Is there any physical difference (in mating and such) between 50 and
75 ohm BNC connectors? I'm using them at low speeds, so the actual impedance doesn't matter.- posted
8 years ago
BNCs
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8 years ago
Den onsdag den 19. august 2015 kl. 22.31.37 UTC+2 skrev John Larkin:
google says bnc made after ~1978 will mate 75/50 with no problems or damage, before that the center pin had different diameter
-Lasse
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8 years ago
I have a customer that's whining that some of our panel connectors are
50 and some are 75.
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I guess we used what was in stock. The 50s are bulkhead mount and the
75s are on a PC board.We have all sorts of cables and connectors here in the lab, and I never saw a problem mixing them.
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8 years ago
On Wed, 19 Aug 2015 13:31:31 -0700, John Larkin Gave us:
I was using a BNC to N today and my only worry was that the big honkin adapter was going to weigh down and tear out the BNC jack from the panel it is in.
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8 years ago
Connectors made this century should fit without damage. The problem used to be that the center pin for 75 ohms was smaller than the 50 ohm center pin. Shove a 50 ohm center pin into the smaller 75 ohm receptacle, and it would spread the connector. Similarly, the smaller
75 ohm center pin would fit loosely in the 50 ohm receptacle. All that changed at some time in the distant past, and all the pin and receptacle sockets now being identical based on the 50 ohm dimensions. 50 ohm:
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75 ohm:
I believer that may you have a TDR. Connect it to the connector in question maybe you can see the impedance bump. I've never tried this and have no idea what you'll find.
Incidentally, 50 ohm BNC connectors were used with 93 ohm RG-62a/u coax cable for AT&T Starlan systems, and nobody complained about the almost 2:1 impedance mismatch.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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8 years ago
Kinda hard to compare them seperately. Combined JPG:
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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8 years ago
On Wed, 19 Aug 2015 20:53:42 -0700, Jeff Liebermann Gave us:
Perhaps because the impedance is a function of the actual cable itself, not the connector. This is why an SWR meter with the old CB radio connections still works with adaptors for N type LMR 400 cable interconnects and equipment. Or SMA for that matter. The connector families were simply in place to signal the user as to what he was about to perform work with and keep him from screwing up royally.
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8 years ago
I always remembered RG-62 was Arcnet, was Starlan the same? But you are right that the freqs involved were low enough that the connector impedance to cable mismatch was neglible,
piglet
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8 years ago
On Thu, 20 Aug 2015 09:37:57 +0100, piglet Gave us:
2Mb/s "cheapernet" (10BASE2) was on coax using BNC as well.- Vote on answer
- posted
8 years ago
A 50-ohm connector on a piece of 75-ohm coax looks like a shunt capacitor, and a 75-ohm connector on 50-ohm coax looks like a series inductor. The reflections get annoying with fast edges / high frequencies. In John's case, the connectors are near one end of the transmission line, which makes reflections a lot less annoying.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
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8 years ago
Were the laws that define the characteristic impedance of a coaxial line changed in 1978?
Apparently I have lots of BNCs that were made before 1978 because I have seen the connectors and receptacles with thinner pins many times. (and sometimes the result of fitting the wrong combination, either a damaged receptacle or a loose contact)
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8 years ago
At low frequencies, the connector is pretty much invisible. When the wavelength of a signal starts approaching the length of the connector, the impedance of the connector becomes important. The connector is just a short section of the transmission line system.
The signals I'm making in this case are a few MHz, so the distinction between 50 and 75 ohm connectors is nil. Well, was nil until the customer got worked up.
-- John Larkin Highland Technology, Inc lunatic fringe electronics jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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8 years ago
On Thursday, August 20, 2015 at 3:16:53 AM UTC-7, DecadentLinuxUserNumeroUno wrote: ...
...
Starlan used Cat 3 twisted pair phone cable, not coax. It predated 10baseT which ran at 10Mbps.
As the name suggests 10BASE2 ran at 10 Mbps with a 200 metre maximum link length running over 50 ohm RG58. 10BASE5 was the normal ethernet with the yellow cable and vampire taps.
kevin
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8 years ago
laws didn't change ;)
but apparently around that time the manufacturers decided that adding a bit of plastic and making the mating part of the center pin the same diameter they could make them mechanically compatible and still get the right impedance
-Lasse
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8 years ago
On Thu, 20 Aug 2015 09:15:16 -0700 (PDT), kevin93 Gave us:
Yes, as did Arcnet (predated). I was around when Tandy developed it.
Ideally, sure..
Many of the coax interface cards ran at 2MB/s even though the thing was 10BASE2.. Also segments were mostly all less than 30 feet. And there is always overhead so none of them ran "at" the speed the spec declared.
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8 years ago
Something did happen in 1978
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-- John Larkin Highland Technology, Inc lunatic fringe electronics jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
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8 years ago
The CB connector would probably be a UHF connector, also known as a PL-259 and SO-239. These connectors are nowhere near 50 ohms. The other connectors you mentioned (N, SMA) are usually 50 ohms (although they can be obtained in 75 ohms).
While the wrong impedance connector might cause an "impedance bump", it's unlikely to cause any reflection problems being close to the source. The problem is standing waves along the coax between the source and the bump, where the voltage across the coax might vary between peaks and nulls at 1/2 wave intervals. At low frequencies, that's highly unlikely to happen with perhaps a few cm of coax between the signal source and the connector "bump", where 1/2 wavelength is much longer than the coax cable.
I suggested that John L. try a TDR, because it's possible for his instrument output impedance to also not be quite a perfect 50 ohms. Why worry about a small mismatch caused by a connector, when you can have a larger mismatched caused by the instrument? That would be amusing, but not very important as a source impedance mismatch doesn't have much of an effect.
Huh? How does a connector signal the user?
Connectors have families? No wonder my collection of small connectors seem to be growing.
Also, when dealing with coax connectors, one installs them by "screwing down" not by "screwing up". It works the same with for royalty and commoners.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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8 years ago
Argh, memory fault. Y're right. Starlan was over CAT5 and Arcnet over twisted pair, fiber, or RG-62a/u.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
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8 years ago
On Thu, 20 Aug 2015 11:37:34 -0700, Jeff Liebermann Gave us: snip
That's just screwed up, man.
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8 years ago
When you operate a screwdriver, what's the natural position for the screwdriver? Up or down? Down of course. Therefore, you should be screwing down, not screwing up. While this may sound like a minor semantic differences, there are those that tend to take such things literally. To avoid complications, like someone pointing the screwdriver up instead of down, I recommend that you use the technically correct terms. However, royalty can either screw up royally, or screw down royally. The results are usually identical.
Footnote: Please note that screwdrivers operate equally well on right and left hand screws. They are also available with replaceable tips. Therefore, the correct term would be an omnidriver or omniscrewdriver.
While you're pondering that, please consider that the correct spelling of "socket" is actually "sockette" meaning a small "sock". I'm not sure exactly where the transition in size from sockettes to socks. Asking for where to find the sock section at the hardware store has yielded some odd responses.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558