2. The receiver's bandwidth must be wide enough to pass the modulated spectrum bandwidth to avoid
distortion in the demodulated baseband signal. This means that for single sideband, the receiver's bandwidth
need be only as wide as the baseband. For receivers using angle modulation, the receiver's bandwidth can be
many times the width of the baseband, depending on the chosen magnitude of the modulation index. The
receiver's bandwidth should not be wider than the minimum needed to pass the modulated wave. By using a
minimum bandwidth, the receiver's noise power is minimized. The bandwidth of the receiver has a greater
influence than any other receiver parameter on the receiver output S/N. The noise content of a demodulated
output signal is related directly to the bandwidth of the input signal.
3. The efficiency of any demodulator is measured by its ability to produce the highest quality output S/N with
the least possible input carrier-to-noise ratio (C/N). The improvement factor (Fm) has been developed as an
indication of the efficiency of any demodulator.
4. The Fm is defined as the baseband S/N output divided by the C/N input.
Fm = S/N (output)
C/N (input)
It has been pointed out that achieving the desired S/N output quality with the highest possible Fm is extremely
desirable. The maximum possible Fm for AM is 1. This is realized with single sideband (SSB). The largest
Fm for double sideband is one-half. For angle modulation, the Fm can be much larger than 1, as shown in
Figure 3-1. For comparison, the broken line shows the S/N output to be exactly equal to the C/N input for SSB.
The FM Fm (in dB) is the S/N difference between the FM line and the SSB line. It is seen from Figure 3-1 that
the larger the modulation index, the greater the Fm for FM.
However, FM has a carrier-to-noise threshold which must be exceeded for the full modulation Fm to be in
effect. For example, again from Figure 3-1, a modulation index (M) of 2 has an Fm of 6 (8 dB), but the C/N
must exceed an 18-dB threshold.
When M is 10, the Fm is 150 (22 dB) and the threshold is 28 dB. For C/N greater than the threshold, the Fm is-
Fm = 3M2
2
As the carrier-to-noise level decreases below threshold, in addition to the rapid degradation of the Fm, the
audible-noise character in the baseband output changes from a fine-grain "hiss" to an erratic sputter of "pops."
In practice, the threshold demarcation is gradual rather than sharply discontinuous, as shown in Figure 3-1.
SS0031
3-12
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