to-noise thresholds for the improvement ratio to be realized. A communications link must achieve a satisfactory
baseband S/N quality at the lowest possible C/N 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 (PLL) detector. The FMFB and PLL
circuitry differ considerably, but the performance is essentially the same for both circuits.
2. 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 limit. 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 3-2.
Figure 3-2. FMFB block diagram.
3. Assume for the moment that switch S in Figure 3-2 is in the "open-loop" position and that a large deviation
FM wave with modulation index M11 is applied to the input terminal of the mixer at point A. At the same time
an identical FM wave, but with a
1
Modulation Index is the ratio between the maximum frequency deviation and the maximum frequency of the
modulation signal.
3-15
SS0031
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