Electronic – Gain-Phase measurement without dedicated equipment

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I was wondering if it is possible to make Gain-Phase measurements without dedicated equipment like the venerable HP4194A or a more modern Bode 100.

As far as I understand, a Gain-Phase analyser is a device used to measure the transfer function of a DUT (Device Under Test) like this:

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LF-OUT generates a sinusoidal signal, R measures the input and T measures the transmitted signal. Computing T/R (taking care of magnitude and phase, of course) one can computed the transfer function.

If I don’t have such a dedicated equipment, I thought I can make the same measurement simply with a function generator and an oscilloscope like in the following picture:

enter image description here

Then I can compute the transfer function as Ch2/Ch1 (taking care of magnitude and phase, of course) Would this setup replace a Gain-Phase analyser? Or am I missing something?

Many thanks in advance!

Best Answer

Just use a dual-trace scope and a function/sine generator; and trigger the scope sweep from the function generator's "sync" output.

At each frequency of interest, adjust the scope's "variable time" knob (you do have one, right?), to use exactly NINE horizontal divisions per input sin cycle; thus you have exactly 40 degrees per division; most scopes have either 4 or 5 small time-tics per major division, thus you can easily interpolate to 1 or 2 or 4 degrees.

ADVANTAGES? you get to WATCH how the circuit performs over the tested frequency range;

(1) if the time delay is not absolutely stable at each frequency, you probably have spurious oscillation, to be debugged. Maybe ensure the bypass caps are installed, so the 2 meters of wire from powersupply to circuit is not causing problems

(2) you get to look for clipping at all frequencies

(3) at some frequencies, because of poles and zeros, you'll see very small output; is the RANDOM NOISE AND POWER SUPPLY NOISE about what you predicted?

(4) you get to discover SLEWRATE limiting

(5) you get to see blatant cases of distortion; 2nd order causes lopsided output sine shapes; crossover distortion in a class_A? cannot happen, right? but if you do?

SUMMARY: you are responsible for all modes of behavior of your system. Be responsible. Examine the waveforms. Otherwise your boss will have to hire a consultant later, to clean up the mess you made.

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