Ian Jackson wrote:

In message , Michael A.
Terrell writes

Ian Jackson wrote:

In message , Michael A.
Terrell writes


The 20 dB isolation means that the return loss is more than 20 dB.

No it doesn't. It means that the isolation - the loss between two
outputs (with the input terminated) - is 20dB.

Sigh. How do you think you measure that isolation?

Sigh as much as you like - isolation and RLR are definitely NOT the same
thing, and you don't measure them in the same way.


You use the same equipment, but connect the ports in a different
order.


This is to prevent the local oscillator from one TV causing interference
to another TV.

True - but, more generally, it is to minimise the effects of something
connected to one output affecting another output.

What are you connecting that doesn't have a TV tuner? FM radios are
supposed to use a single port -20 dB tap.

That could make the FM level uncomfortably low.


A 0 dB or higer signal will overload the front end of a FM radio. O
dBM is 1000 microvolts @ 75 Ohms. FM radios are supposed to work at full
quieting 10 microvolts. Some of the -20 dB taps were labeled as FM Taps
and were supposed to have a bandpass filter on the output port but none
of the ones I tested in the lab did.


It's better to separate
the FM from the TV signals via a bandpass diplexer filter. This is/was
common practice in Europe. Of course, in the USA, you have always had
your Channel 5 (running right up to the LF end of the FM band).


No, it is Channel 6, with the aural carrier at 87.7 MHz. That is .3
Mhz below the bottom edge of the 88 to 108 MHz FM Broadcast band.


Also,
these days, you could have digital TV signals starting immediately above
the FM (and still meeting the 'no interference' regulations for Air
Traffic Control radio. Do any cable systems still actually carry FM
radio?


Are you kidding? They were dropped as soon as the franchises stopped
asking for FM and wanted more video channels. They gained three midband
channels without rebuilding the system.


As well as protecting
against local oscillators, it also protects against incorrect loading
(ie if an output port is not correctly terminated in a 75 ohm load). It
should not matter (too much) if an output is left open circuit, short
circuited, has a length of cable connected to it but going nowhere, etc
etc.

Unterminated ports are not allowed in properly designed systems.
PERIOD.

They may be installed correctly terminated, but you can bet your bottom
dollar a lot won't stay that way!


Not if it's maintained by morons. But then, those systems were never
in spec.



I was the engineer at a CATV system with over 10,000
active drops. There are hundreds of myths and half truths about the
hardware used for CATV systems.

Was your statement above (about RLR) one of these? [Sorry. The Devil
made me say that! ;o))]

I find 2 ports with -6.5dB loss and one port with -3.5dB, which makes
more sense. (Actually, it should be -7.0dB loss).

While most 3-ways are typically 3.5/7/7, there are one or two around
with equal outputs, of appx 5.5dB (although, personally, I can't really
see much need for them).


You would, if the use of the 3.5/7/7 would require a second drop from
the line tap on the street.

When I said "much use", I meant "MUCH use". While a computerized CATV
system planner may flag up an out-of-spec condition, in practice, there
may be less than 1dB difference between a nominal 5.5 and 7dB.


So? If the franchise calls for all drops to be above 0 dB, one dB is
out of spec.


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