Figure 3-3. RF carrier-to-noise threshold.
7. Noise operates on an angle-modulated signal in such a way that only some of the noise energy angle-
modulates the carrier and only the angle-modulated portion of the noise is reduced by the feedback. For
moderately high input angle-modulation indexes, requiring only a moderate amount of loop-feedback gain,
networks can be designed which will ignore the amplitude-modulated portions of the noise. As the modulation
index of the input signal is increased, the amplitude portion of the noise modulation causes the threshold to rise
to some intermediate value between the 6-dB lower angle-modulation limit and the high threshold it would have
had without feedback.
8. An example of the action of FMFB can be seen in Figure 3-4, page 3-18. Let A indicate the normal amount
of carrier deviation that would occur in the IF signal if FMFB were not used. Noise would cause unwanted
carrier deviations as shown. Introduction of FMFB will minimize carrier deviation due to noise as shown in
waveform B.
3-17
SS0031
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