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Old 10th Aug 2018, 3:54 pm   #48
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Back terminated cable?

Most video gear, if not all, uses 75 ohms and typically there is a low Z output stage and a 75R resistor in series (so in these cases the impedance of the output amplifier always matches the cable) to the typical BNC or RCA video output socket. If the far end of the cable is unterminated with a 75R resistor (often there is a switch on a video monitor to select high Z) the signal level doubles, but it was helpful if a few video monitors were daisy chained then the last one terminated it in 75R.

Most TV related RF test gear is also 75R out, for example the PM5519 pattern generator video and RF outputs are also 75R. If you look into the RF out connector on the generator, with a meter, you will find it measures as a 75R resistor, regardless of the setting of the attenuator. It has a very clever enclosed linear attenuator that maintains a stable input and output impedance as it is rotated.

Also most TV related antenna coax is 75R, and the antennas have baluns, it replaced the early 300 R ribbon balanced line, which incidentally, has got difficult to get now. I needed some for a restoration of a tuner and had to scavenge it from an old set of rabbit ears.

Most commercial RF gear, Comms radios etc, various transmitters appear to have adopted 50R.
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