HP/Agilent/Keysight 33120 amplitude jump at 78.125 kHz

[electronomicon]

Hi folks,

I recently did a performance test on my 33120 A function generator. Everything was fine except for the amplitude flatness (about 150 mVrms off over the whole tested range).
I did an AC amplitude flatness adjustment according to the datasheet but noticed quite the strange behaviour afterwards. The amplitude is now fairly flat over the whole range, except for an odd jump of about 120 mVrms occuring between 78 and 79 KHz, which is of course outside of the device specifications.

I measured a deviation of 3.7 mVrms from 1 kHz to 78 kHz, 120.2 mVrms from 78 kHz to 79 kHz, and 10,0 mVrms from 79 kHz to 300 kHz (the AC voltage frequency limit of my 34401A multimeter).

At first, I thought this might be a calibration error, but there is no calibration step happening at 79 kHz (only 1 kHz and 100 kHz) and the problem persisted between calibration procedures.

Does anyone have an idea what might cause this or where to start looking? I suspect a fault somewhere in the device, but I'm a bit lost on how to get at this right now.

[Someone]

Do you have other instruments to rule out that multimeter being the source of the error?

[electronomicon]

Yes, the jump also shows up on my UT61E+ handheld multimeter and on my oscilloscope.

Edit: I decided to inspect the jump more precisely. The jump appears exactly when switching between 78.124,999 kHz and 78.125,000 kHz.

Edit 2: Just a quick update, thanks to this video I got an idea where to start diagnosing the fault.:


Currently I narrowed it down to appearing somewhere between the AMP_CTL signal (measured at R413, stable) and the Waveform DAC output (measured at R405 and R406, 30 mVrms).

[electronomicon]

After some more probing, the problem manifests itself around U410 (a TL074 op-amp).
The output of U409 (the dynamic flatness correction DAC) is stable, goes into U410A and out comes the voltage jump (37,5 mVdc at that stage) which then moves further down through U410B, U314 (another TL074 op-amp), and Q401 to end up on the DACOUT lines, at least that’s what it looks like to me.
I’m still a bit sceptical, though, towards U410 being the culprit. This is quite strange behaviour for an op-amp. Also, the problem only shows up on sine wave output, not on square and triangle waves.
But if it didn’t come from U410, the only other option I can see is U407 (the main waveform DAC) somehow backfiring into the amplitude adjustment circuitry, which seems quite unlikely, looking at the schematic.

Any thoughts on this? I’ve attached the section of the schematic with my measurements for reference.

Edit: I replaced U410, problem remained. I recalibrated it two more times and got it to barely stay within the specified limits*, still just because of the single jump, otherwise it’s really flat. Given I don’t really have adequate equipment for measuring high frequency AC at hand, I’ll just leave it at this for now. That’s actually good enough for my use cases.
I’m still curious, though, what causes this weird jump …

* Depending on which device I use … the 34401A reads a delta of 0.0694 Vrms from 1 to 100 kHz, my 54645D oscilloscope reads 0.021 Vrms. Specified is a maximal error of +/- 0.03 Vrms.