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Old 28th July 2005, 03:40 PM   #1
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Question True 75ohm RCA Type Plug Possible?

Is there such a thing as 75Ohm RCA plug? All along i was told that the way RCA plugs are structured make it impossible to be of a true 75Ohm characteristics impedance. But the famous WBT says that their WBT0110 NexGen plugs are 75Ohm.

http://www.wbtusa.com/nextgen/wbt0110cu.html

http://www.wbtusa.com/nextgen/0110SpecSheet.pdf
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Old 28th July 2005, 03:45 PM   #2
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Default Re: True 75ohm RCA Type Plug Possible?

Quote:
Originally posted by commstech
Is there such a thing as 75Ohm RCA plug?
No.

Don't ask that twice, Jocko is here.
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Old 28th July 2005, 04:04 PM   #3
badman is offline badman  United States
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My understanding is that, when mated with a WBT nextgen Female, the nextgens ARE true 75 ohm characteristic impedance.
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Old 28th July 2005, 04:19 PM   #4
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What makes the impedance of a plug (or a cable) is the distance between the central pin (or conductor) and the shield.
On ANY RCA plug the central pin is too thick, it's impossible to make a 75ohm RCA plug. It's more like around 30 ohm.
The rest is marketing talk.
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Old 28th July 2005, 04:41 PM   #5
gmarsh is offline gmarsh  Canada
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It's possible to increase the impedance of an RCA by not using a 100% shield around the center pin, reducing the shunt capacitance. It looks like WBT did this on their plug - there's only three shield contacts, and it looks like the space between them is greater than the width of the contacts themselves.

But the moment you plug this thing into a jack... goodbye 75 ohms. Unless of course there's a matching RCA jack, in which case your equipment has become so specialized that you should have been using BNC anyway.
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Old 28th July 2005, 05:33 PM   #6
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interesting points....

lets say the jacks and plugs are actually structured as 75ohms...will the copper tracks thereafter the jacks need to be specially treated to maintain this impedance throughout the equipment>?
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Old 28th July 2005, 06:20 PM   #7
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Default It is absolutely possible. Sorry, carlos..

Nextgen did it by decreasing the shield.

This asymmetry in the shield currents increases the net inductance at the connector by preventing the coaxial cancellation of the magnetic fields outside the shield cylinder. In addition, by reducing the shield total in this fashion, decreases the net capacitance at the connector.

Since Z = sqr(L/C)....increasing L increases impedance, and decreasing C also increases impedance.

They have line males and chassis females which have the same feature, as they spiralled the shield pair in one direction for the males, and the other direction for the females. By using this design, they maintain physical compatibility with all existing rca jacks and plugs, but will remain true 75 ohm only if nextgen wbt's are used together..

Their spiral shield contact solution is useable up to frequencies which have slew features about 1 to 10 times the connector length, that being about 1/5 inch to 5 inches. Above those frequencies, the design they embrace will begin to show lumped element mismatches.

There is a more elegant solution which affords far higher bandwidth impedance match. But to introduce it would undermine all the wbt product, obsoleting it entirely. And, it costs far too little to be a niche product, better as an OEM one..

Cheers, John

PS...sorry, I can't find the magnetic field analysis jpeg I ran which shows how the fields are arranged..guess it's at home...
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Old 28th July 2005, 06:23 PM   #8
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Quote:
Originally posted by commstech
lets say the jacks and plugs are actually structured as 75ohms...will the copper tracks thereafter the jacks need to be specially treated to maintain this impedance throughout the equipment>?
Yes, in fact it is actually more important than the jack itself.

The reflection of signal back is a function of the length of the discontinuity, the longer the discontinuity (incorrect impedance), the lower the frequency that will be affected.

John
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Old 28th July 2005, 06:58 PM   #9
Mikett is offline Mikett  Canada
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http://www.canare.com/index.cfm?obje...A54F63C6A9870D
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Old 28th July 2005, 08:46 PM   #10
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No, it is not ******* possible.

Jocko
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