Quadrature FM Demodulator / Detector Tutorial

- details of the quadrature FM demodulator or detector used in many integrated circuits for demodulating frequency modulated signals.

Another form of FM demodulator that is widely used is called the quadrature detector.

This form of FM detector only requires the use of one coil and a handful of external components, making it ideally suited for use within integrated circuits.

In addition to its convenience of use, the circuit offers a high level of performance with excellent linearity.

FM quadrature demodulator basics

The basic format of the quadrature detector is shown below.

It can be seen that the signal is split into two components. One of these passes through a network that provides a basic 90° phase shift, plus an element of phase shift dependent upon the deviation.

The original signal and the phase shifted signal are then passed into a multiplier or mixer.

Quadrature FM Detector / demodulator circuit
Quadrature FM demodulator circuit

The mixer output is dependent upon the phase difference between the two signals, i.e. it acts as a phase detector and produces a voltage output that is proportional to the phase difference and hence to the level of deviation on the signal.

Mixer phase response for quadrature FM demodulator
Mixer phase response for quadrature FM detector

If the operation of the system is designed to ensure that the deviation remains well away from the ±90° points, then the linearity remains very good.

In terms of performance, the quadrature detector is able to operate with relatively low input levels, typically down to levels of around 100 microvolts and it is very easy to set up requiring only the phase shift network to be tuned to the centre frequency of the expected signal. It also provides good linearity and this results in low levels of distortion.

Coincidence FM demodulator

Often the analogue multiplier is replaced by a logic AND gate and the input signal is hard limited to produce a variable frequency pulse waveform.

FM coincidence demodulator using an AND gate
FM coincidence demodulator using an AND gate

The operation of the circuit is fundamentally the same, but it is known as a coincidence detector. Also the output of the AND gate has an integrator to 'average' the output waveform to provide the required audio output, otherwise it would consist of a series of square wave pulses.

Quadrature detector advantages & disadvantages

The quadrature detector offers significant advantages for many circuits, but as with any decision, a number of different advantages and disadvantages have to be considered when selecting a given circuit for FM demodulation.

Advantages Disadvantages
  • Offers good level of performance and including linearity.
  • Can be incorporated into an integrated circuit.
  • Requires the use of a coil.
  • Some designs may require setting during manufacture.

Despite the disadvantages, the quadrature FM detector is the circuit of choice for many radio receivers these days.

By Ian Poole

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