Impedance of connectors

[percy]

I need to be educated on the subject. What exactly determines the impedance of a particular type of connector ? For example, BNC is 75ohm, RCA is 200(?), etc...
I am curious to know what could be the impedances of typical 1/4" and 1/8" stereo plugs ?

[lndm]

This impedance is of interest only at RF where the signal doesn't travel down the cable per se, but traverses the inductance and capacitance of the cable/connectors. At audio frequencies we would refer to these inductances and capacitances of the cable to be stray reactances, and we'd probably wish they didn't exist. At RF they are just a fact of life, and have to be worked with, but at AF they normally are not significant.

To sum it up, at audio frequencies, we are normally only concerned with just the resistance of a cable (if even). There is no 'impedance' to worry about.

Apart from this, phono cable capacitance and capacitance from long runs of professional cable, can be a concern but these are different to cable impedance.

[tubetvr]

Hi,

The impedance of any transmission line is determined by the relation between inductance and capacitance at any point along the line. The definition of characteristic impedance is SQR(L/C) for a lossless line.

The relation of L and C in a coaxial connector is determined by the mechanical dimensions, As an example a thin inner conductor gives a lower impedance than a thicker one and so on.

In order to determine a characteristic impedance of a connector, (if the value should have any meaning) it should ideally have the same impedance in any point which is what you are trying to achieve in connectors for RF, the higher frequency the more important it is that this requirement is fulfilled, as an example it is more difficult to use angled connectors at higher frequencies as it is very difficult to make an angled connector that have the same impedance at any point.

For Audio it is not that critical but I think it is in principal wrong to say that a RCA phono connector is 200ohm as in reality the impedance varies along the connector and between manufacturers, it is different with BNC which are made for a certain impedance, (either 50 or 75 ohm, both types exist). For 1/4" or 1/8" stereo plugs I don't even want to guess.

However is it really important which impedance a connector have in an typical audio system? The answer is no for 2 reasons:

1, the typical Audio connection is where the signal source have low impedance, (ideally a voltage source) and where the load is high impedance, in this kind of system characteristic impedance has no meaning.

2, For an impedance matched system for Audio any fault in characterstic impedance introduced by a connector, (which will be seen as an extra minute amount of inductance or capacitance at a ceratin point) will in reality be so small that the effect is negligible.

Best Regards Hans

[hacknet]

hi guys!

i've never got to understand this impedance concept and i'm extremely curious now..


tubetvr,

am i right to say that the only the connectors have a say in this characteristic impedance?

[I_Forgot]

Here is a reasonable explanation of the concept.

http://en.wikipedia.org/wiki/Impedance_matching

I_F

[phn]

Tube gear made in the old days was designed for 47kOhm impedance. Hence the high-impedance, low-capacitance coax cable. (Back then they called it diode cable.) And because of that we got the RCA connector. So it's the cable, not the connector, that primarily matters.

Digital gear works different.

[tubetvr]

am i right to say that the only the connectors have a say in this characteristic impedance?

No, not at all, it goes for the whole transmission line. i.e connectors and cable.

But as I also said the characteristic impedance doesn't matter for audio as the line is not terminated in a defined impedance but as I said you have a voltage source and a high impedance load. What matters is the total capacitance of the cable and connectors compared to the source impedance, in this case the capacitance of the connector is so small that it can be ignored.

Regards Hans

[hacknet]

i'm thinking of this concept when impliement bncs in spdif transmission, i see there are 50ohm and 75ohm connectors, what difference would these have? what is the essense behind the need for these impedances? bandwidth or is it to minimize losses?

[tubetvr]

Tube gear made in the old days was designed for 47kOhm impedance.

You are refering to the load impedance that in tube gear can be anything form 10s of kohms up to Mohm. However the load impedance is not that important but the source impedance is, even in tube gear this can be quite low, say 100s of ohm up to 10s of kohm, (a cathode follower is easy to make with standard tubes and this will give source impedance of 100s of ohms)

The RCA connector is not designed for any specific impedance or for low capcitance, it is only designed to be coaxial and low cost. The capaciatnace of a connector doesn't matter as PHN says, and example a coax cable have 200pF per meter, and RCA connector is approx 4cm, then even if the RCA have double capacitance per length, (it is unlikely that is the case) then the RCA will give approx 8pF more than a cable of same length.

Regards Hans

[AndrewT]

Hi Hack,
digital is a different ball game.

All the essentially square wave signals passing down the line like to see a constant impedance to very high frequency.

You need to avoid reflections by ensuring correct source and load impedances.

You also need to preserve the squareness of the waveform as best you can to allow easier decoding, with fewer errors, at the load end. This requires a flattish reponse out to about ten times the bit rate.

Since all the above effectively requires a transmission line, you are now into line drivers and line receivers with correctly matched cables, connectors, sources and loads

BTW how does one differentiate between a 50ohm BNC and a 75ohm BNC?