Re: Questions on impedance matching


Kevin Wood G7BCS
 

Hi David,

The situation at Audio is slightly different because the wavelengths in
cables are very much longer so the problems of strict impedance matching
and reflections occurring where there are mismatches only really occur
when you're talking about the distances used in telecommunications.

With a couple of exceptions, in audio one tends to keep the output
impedances low and the input impedances high (say a ratio of 10:1 or more)
such that effects of loading your source are minimal.

The exceptions would be devices such as moving coil phono cartridges or
microphones where the signal levels are so small that you might use a
transformer to better match the impedances and increase power transfer.

Another area is when passing digital audio in SPDIF / AES3 formats where
reflections could cause problems unless impedance matching is carried out,
but again, this is likely to be experienced only with "longer than
domestic" transmission lines.

The idea with an oscilloscope is for the input impedance to be high so
that connecting it to monitor a circuit has a minimal effect on its
operation. In this case, there simply is no need to achieve maximal power
coupling between your generator and the instrument. Significant power
coupling might change the operation of the circuit you are trying to
diagnose!

73 for now

Kevin

Hi all,

I have some questions on impedance matching that are borne out of some
recent threads regarding challenges in getting a proper oscilloscope
reading off an old signal generator.

I've been spending a disproportionate amount of time learning about
impedance matching. I've learned the math (even developed a spreadsheet to
game various scenarios), developed a pretty good understanding of the
Maximum Power Transfer Theorem, and I get the principles of reflection.
But here are my questions.

(1) While it's always best to shoot for minimal reflection (optimal power
transfer), I've seen plenty of references to an acceptable over/under
range. In other words, not a direct match. Are there any thoughts on what
that range is?

(2) There are various techniques to impedance matching... resistors and
transformers.. applying capacitive reactance... what has worked for you in
the past? For example, if you go the reactance route, you minimize loss of
power (vs. resistive), but then you tie yourself to a narrow range of
frequencies. That's good if you have a relatively fixed use case. But what
if you have a range of used cases?

(3) My main use case right now is working in the audio range with guitar
amps, working with signal generators and so on. So, a safe diagnostic
range is 80-5000Hz (I know overtones can run up to 20KHz). But let's say
I'm matching the output of an old 600 ohm signal generator to the 1
megaohm input of my Tek oscilloscopes (which all have the same input
impedance). That's an impedance ratio of 1000000 to 600 which works out to
1666:1. By the official math, minimizing reflection via transformer (if I
chose to do so) would require a 40:1 ratio between the primary and
secondary windings (or the equivalent accomplished in stages). I'm looking
for suggestions on this "fixed" case.. but would also like to hear about
any flexible solutions (for example, to accommodate a wide range of output
impedances).

Thank you.


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