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Re: test for transients

mfc,
an important thing for a test like this is to know what to expect to happen so you can interpret the results. I can boldy claim that I've no idea what to expect which frees me to conjecture ..... The intermods from the square wave harmonics will all fall on top of the harmonics themselves, some will add and some subtract. If you subtract out the harmonic series for an ideal square wave you should se a difference but I think it will be impossible to interpret the results. Maybe you could run the same test for a range of input transients of differing amplitudes and rise times and plot the output harmonic amplitudes in input intercept fashion - you should expect to see 3rd order products rising at 3x the fundamental rate (log, or dB y-axis, log freq axis), 5th order rising at 5x etc.
You may find a region where this breaks down indicating that the non-linearity has changed with input level of slew rate. Again, interpretation may be difficult beyond setting that input condition as an upper bound for 'good' amplifier behaviour.
I mention different input amplitudes and different slew rates because I've always assumed that TIM is due to amplitude limiting at the input to an NFB amp. during the open loop rise time. Others use it to mean slew rate limiting (it's been so long since I read the papers that I can't remember which definition Otalla used but I think Daugherty & Greiner were concerned with amplitude limiting. But then they're both related by power bandwidth so maybe it doesn't matter).

I'd suggest making a harmoinc intercept plot with just a sine input and see if the there is a break point where the slopes change and then compare to any results you take with square waves.

On a related note, I recal a couple of papers from Japan testing amplifier non-linearity using an assymetrical square input pulse with no DC component (the input pulse looked like a top-hat with the brims negative, if that makes sense). Non-linearity was assessed from the frequency response of the output FFT. If I recal this test showed that audio amplifier non-linearity was of the same order as loudspeaker non-linearity whereas sine testing would say the amps are significantly better. I'll look out a reference for you.

Regards
13doW

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