Help with HP 4951B protocol analyzer + 18179A

Tony Duell ard at
Sat Feb 18 12:55:58 CST 2006

> > Sounds a little off.  Cables are important, with impedance varying
> > with diameter, _Terminators_ are important, coming in the 50, 75
> > Ohm varieties mentioned, but the connector itself being 50 or 75
> > Ohm?  First I've heard of this.
> True and important. Using the wrong impedance connector or cable end
> will cause a (sometimes insignificant) missmatch. The bigger problem
> though is that mating a low impadance plug with a high impedance socket,
> e.g. 50 ohm -> 75 ohm, will probably damage the socket due to the larger
> internal conductor diameter of the plug. Mating a high impedance plug
> with a low impedance socket, e.g. 75 ohm -> 50 ohm, will probably give
> a poor contact due to the smaller inner conductor diameter of the high
> impedance plug.

I'd read that too, but somebody sent me a private e-mail saying that the 
mating diameters of the inner pins of both types of connector were the 
same, it was the outer diameter of the socket contact that differerd (so 
as to give the right characteristic impedance with the dielectric used).

And therefore there's no mechancial problem in inter-mating the wrong 
connectors. And for this application there's no electrical problem either 
(the other end of the cable has an RCA phono plug on it, well known for 
not having a defined characteristic impedance). Any mismatch at the BNC 
end would cause no problems.

I feel like getting a couple of plugs, one of each type, and sticking a 
micrometer on the centre pin. And I am not going to trust the no-name 
ones from the local hobbyist shop either (who knows what spec they're 
made to!). Then I'll know (hopefully) for sure.


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