A CATV headend places multiple TV and FM (from 30 to 125) carriers on a broadband coaxial cable trunk and distribution system. The objective is to deliver a signal-to-noise ratio (S/N) of 42-45 dB at a subscriber's TV set. From previous chapters we would expect such impairments as the accumulation of thermal and intermodulation noise. We find that CATV technicians use the term beat to mean intermodulation (IM) products. For example, there is triple beat distortion, defined by Grant (Ref. 1) as "spurious signals generated when three or more carriers are passed through a nonlinear circuit (such as a wideband amplifier)." The spurious signals are sum and difference products of any three carriers, sometimes referred to as "beats." Triple-beat distortion is calculated as a voltage addition.
The wider the system bandwidth is and the more RF carriers transported on that system, the more intermodulation distortion, "triple beats," and cross-modulation we can expect. We can also assume combinations of all of the above, such as composite triple beat (CTB), which represents the pile up of beats at or near a single frequency.
Grant (Ref. 1) draws a dividing line at 21 TV channels. On a system with 21 channels or fewer, one must expect Xm to predominate. Above 21 channels, CTB will predominate.
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