If you have some time, I would appreciate when you can test the last modifications (calibration and ZLoad mode) on your system.
Oh, I forgot to respond: I did do the calibration and Z-mode yesterday.
Very nice, that low .1Hz nearly DC routine to calibrate using multimeter. Very nice and simple.
After calibration, when I next checked by putting out a 50Hz 1.41Vpp and 14.1Vpp sine on a TrueRMS multimeter, the voltages were slightly off, but better then before. From memory, I think the DDM gave 0.498 and 4.89Vrms, but I lost the piece of paper I wrote it down on. I think being within 2.5% is good enough for a device like this.
But still, While I appreciate the simplicity of the .1Hz nearly DC routine, I wonder you should not build the routine on a 50Hz sine.
The .1Hz routine only being usable for offset. And a 50Hz signal for amplitude. (Note that for a good sine, Vac and Vrms are the same, so a TrueRMS DMM is not even needed)
The DSO (DS1070Z-S) disagreed quite a bit with the DMM. Forgot the numbers, but at 50Hz at least 10% off. I assume the DMM is correct. But the DMM cannot be trusted above 400Hz or so, for 1MHz or 10Mhz signal it probably does not even see a signal at all.
So I am not sure if you plan to extent the calibration toward attenuation, but in that case, we'd have to come up with something smarter.
By lack of a smart idea, the only thing I can come up with is to read and use the error of the DSO at 50 Hz and next read the DSO at 1MHz, and first apply the 50Hz multiplier.
But not sure if that is reliable. Maybe we should for now not bother with attenuation correction.
I suspect you already went through this reasoning anyway.