Q.Explain the operation of Quadrature FM detector.

A quadrature FM detector recovers audio from an FM signal by converting the frequency changes into phase changes. The signal is passed through a network that gives a 90 degree phase shift at the centre frequency; a phase detector then multiplies the shifted and unshifted signals, and its average output is proportional to the frequency deviation. …

[!TLDR]

A quadrature detector converts FM frequency variations into phase variations using a 90-degree network and a phase detector, whose output follows the deviation and hence the audio.

Principle: In FM the information is contained in the variation of the instantaneous frequency. A quadrature detector recovers this by first converting the frequency variation into a phase variation and then detecting that phase.

Operation:

1. The incoming FM signal (at the IF, 10.7 MHz) is split into two paths. One path is the direct signal.
2. The second path is passed through a quadrature network, consisting of a small (high-reactance) capacitor in series with a parallel LC tank tuned to the centre (rest) frequency. This network is designed so that at the centre frequency the signal it delivers is shifted in phase by exactly 90 degrees relative to the direct signal - hence the name quadrature.
3. Because the phase shift produced by a tuned circuit depends on how far the frequency is from resonance, when the instantaneous frequency deviates above or below the centre frequency the phase shift becomes more or less than 90 degrees, in proportion to the deviation.
4. The direct signal and the quadrature signal are applied to a phase detector (product detector / multiplier). The average value of the product of two sinusoids depends on the cosine of the phase angle between them; near 90 degrees this varies almost linearly with the phase change. …