At the centre frequency, the two diode voltages are equal and the output is zero. When the frequency moves up, the phase shift of the secondary changes, one diode receives a larger voltage and the other a smaller one, and the output swings positive. A move down makes it swing negative. The characteristic curve has the shape of the letter S, with a nearly straight middle section that is the useful working range. The width between the peaks of the S curve sets the maximum deviation that can be handled without distortion.

The Foster-Seeley circuit responds to amplitude changes as well as to frequency, so it needs a good limiter in front of it. The ratio detector is a variation in which one diode is reversed and a large capacitor is connected across the output. The capacitor holds the sum voltage constant and the circuit measures the ratio of the two diode voltages, which gives it some natural immunity against amplitude changes. The ratio detector was popular in valve and early transistor FM radios and televisions because it saved a limiter stage. Both types are rare in new designs, but they are still found in older equipment and in kits, and they still work well when properly aligned.

The Quadrature Detector

The quadrature detector, also called the coincidence detector, is the one inside most modern FM chips, such as the ones that have been used for years in scanners, handhelds and the receivers of cheap FM transceivers. The limited signal is fed to a phase detector in two ways: directly, and through a phase-shifting network that delays it by 90 degrees at the centre frequency. This network is usually a small capacitor and a tuned circuit with a single coil. When the frequency moves, the phase shift of the network changes in proportion, and the phase detector, which is really a mixer of the type described in the article on the double balanced mixer, produces an output voltage proportional to the shift. That voltage is the audio.

The advantage of the quadrature detector is that a single coil sets the centre, and a screwdriver adjustment to zero the DC output is all the alignment required. It copes well with the wide deviation of broadcast FM and with the narrower deviation used in amateur narrowband FM. The coil’s loaded Q sets the linearity and the sensitivity: a low Q gives a wide, linear range with less output, and a high Q gives a larger output over a narrower range and more distortion on strong modulation. This is why a radio built for 12.5 kilohertz channels sounds distorted, and the audio is quiet, when it is used with the wider deviation of a 25 kilohertz system.

The PLL Demodulator

A phase-locked loop contains a voltage-controlled oscillator that is steered by a phase detector so that it follows the incoming signal in frequency. When the incoming frequency moves, the control voltage that keeps the oscillator in step must move too, and that control voltage is a copy of the modulation. The loop therefore acts as an FM demodulator with excellent linearity and a good threshold, because it follows the signal as a narrow tracking filter and does not respond to noise outside its loop bandwidth. The phase-locked loop is the same circuit that creates the frequency of the synthesizer in your transceiver, which makes it a pleasant surprise that the same building block can also recover the audio.

The PLL has its limits. The loop must be fast enough to follow the highest audio frequency at the full deviation, and slow enough to reject noise, and the two requirements are in conflict. The capture range, the range of frequencies from which the loop acquires lock, must also cover the drift of the incoming signal. PLL detectors are common in integrated circuits for FM broadcast and in some amateur receivers for weak-signal FM, and the same idea has moved into software where an algorithm tracks the phase.

Digital Demodulation in SDR and DSP Radios

In a software defined radio or a DSP receiver the signal is sampled as a pair of numbers, I and Q, that describe the phase of the carrier at each moment. FM demodulation then becomes a calculation: find the angle of each sample and subtract the angle of the previous one. The difference is the frequency, and it is the audio. There is no tuned circuit to align, no centre to adjust and no analog distortion, which is why the FM in an SDR can be so clean. The receivers described in the article on direct sampling and superheterodyne radios use this approach. The same calculation, with different filters, serves for wide FM, narrow FM and the packet and data modes that ride on FM.

The squelch, the de-emphasis and the CTCSS decoder are then separate pieces of the program, and they can be improved with a software update. This is part of the reason why modern hand-held radios often sound better than older ones with a similar front end.

Alignment and Troubleshooting

With a discriminator or a quadrature detector the standard alignment is to inject an unmodulated signal at the intermediate frequency and adjust the coil for zero DC output, using a voltmeter. Then check by changing the signal slightly in both directions, and the voltage should go positive and negative symmetrically. If the audio is distorted on strong signals and clean on weak ones, the coil is not centred or the limiter is not saturating. If the audio is quiet and noisy, the IF filter may be too narrow or off frequency, as discussed in the article on receive filter bandwidth.

A common fault in older sets is a weak or missing audio together with a loud noise between stations. Check that the limiter stages are alive and that the detector diodes are not leaky. In a ratio detector the large capacitor can dry out and cause a buzz. In a modern IC-based receiver, the quadrature coil is often the only adjustable part, and a drifted or replaced coil is a suspect when the sound is thin.

The Bottom Line

An FM detector converts frequency swings into audio, and it is preceded by a limiter that removes amplitude variations and gives FM its immunity against impulse noise and its capture effect. Slope detection is the simplest idea, the Foster-Seeley and ratio discriminators use a centre-tapped transformer and two diodes, the quadrature detector multiplies the signal with a 90 degree delayed copy, the PLL demodulator uses the control voltage of a tracking oscillator, and digital radios compute the phase difference between samples. Each has its place, from vintage receivers to an SDR, and the common requirements are a correctly centred response, enough limiting and a bandwidth that matches the deviation of the signal.

Frequently Asked Questions

What does an FM detector do?

It converts changes in the frequency of the received carrier into a voltage that follows the original audio, while ignoring changes of amplitude.

Why is a limiter needed before the detector?

It removes amplitude variations caused by noise and fading, so that only the frequency information reaches the detector. It also produces the capture effect.

What is the difference between a Foster-Seeley and a ratio detector?

Both use a centre-tapped transformer and two diodes. The ratio detector has one diode reversed and a large capacitor, so it is less sensitive to amplitude changes and can work with less limiting.

Why does narrowband FM sound quiet on a wideband radio?

The deviation of the signal is smaller than the radio expects, so the detector produces a lower audio voltage for the same modulation.

Is a PLL detector better than a quadrature detector?

It can be more linear and have a better threshold on weak signals, but it is more complex. A quadrature detector is simple, cheap and good enough for most voice work.

How is FM demodulated in an SDR?

The software computes the phase angle of each I and Q sample and takes the difference from the previous sample. That difference is the instantaneous frequency, which is the audio.