Coax (BNC) cable impedance question - for level not clock

[Blue Jinn]

I have a signal generator with BNC outputs, and a couple of AC voltmeters with only BNC connectors. I've a mix of RG58 (53.5Ω), RG59(73Ω), and might even have RG62 (93Ω) . Does it matter?

I've no idea what the output impedance of the signal generator a Wavetek 186. I've managed to hook it up to both a Fluke DVM and the HP, (they're not tube but solid state) to get a .316v sine wave, and it seems as long as the voltage of the sine wave at various freq's is correct that would be the the focus. I've a mix of RG58 (53.5Ω), RG59(73Ω), and might even have RG62 (93Ω). Does it matter for this purpose?

NOTE: My understanding is for clock & SP/DIF you need 75Ω. Second question, is RG59 close enough?

 

[rob aylestone]

In radio frequency circuits, the correct impedance is vital, but in audio frequency use, im not sure any of my test gear could detect the difference. In digital audio, the key feature is not voltage, its not rounding off the edges of square waves. I would guess your generator is 50 ohm output, most are. I would suspect that unless you need the sort of accuracy that means your meters go back twice a year to the manufacturer for recalibration you just wont notice anything if the interconnect has DC continuity. A lot of test generators have banana plug outputs and bnc converters!

 

[ecc83]

It's the wavelength innit! Given by, in theory c/f where c is speed of light and f the highest frequency of interest. So for 20kHz that is 15,000 mtrs. Signals* don't quite go at c in cables and it depends on the cable but you can see that 20kHz is going to be well over 10km!

At audio frequencies then the impedance of a cable does not matter for other than very large distances. For video and S/PDIF it can. Both should use the correct connectors and cable, usually 75R and be terminated at the end of the line. Otherwise video gets "reflections" repeat vertical lines and S/PDIF can drop out and crackle. (in practice I have never had a problem with S/PDIF using RCA plugs which are not 75 Ohms)

But, some signal generators, old ones mainly, have an output attenuator designed to deliver a constant, 600 Ohm source impedance and that will only deliver the correct voltage into 600 Ohms. Best to check.

We are blessed now with super accurate digital AC meters for peanuts but, most have a limited frequency response. Many are a dB or two down at 2(yes TWO!) kHz an some as low as 1kHz. Even the "true rms" types cannot be trusted past 400Hz or so. A spot check at 100Hz and 10kHz will tell you the unvarnished.

*Note, SIGNALS, it is the WAVE that travels in the cable not "electricity". You can learn more by studying Mr Maxwell's famous equations but don't ask ME questions...WELL past my pay grade!

Dave.