Subject: Re: How to check for a 50 or 75 ohm connector
Summary: What do you think of these ping times eh?
Reply-To: You can't see me, and I pass right through planets...
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Date: Sun, 22 Dec 2002 11:07:58 GMT
NNTP-Posting-Date: Sun, 22 Dec 2002 06:07:58 EST
On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
>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.
For George: It is based on the cable that the connector is meant to
be used for. You cannot electrically measure it, you must physically
measure the cable side of the connector, and match those particulars
up with your coax. Basically, the center conductor's insulator
diameter changes, as well as the material, and the overall cable
diameter changes. In fact, THAT is how the impedance is determined.
>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.
The connector does not possess the impedance. That reference is
really about the transmission cable that gets used. Connectors can
differ between them, but the cable is what really determines the
>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
You would get NOTHING from a connector alone. It would require a
CABLE, terminated properly at both ends with no kinks, and THAT is
what the meter would test. The connectors themselves are shaped and
sized merely to keep someone from mismatching connection cabling.
>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.
As far as I know, BNC is BNC is BNC, and the ONLY differences are
the size of the rear portion of the connector for different sized
cable. THAT would determine the proper cable for the connector, and
*that* cable would determine the impedance. So, by default of
application, the connectors differ. On the front, connection side of
them, they are all the same, however. There are "hybrids" that will
claim some super duty or used gold plated parts, but BNC nomenclature
is the same. It has to be able to be hooked directly in to ANY BNC
female ANYWHERE. So they are certainly ALL the same on the connectors
The cabling determines the impedance. RG-59 is 75 ohm, RG-58 is 50
ohm. Then, there is RG-6 (75ohm), RG-174, etc. ALL have BNC
>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
Sure it would. At several GHz... maybe. At Mhz frequencies the
connector contributes little to the overall impedance of the cable
run. That is determined practically solely on the cable run itself.
What can be determined from different connectors is how well they
contain what signals they carry. That would be RFI leakage at the
terminations. That is why the connectors are "shrouded" in nature by