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 17:56:18 GMT
NNTP-Posting-Date: Sun, 22 Dec 2002 12:56:18 EST
On Sun, 22 Dec 2002 11:45:04 +0000, Don Pearce
>On Sun, 22 Dec 2002 11:07:58 GMT, DarkMatter
>>On Sun, 22 Dec 2002 02:15:12 GMT, Chris Carlen
>> Gave us:
>>>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.
>You are getting to different issues hopelessly confused here. The
>cable end of the connector is available in many different styles, to
>suit all sorts of different cable types. That is true. The mating end
>of the connector is where you will find the specification for 50 or 75
>ohms. They are different in design.
Absolutely incorrect. A "BNC" type connector is a specification for
the CONNECTOR side of the terminator. The CABLE side varies for
different cable, and it is THAT cable that determines the impedance of
the "jumper" or "cable run".
>A connector will normally have a part number stamped onto it. That
>part number will determine the appropriate cable type that will fit
>it, and somewhere in that part number will be a 50 or a 75 to tell you
>the impedance of the mating surface.
Different BNCD connectors have different part numbers such that they
match up to different impedance cables. The PLUG side, however is the
same for ANY given connector type.
One can purchase the simple "consumer electronics" "F" style
connector in a number of cable termination sizes. Depending on WHICH
cable gets used, the impedance will differ. The PLUG side, however is
ALWAYS absolutely identical.
Here is an example that proves it quite simply.
One must connect the two segments in order to hit the site.
It clearly shows that the connectors are made for different cable
types, and that is what defines the impedance. It also clearly shows
that the front, PLUG side of the connector is always the same.
In order to plug into a BNC Female JACK, ALL of the BNC MALE PLUGs
HAVE to be the same.
>>>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
>The connector does indeed contain the impedance.
No. Absolutely incorrect. The connector contains the physical
changes on the CABLE insertion side to accept different impedance
cabling. The PLUG side is ALWAYS the same.
> The design is such
>that the correct impedance is maintained as far much as possible
>throughout the mating connector pair. BNC isn't terribly good in this
>regard, but the connectors certainly do contain the impedance.
>>>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
>The two impedance connectors are of quite different design visually at
>the mating end, and there is no need to resort to test gear to
At the cable mating end, yes. At the PLUG to JACK side, no.
The PIN, AND the GROUNDED SHROUD that encircles the center PIN MUST
have the same dimensions in order to plug into a BNC Female JACK.
On the BACK side of the connector, the opening for the insulation of
the CABLE changes, and the opening for the overall diameter of the
cable varies. AGAIN, ALL on the cable side ONLY.
>> 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
>Quite wrong. You have been misinformed.
Give me examples of it then. If this debate were for money, I'd put
you in the poor house.
>> 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
>Well, the system design determines the impedance. You then use cabling
>and connectors of appropriate impedance.
You missed the discussion completely. If one incorporates a Female
BNC into a design, the EXIT cable on the BACK side of said connector
determines the impedance of that *jumper* to the circuitry inside.
The cable, NOT the connector. The ONLY reason the BACK side of the
connector differs is to accept different impedance cabling for
termination. The FRONT, PLUG receptacle side, however has to match
the industry standard for BNC, which is impedance independent.
This is common sense. I cannot believe you have misled yourself in
your thinking, or that you have somehow misinterpreted the data.
>>>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.
>This is true. Long lengths of cable certainly have much greater
>effects than short lengths of connector.
The cable is the SOLE determining factor.
Try again. Review your data, and make information for yourself out
of it. Unless disseminated properly, it is mere data. It is not
INFORMATION until it gets examined, and absorbed properly.