Displayed waveform is out of phase with actual

I’ve been looking at clipping performance of several preamp designs and noticed that the waveform that is displayed bu the QA401 application is 180 degrees out of phase with the waveform I see on the scope. So if I’m seeing the positive peak clip on the scope, the QA401 tells me it’s the negative peak, Just as an idiot check, I’m driving a series circuit of a 1k resistor into the Anode of a small-signal diode, and looking at the output waveform at the anode. As I would expect, the scope shows the positive peak clipped at about 680mV. The QA401 is telling me that the negative peak is clipped however. I’m using the Left channel + output and measuring on the Left + input, with the - input shorted.

I’ve checked my cables and verified the polarity is correct, and the scope is not inverted. The two signals are in phase on the scope - so if there was a phase reversal in one cable, I would see it.

The Right channel of the QA401 does the same thing.

Hi @voltist, confirmed on this end. It will be fixed in the next release. Thanks very much for reporting!

Thanks, good to know I’m not going crazy. Hard to believe I’m the first person to notice this.

When will the next release take place?

Probably the next release will be in the next few weeks or so…

Hi @voltist, release 1.910 is up and should address the issue you saw. I verified with an external scope AWG on input, and input/output of the QA401 also match. Should be very low risk change but it still needs a bit more validation.


Ugh, hold off on this, something is off

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This won’t be an easy fix to make. For now, this won’t be fixed. You’ll just need to note that the displayed phase of external signals in time domain is inverted.

That’s really disappointing.

You can try release 1.913, which has time domain display inverted. This inversion is only on the display. But it will display the correct slope of an externally applied ramp or, in your case, an asymmetric clipping. The down side is that is displays the polarity of generated signals as inverted. But since the QA401 generates sines only, the only discrepancy you’ll see is on the ramp of the sine. In the plot below, notice the first excursion of the burst is shown as negative going. In fact, it’s positive going. All maths are the same.

So, the tradeoffs are related to the hardware. We can do 1) or 2)

  1. Generated phase is displayed inverted (but unchanged for actual generation), but externally applied signals are shown correctly.

  2. Generated phase is display correctly, but externally applied signals are shown inverted.

For both, the issue is related only to display. All other maths remain the same.

Normal behavior is 1). We are trying 2) on release 1.913 to understand what other issues might arise.

Thank you, I’ll test it out. I appreciate the help.

Running 1.913 Unfortunately the phase measurement says it is 180degrees. I can get an in-phase measurement if I use the L- output instead of L+

It’s also reporting 6dB of gain - but I’ve verified that my UUT is at unity. - NEVERMIND - just found that the external gain was set in the software at -6dB. Also just updated to 1.915 - the phase error seems to be gone, it’s now correctly reporting 0 degrees. I’ll continue with this version for now.