RECEIVER CIRCUITS
Section I. RECEIVER CONTROL CIRCUITS
3-24.
FREQUENCY-MODULATION FEEDBACK
a. Angle modulation is a technique used to achieve improvement factors greater than 1. This means that
at the receiver's input the demodulated or baseband signal-to-noise ratio is better (higher) than the carrier-to-noise
ratio. For ordinary demodulation (without feedback) of an angle-modulated signal, the carrier-to-noise ratio at the
input must be greater than the moderately high carrier-to-noise thresholds for the improvement ratio to be
realized. A communication link must achieve a satisfactory baseband signal-to-noise ratio quality at the lowest
possible carrier-to-noise power. The relatively high threshold of carrier-to-noise ratio that an angle-modulated
signal must exceed for satisfactory demodulation of the signal with conventional FM detectors is therefore an
obvious disadvantage. However, in recent years two demodulation techniques have been developed that can
demodulate satisfactorily down to considerably lower threshold levels, and that retain the improvement factor of
angle modulation at signal levels above the threshold. One demodulation technique utilizes frequency-
modulation feedback (FMFB). The FMFB circuit is also called a threshold extension circuit and a signal
enhancer. The second technique employs a phase-lock-loop (PL) detector. The FMFB and phase-lock-loop
circuitry differ considerably, but the performance is essentially the same for both circuits.
b. The received signals are at relatively low power levels. Therefore, the amount of noise that
accompanies the received signals is of prime importance. This noise may be reduced by narrowing the receiver's
bandpass, but this method would also introduce distortion should the deviation of the angle-modulated signal
exceed the bandpass of the receiver. A more acceptable method is to degenerate the signal automatically when
the deviation exceeds certain limits. In effect the bandpass appears to be narrower to the higher frequency (more
troublesome) noise signals. Both frequency and phase modulation have a carrier that deviates (higher and lower,
or ahead and behind, in phase) with the complex wave shape of the baseband signal but, practically, it also
contains noise. The greater the deviation, the greater the bandwidth occupied by the spectrum of the modulated
carrier. A functional diagram of an FMFB circuit is shown in figure 110.
c. Assume for the moment that switch S in figure 110 is in the "open-loop" position and
that a large deviation frequency modulated wave with modulation index MI* is applied to the
input terminal of the mixer at point A.
At the same time an identical FM wave, but with a
slightly reduced deviation (modulation index M2), is applied to the other terminal of the
mixer at point B.
The mixer output will be the sum and difference frequencies of the two
*Modulation Index is the ratio between the maximum frequency deviation and the maximum frequency of the
modulating signal.
344 L3
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