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 signal-to-noise ratio. The noise content of any demodulated output
signal is related directly to the bandwidth of the input signal.
c. The efficiency of any demodulator is measured by its ability to produce the highest quality output
signal-to-noise ratio with the least possible input carrier-to-noise ratio. The improvement factor (Fm) has been
developed as an indication of the efficiency of any demodulator.
d. The improvement factor (Fm) is defined as the baseband signal-to-noise ratio (S/N) output divided by
the carrier-to-noise (C/N) input.
It has been pointed out that achieving the desired S/N output quality with the highest possible improvement factor
is extremely desirable. The maximum possible improvement factor for amplitude modulation is 1. This is
realized with single sideband (SSB). The largest improvement factor for double sideband is one-half. For angle
modulation the improvement factor can be much larger than 1, as shown in figure 117. For comparison, the
broken line shows the S/N output to be exactly equal to the C/N input for SSB. The frequency-modulation
improvement factor (in db) is the S/N difference between the frequency-modulation line and the SSB line. It is
seen from figure 117 that the larger the modulation index, the greater the improvement factor for frequency
modulation. However, frequency modulation has a carrier-to-noise threshold which must be exceeded for the full
modulation improvement factor to be in effect. For example, again from figure 117, a modulation index (M) of 2
has an improvement factor of 6 (8 db) but the C/N ratio must exceed an 18-db threshold. When M is 10, the
improvement factor is 150 (22 db) and the threshold is 28 db. For carrier-to-noise ratios greater than the
threshold, the improvement factor is
As the carrier-to-noise level decreases below threshold, in addition to the rapid degradation of the improvement
factor, 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
117.
4-17.
BASEBAND AND BANDWIDTH CONTROLS
a. The transmitter in a typical microwave station is used to convert the baseband input from the terminal
equipment into an angle-modulated signal whose carrier frequency is in the range of 50 to 90 MHz. The receiver
is used to convert the angle-modulated signal back to the baseband signal prior to applying the signal to the
terminal equipment.
344 L4
148
Previous Page
