@Tautech thanks for the offer to investigate. I have found one possible reason already: With HW-acceleration on, you can’t see the individual dots (in dot mode) anymore when in normal or peak detect mode. See my findings below…
For some reason I don’t care so much about all the additional bells and whistles of a modern DSO. I consider them (sometimes very) nice to have, but can’t get upset when not everything works as I’d like. Has probably to do with the fact that I have used analog scopes for decades and only switched to DSO once reasonable sample speed and memory depths became affordable.
So I can forgive bugs in the FW, as long as the basic functionality is solid and the darn thing doesn’t crash. In this regard I had little complaints even with the R1.0 FW. Yes, the logic channels were rather simplistic and buggy, and I really could not understand why the serial decoders would only work with the analog channels.
And then I also thought it was a shame there was no ETS (equivalent time sampling), considering the sample rate drops to 1GSa/s with both channels in a group in use and the bandwidth of the analog frontend, which easily allows signals >500MHz to make it through to the ADC.
Apart from that, the basic operation was quite pleasing. I want good triggering, low noise and high signal fidelity – all combined with a responsive user interface and decent screen appearance (no ugly fonts and loud colors like some cheap scopes have). The SDS2000 R1.0 did deliver on that. The trigger is about the best I’ve ever seen on any scope and the noise performance is pretty impressive. Frequency response is almost perfectly flat, dropping to just 1.3~2.2dB at 300MHz, depending on the vertical attenuator setting (except 2mV/div, where BW is limited to 20MHz) and all channels behave very similar, differences at 300MHz are pretty much within +/-0.5dB. Phase errors and jitter were also not an issue at all.
However, display mode had to be ‘Vectors’ and sin(x)/x reconstruction filtering had to be turned on at high frequencies in order to get a reasonably undistorted signal trace, which was a shame, as otherwise this scope works very well in dot mode without sin(x)/x, where the trace gets a nice analog like appearance and intensity grading is aided by the actual luminance variation caused by the densitiy of the dots. Generally, for frequencies >420MHz the trace started showing double contours, thus making this scope unusable for looking at signals far above its bandwidth. I didn’t consider this a problem, as most analog scopes wouldn’t even trigger at frequencies higher than 30% above their nominal bandwidth.
I have discovered a major improvement with R2.0 in this regard.
It is now possible to view frequencies >500MHz even when in dot mode and without sin(x)/x. In this mode, the signal gets only slightly distorted at frequencies above 300MHz. With sin(x)/x on, the signal remains almost perfect up to 530MHz and starts showing double contours above. This is of course with one channel only and 2GSa/s sample rate. At 1GSa/s, the screen trace is usable up to some 340MHz with sin(x)/x on, and only 200MHz at best without.
Doing these tests, I also discovered an unexpected effect of the ‘Fast’ acquisition mode: not only is it much faster (as the name suggests), but also outputs it a lot more dots on the screen. Probably a bunch of acquisitions is accumulated and output together, which makes a nice line because of the continous phase-shift in each individual acquisition. So with the HW-acceleration in FW R2.0, there is really no need for using vector mode, even at high frequencies. You always see a nice solid trace. This only works in ‘Normal’ and ‘Peak Detect’ acquisition modes though. Surprisingly, on ‘Average’ and ‘Eres’ there are only the true sampled dots visible.
After short investigation the reason for this has been revealed: there is no HW-acceleration available for these modes. Hence no additional dots, but also the waveform update rate drops down to about 12 per second. Don’t consider this to be a problem, since I very much doubt someone would deliberately select one of these averaging modes when high waveform update rates were important for the job…