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From: Boris Mohar
Subject: Re: How to check for a 50 or 75 ohm connector
X-Newsreader: Forte Agent 1.92/32.572
Date: Sun, 22 Dec 2002 22:22:29 -0500
NNTP-Posting-Date: Sun, 22 Dec 2002 22:24:43 EST
Organization: Bell Sympatico
On Sun, 22 Dec 2002 23:23:38 GMT, DarkMatter
>On Sun, 22 Dec 2002 23:13:11 +0000, George Shaw
>>Ok thanks to ALL that have replied so far, interesting information.
>>I have looked at known 50 and 75 BNCs I have here from a reliable
>>source and can see NO difference in the sizes.
>>If a measuring equipment was available, what would be used to test the
> A time domain reflectometer. Fluke and many others makes cable
>testers for LANs and other communication systems, but I do not know if
>this class is covered, and the tester is very pricey.
> You could lease one from an equipment vendor for a month, but it the
>application that critical? You could have had a fedex shipment by now
>of the type you need, and would still have your hair, AND would be
>able to compare upon receipt thereof.
> Normal test gear, such as meters and scopes will yield no
>discernable differences. If you are not doing Ghz applications, you
>only need to worry about matching up the physical side. The cable.
Little do you know. You are pulling this out of your but without any
direct knowledge or experience. My first intro to the TDR was about
thirty five years ago when the instructor hooked up one up to a peace of
cabling that consisted of various lengths that were interconnected with
various connectors. He proceeded to demonstrate the importance of good
impedance match between the cable and the connector as each of the splices
showed up on the TDR. He than pinched a cable slightly and that slight
deformation showed up also. Every connector represents a impedance
discontinuity. The whole point is to minimize that. Often you work with
signals of low enough frequency and cables that are short enough so that
these things are not obvious but believe me they are there and they
sometimes matter. I work on ground penetrating radar where we send a 1kV
2ns rise time pulse to antenna. Any little discontinuity on the line
shows up as false echo and really messes up the return signal.
You can make a reasonable TDS out of a fast pulse generator and a decent
scope. Go convince yourself.
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