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Impedance of BNC connectors/cables

This is not a LabVIEW, but a basic electrical question. I think many LabVIEW guys have an EEE background. Please forgive me for my ignorance if the question sounds too simple to you.

Impedance values (either 50 Ohm or 75 Ohm) are specified for BNC connectors/cables. What does this impedance value mean? I know the resistance value of a BNC cable is effectively 0 Ohm. My guess is that the value must be the impedance at a particular frequency, but I am not sure. Can someone clarify? Also, does this impedance value matter in data acquisition?

Thanks.

XLE
Message 1 of 5
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Re: Impedance of BNC connectors/cables

XLE,

The characterstic impedance of a cable or connector is related to the propagation of an electromagnetic wave along the cable. Maxwell's equations define the relationship between the electric and magnetic parts of the wave. You can think of this as the voltage across the cable and the current flowing through it. The ratio of voltage to current is the impedance. At DC the series resistance in short cables is usually negligible, as you suggested. At radio frequencies the time it takes for a signal to travel along the cable can be a significant portion of one cycle of the signal and, thus phase shifts occur. If the source and load have the same impedance as the characterstic impedance of the cable, no reflections will occur at either end.

As a practical matter the impedance of the cable and connectors matter very little at low frequencies and DC. Connectors with different impedance ratings will differ slightly in dimensions and may not mate, so you need to stay with one impedance. 50 ohm BNC connectors are more common. 75 ohm cables are used mostly in video systems.

Lynn
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Re: Impedance of BNC connectors/cables

Lynn

Thanks for the informative reply, I have a much better picture now. Can you tell me how low is "low frequencies and DC"?

Is 50kHz a low frequency? It is pretty high to a mechanical engineer like me, but it is probably nothing in the RF band.

Thanks.

XLE

Message 3 of 5
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Re: Impedance of BNC connectors/cables

The cable impedance can be significant when the cable length is greater than approximately one-tenth of a wavelength.  Impedance mismatches causes reflections which will generate a standing wave within the cable.  As long as the cable is much shorter than the wavelength of the signal, the reflections will not cause a problem.  At 50 kHz, a wavelength is around 6000 meters so I'm guessing you won't have a problem.
Randall Pursley
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Re: Impedance of BNC connectors/cables

XLEagle wrote:
> This is not a LabVIEW, but a basic electrical question. I think many LabVIEW guys have an EEE background. Please forgive me for my ignorance if the question sounds too simple to you.
> &nbsp;
> Impedance values (either 50 Ohm or 75 Ohm) are specified for BNC connectors/cables. What does this impedance value mean? I know the resistance value of&nbsp;a BNC cable is&nbsp;effectively&nbsp;0 Ohm.&nbsp;My guess is that&nbsp;the value&nbsp;must be the impedance at a particular frequency, but I am not sure. Can someone clarify? Also, does&nbsp;this impedance value matter in data acquisition?
> &nbsp;
> Thanks.
> &nbsp;
> XLE&nbsp;&nbsp;

The impedance of a cable is the impedance seen looking into the near
end, when the far end is terminated in that impedance. So if the cable
is 50 Ohms, and you terminate the far end in 50 Ohms, then the near end
will look like 50 Ohms.

If you take a 50 Ohm cable, and terminate the far end in 75 Ohms, then
the impedance seen looking into the near end will depend quite a lot on
frequency. But as long as you terminate the 50 Ohm cable in 50 Ohms, the
frequency you measure will be close to 50 Ohms, irrespective of frequency.

Read up on transmission lines, if you want to know more.

--
Dave K

http://www.southminster-branch-line.org.uk/