In message , Bill Wright
writes
Michael A. Terrell wrote:


Do you know that the channel combiners in a CATV head end
were wired in odd and even banks, on separate groups to prevent IMD
caused in the passive mixing?

Even now if we use passive filters to combine channels we try to
arrange the filters as follows (for example)
Filter 1: 21, 25, 29, 33 etc
Filter 2: 22, 26, 30, 34 etc
Filter 3: 23, 27, 31, 35 etc
Filter 4: 24, 28, 32, 36 etc
The filter outputs are combined using passive combiners. The idea is to
prevent interactivity between adjacent channel filters.

Ah but, unless they really have to, most large cable TV networks don't
use filter combiners. This keeps the whole of the headend combining
system wideband. Of course, you can only do this if the modulators are
themselves sufficiently clean in respect of out-of-channel unwanted
signals and noise.

Normally, the losses associated with wideband combining are not a
problem. At least one manufacturer made a purpose-made 8-port (and later
a 16-port) combiner, with insertion losses of around 20 and 24dB
respectively. They were designed specifically to provide high isolation
between input ports. A 48-channel headend could be made using four
16-port combiners followed by a standard 4-way turned-around splitter,
so the total loss was around 28dB. As the modulator outputs were around
60dBmV, the combined spectrum level was 32dB - which was more than
enough to allow splitting to provide multiple feeds for the inputs of
individual trunk-line launch amplifiers.

Of course, there are also losses in the interconnecting cables, in-line
injection points for test signals, monitor points etc, so if required, a
special low-distortion level-raising post-amplifier might be used to
ensure that plenty of signal was available. The last thing you want in
large cable TV headends is to be scraping the barrel for signal level.
--
Ian