From: Chris Carlen
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Subject: Re: How to check for a 50 or 75 ohm connector
Date: Sun, 22 Dec 2002 02:15:12 GMT
NNTP-Posting-Date: Sat, 21 Dec 2002 18:15:12 PST
George Shaw wrote:
> I have a large box of mixed BNC connectors/tees etc.
> Using the simplest equipment possible (multimeter?) HOW do I check if
> the connector is for 50 Ohm or 75 Ohm coax connection. There are no
> external markings.
I find it very difficult to accept the suggestions that 50R vs. 75R
connectors have different diameter pins, as that would imply that the
"BNC" connector has different mechanical standards for different
impedances. If this is truly the case, then I will be eager to learn
that it is so.
But until I'm convinced, I would hesitate to make any certain judgements
about the connector impedances. Also, the appearance or presence of an
insulating material is of little value in determining the impedance as well.
I would instead rely on a TDR (time domain reflectometry) type of
Take two pieces of 50 ohm cable, and put one of your connectors in the
middle, joining the two. Set up a signal source to drive one end,
preferably with output impedance equal to that of the cable and put a 50
ohm T on that end. Terminate the end of the cables with a 50 ohm
terminator. Now probe the open T at the generator with a proper high
impedance RF scope probe.
Feed square waves into the cable and look at what's on the scope. A
slight reflection from the termination may be visible from a slight
mismatch at the end. Also, a small blip from the connector in the
middle may be visible. If you swap known 50 ohm vs. 75 ohm connectors
in the middle position, you should see the size of the reflection blip
from the middle connector change in size and character.
When you know the way a 50 ohm connector looks vs. a 75 ohm, then sort
all your connectors.
This is my untested idea. I'm not an expert on the subject, but it
follows from what I have learned from transmission line theory that this
Christopher R. Carlen
Suse 8.1 Linux 2.4.19