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In Reply to: nope, trust me posted by audioengr on December 10, 2002 at 19:43:05:
Somewhere around 1/10 of a wavelength driving and sink impedences do start to matter.What that amounts to at audio frequencies is pretty boring, but when we're talking about a digital system that must respond to 20 MHz it starts to get a bit more touchy, at least for the cable and termination side.
The driving side may not care about SWR, of course :)
JJ - Philalethist and Annoyer of Bullies
Follow Ups:
The spectrum of a trapezoidal wave is a series of frequencies extending to GHz.
Thanks for adding something to this, JJ. Last night I got out my TDR meter and made some simple measurements. First, the TDR meter works best at 50 ohms, so I used that as my reference. IF I had a 50 ohm source, 50 ohm cable and a 50 ohm termination, everything was virtually perfect. But IF I added a additional 50 ohm termination at the input of the TDR, to make the effecitve source drive 25 ohms, then I got reflections. IF I used a 75 ohm cable, instead of a 50 ohm cable, then I got reflections, etc etc. This was with 1/2 or 1 meter of cable. Therefore, even a 1/2 meter length of the wrong impedance cable, will change the signal.
It is also important to note that RCA connectors CANNOT be 75 ohms, because the intrinsic ratio between the inner pin diameter and the 'gound' inside diameter of the connector is too small. This has to create reflections or what might effectively look like 'glitches' on the digital waveform.
Ignoring the evidence doesn't do much to improve our understanding of what might cause differences between digital cables.
""It is also important to note that RCA connectors CANNOT be 75 ohms, because the intrinsic ratio between the inner pin diameter and the 'gound' inside diameter of the connector is too small. This has to create reflections or what might effectively look like 'glitches' on the digital waveform.""From that alone, I would worry about the source impedance, as the designed connectors in use will reflect somewhat.
What kind of risetime does your TDR have; risetimes must be a beast, and those kind of slew rates are hard to keep under control, they can tell if your shoes are tied.. Does an RCA connector show as bad a match at the frequencies of the digital audio stream? I imagine the point discontinuity of the RCA would have a frequency dependence component. Perhaps they considered it as a non-issue? Has anybody tried TDR on the standard 75 with RCA's?
Cheers, John
The standard RCA connectors are barely noticed. The 15 nsec risetime is too slow to cause much reflection from a short connector like this. Connector effects are definitely second-order compared to cable losses.
John Es, I use a TEK 1502. It has a 200ps risetime, I think. I do know this: I can easily see the effect of a single RCA-BNC adaptor either at the source or in the line after one meter of cable.
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