Ian Jackson wrote:
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.
The manufacturers of many of the modulators sold for communal TV system
use claim that the output is so clean that filter combiners are
unnecessary. But I use filters anyway. The fact is that I seem to get
better results.


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.

Typically the loss using four six-way filters and a four way combiner
would be about 15dB. Modulator O/P is usually 25dBmV (analogue mods) so
the combined feed is 10dBmV. That would usually be amplified to
something between 40dBmV and 55dbmV before feeding out.

One advantage of using filters is that if some of the RF sources only
provide low level signals it doesn't hurt, noise-wise, to amplify before
combining.

Bill