In message ,
Terry Casey writes
In article , says...
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.
I wonder if they have any active stages AFTER the filter, possibly
because they might be used in installations with less than perfect
matching?
We used to use some tunable modulators, although most of them were
fixed. The idea of the tuneable one was that you had a near instant fix
if one of the modulators developed a fault. The downside was that the
broadband noise from the unfiltered output degraded the headend by 1dB
per modulator ...
The number of tunable modulators was reduced to one - we used it as the
reference for all of our distortion tests which, of course, used a spare
channel, hence no (fixed) modulator ...
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.
Obviously you will add noise to the amplified source but not to the rest
of the system.
Using unfiltered combiners, the tuneable ('frequency agile' in American
parlance) modulators which I worked with were specced to give a CNR of
better than 60dB after 48 headend channels had been combined. Typically,
it was 63dB.
I recall visiting one particular headend in order to prove to that we
met the spec, and to explain why 'Brand X's' modulators gave more like
70dB or better. I spent the day making lots of measurements on a goodly
selection of the 48 channels, and in the end, everyone was satisfied
that the usual 63dB was being obtained.
The reason that the Brand X units were so much better was quite simply
that they incorporated individual channel filters. The disadvantage was,
of course, that they were not instantly tuneable to any network channel.
But, as Terry says, what a headend operator could do was to have a few
fully tuneable modulators to do system measurements with, and to use as
backup units.
Although the competitor's equipment undoubtedly gave around 10dB better
SNR than ours, individual channel filtering was something we had ditched
two generations of equipment earlier, as it had been decided that there
was absolutely no advantage in getting a 70dB CNR at the headend when
the rest of the network was going to degrade this to something like 48dB
(or even less). The overall impact of changing the headend launch CNR
from better than 70dB to our typical 63dB would have been negligible.
Nevertheless, I believe that later on, all our gear was replaced with
the Brand X units.
--
Ian