Quote:
Originally Posted by CGabriel
The characteristic impedance of a cable has no relevance to signal transmission unless the cable is operating as a "transmission line". What determines the mode of transmission is based on two factors: transmission frequency and the length of the cable. For a cable to operate as a transmission line the length of the cable must be close to the wavelength of the transmission frequency.
So, to be clear, if a cable is not operating as a "transmission line" then the characteristic impedance (50 or 75 or any other) is irrelevant. If it does not operate as a transmission line then the normal characteristics of DC resistance, capacitance and inductance become the primary factors in signal integrity.
10 Mhz is a significant transmission frequency and it is very likely that the cable could be operating as a transmission line. Here is some sample math:
Frequency = 10 Mhz
Velocity over copper wire = ~70% C
It is well known that a cable that is 1/4 wavelength of the wavelength does operate as a transmission line. A cable that is 1/10 the wavelength MAY be influenced but this is at the outer limit of transmission line operation.
So given the above: A cable will definitely operate as a transmission line if the cable is 5.25 meters or longer. And it MAY be influenced with a length as short as 2.1 meters.
Summary: If cable the length is very short as in .5 to 1 meter you should have no problem using a 75 ohm cable. If the cable is longer than 2 meters I would be looking for a legit 50 ohm cable.
Hope this helps.
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Guys...this is an excellent post and it states some very specific facts that helped me understand this issue alot more.