A plug does not have an impedance, in any meaningful sense. You could measure the impedance of a plug in a particular socket with particular wiring beyond that or a 75 ohm termination wired straight onto the socket. It won't be 75ohms, as RCAs are not intended for constant impedance use. Fortunately it doesn't matter too much for SPDIF. If you are concerned don't use RCA, use BNC.
The RCA plug is a coaxial very small short wave-lenght, but as the coaxial has a nominal impedance given the ratio between two radii in the conductor cylinders, and the log function. If I remember correctly, the impedance was a constant (136 perhaps) and the log10 of the radii ratio (or diameters). Also, has a dielectric between two conductors, and then, a velocity factor.
Just as the coaxial cable.
OK, to clarify: a plug has an impedance but there is no point in measuring it unless you wish to leave it disconnected and reflect all the signals. What matters for normal use is the impedance of the plug/socket combination. When people say that BNC has an impedance of, say, 50ohm what they mean is that a properly connected BNC plug in a properly connected BNC socket has an impedance of 50ohm.
At RF frequencies connectors have impedance and will cause reflections if miss matched with the coax and equipment termination. Check any RF connector supplier and you can find 75 ohm and 50 ohm BNC's, TNC's, pl-259, N types, etc.
In my experience it isn't a big issue for spdif, but opinions very, as always.
In my experience it isn't a big issue for spdif, but opinions very, as always.
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