DG4000 - a firmware investigation

[Blanc]

I see, thank you.
If there is any progress regarding my defective DG4000,
I will open a new thread.
I would be happy if I could contribute to users of the device.

[TurboTom]

Your diagrams make the situation much clearer now. So the inaccuracies don't happen only at low levels but are rather distributed over the whole level range. On my DG4000, I counted seven attenuator relay switchovers, resulting in eight level bands within which the level has to be adjusted digitally by scaling the DAC input values. The ranges on my AWG are (all in dBm into 50 Ohms load, 1dB steps):

-56...-41
-40...-33
-32...-23
-22...-13
-12...-3
-2...+10
+11...+14
+15...+23

Since there are more than seven discontinuities in your figures (seems rather like eight positive and seven negative "errors"), my take would be that it's not related to the attenuator chain but rather to the digital signal path from the FPGA to the DAC. A problem like this should also result in some distortion of the signal. Did you look at the AWG's output signal in the frequency domain (FFT)? The problem may be related to something as simple as a defective BGA solder joint (hard to repair , depending to your available equipment and experience...). You may try applying some "tender pressure" to the FPGA and DAC casings  to check for a change and maybe identify the problematic component. If it's an internal fault in one of these two chips (or maybe even someting sample-memory-related), things will quickly get pretty awkward.
For diagnosing these problems, a ramp waveform may be better suited due to the repetitive, periodic changes of the data lines. But sure enough, this won't be an easy job.

I don't think it's neccessary to move this "hijack attempt" to a new thread. Contribution frequency on this topic has died down since a long time so a few half-way OT posts won't do much harm, in contrary, it may help keep the information on this particular instrument in one place.

[Blanc]

I don't want to hijack this thread.
However I have no other way to send images,
so I'll just a report in response to the previous question.

I don't see any unusual distortions.

[TurboTom]

Sorry I didn't put it clear enough in my previous message I guess:

For the harmonics test, you better chose a sine wave since you want a signal as free of harmonics as possible. Then, while looking at the spectrum, step by step change the level to "scan" through the discontinuities and observe the spectrum for accompanying changes.

If you don't find any, my take would be that Rigol utilizes the DAC's internal current source adjustment for level control and no "external" data manipulation. In this case, there's a good chance the DAC (AD9781) itself is at fault since the internal current sources are configured via a (comparably slow) synchronous serial interface. If this interface itself would cause the problems, it would be quite unlikely that the instrument worked at all (this interface is common to both channels and is used to configure the whole chip).

I mentioned the ramp signal as a good choice for checking individual data lines of the DAC, not so much for the harmonics test. Sorry for the confusion...