Re: Questions on impedance matching


Ted Rook
 

One of the reasons to put an external 600 ohm load on audio generators is because the
output is usually calibrated for the condition that the 600 ohms is present. If it is absent the
actual voltage present at the output terminals is twice the size, +6dB greater, than the dial
states.

On 9 Feb 2018 at 7:37, David Berlind wrote:

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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