In article , Laurence Payne
wrote:


What a load of ****. Do you know what an "ohm" is?

I suspect you think it's a simple measurement of resistance. Not so,
with cables carrying high-frequency signals.

Erm, we may need to make a distinction here to avoid confusion.

The Ohm is the unit of resistance. Nominally comes from the definitions of
the Volt and the Ampere. You can then generalise by allowing the value to
be complex instead of real and specify values that way.

A cable that genuinely has a characteristic impedance of 'X Ohms' will - if
either 'very long' or attached to a load of the same resistance - act just
like a resistive load to the source. Thus if you had ten light years of 75
Ohm cable and fired signals into it, so far as the signal source would be
concerned it would (for 20 years, anyway!) look indistinguishable from a 75
Ohm resistor. Ditto for a shorter lead terminated in the matching
resistance.

However most cables that are called 'X Ohms' tend not to actually have this
value of impedance (resistance) at all frequencies, and hence may have an
impedance which is not always resistive, and varies with frequency. For
domestic digital links this matters very little, though. :-)

Slainte,

Jim

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