So yes, just like other manufacturers' implementations of High Resolution mode, Rigol's implementation acts exactly the same way - and will filter the waveform (and cause anti-aliasing if the effective sample rate is reduced too far for the incoming signal).
The complaint isn't just that "high res" happens post acquisition, but that Rigol's "one pixel per column" algorithm aliases out components that are too high frequency for the current time base.
See that the sample rate is 1Gsa/s in my screenshot, which should leave plenty of effective sample rate for a 100kHz signal.
Agilent produces the results I expected, and with far less sample rate.
As far as I can see, the Rigol is not acting the same way as the Agilent.
What's the point of using high resolution mode for display of an envelope? If you use normal aquisition mode the Rigol display looks exactly the same as Agilent and accomplishes all you could wish for.
"within the same acquisition" means oversampling. Oversampling is done at acquisition time before storing each high-res sample.
Yes there is a filtering effect, but this is firstly applied to the "oversampled" data rather than the final trace samples.
Naturally, now that I know more what it does, it can be a useful tool. One just has to be aware that at 1ms/div + high res, the effective bandwidth (on screen) is like 5kHz.
Rigol's method stores only 8 bit samples. There is never any oversampling. It's not an acquisition mode at all. Any "high-res'ing" is derived, at display time, from what is stored.
It can only add low-pass filtering on top of what's stored, and what's more it doesn't even say how severe this filter is let alone have any parameters.
Agilent's method... need not create any extra filtering beyond the stored sample rate ... and probably ensures it never does by adjusting the bit depth accordingly.
The maths is not the problem. It's the stored trace that's the problem. One method perform oversampling and filters only to the Nyquist point and stores those high-res samples as the trace. The other method just stores 8-bit samples straight from the ADC ... which is nyquist limited and still only 8 bits. Any further processing cuts-off even lower.
I bet Rigol's method could be, but it doesn't let you, flipped on and off - refreshing the display in either Normal or High Res without any new trace acquisitions.
As far as I know, the only time Rigol doesn't let you change the High Res setting on existing data is in record mode. It absolutely does let you change the setting when stopped normally.
There's a practical difference between the two approaches and it affects record mode: You get to store fewer waveforms if you want the high res averaging.
I'm not sure I understand you: on my DSO I haven't noticed any difference in the maximum frames I can record when using High Res mode.
I admit I'm operating on some assumptions there; I'll do some experiments in the next day or two and get back to you.
I wish there were owners of that Agilent DSO who did testing
I foresee Agilent dropping out of the lower end market.
See that the sample rate is 1Gsa/s in my screenshot, which should leave plenty of effective sample rate for a 100kHz signal.
Time base - | Bandwidth (-3db) - | First null in stopband |
10ms/div | ~4.3kHz | ~10kHz |
5ms/div | ~8.6kHz | ~20kHz |
2ms/div | ~21.6kHz | ~50kHz |
1ms/div | ~43.3kHz | ~100kHz |
500us/div | ~86.6kHz | ~200kHz |
200us/div | ~173.2kHz | ~400kHz |
100us/div | ~346.4kHz | ~800kHz |
50us/div | ~692.8kHz | ~1.6MHz |
20us/div | ~1.38MHz | ~3.2MHz |
10us/div | ~2.77MHz | ~6.4MHz |
5us/div | ~5.54MHz | ~12.8MHz |
The maths is not the problem. It's the stored trace that's the problem. One method perform oversampling and filters only to the Nyquist point and stores those high-res samples as the trace. The other method just stores 8-bit samples straight from the ADC ... which is nyquist limited and still only 8 bits. Any further processing cuts-off even lower....
There is no such thing as oversampling when the DSO is already sampling at it's maximum rate. I can't speak about the DS1000Z, but the Rigol DS2000 series can sample at it's maximum 2GSa/s rate down to 2ms/div - it doesn't need to oversample because the sample memory already contains all possible samples that could be captured in a given time frame.
I bet Rigol's method could be, but it doesn't let you, flipped on and off - refreshing the display in either Normal or High Res without any new trace acquisitions.I think this is a clue to Evan's issue. As far as I know, the only time Rigol doesn't let you change the High Res setting on existing data is in record mode. It absolutely does let you change the setting when stopped normally.
There is a distinct acquisition mode change here, it changes the content of the stored samples. The Rigol's don't attempt to perform this function and therefore are not implementing a high-res acquisition mode.
I was only giving an example of what could happen because the stored trace is no different between Normal mode and High Res mode. Which is not the case on scopes that have a true high-res acquisition mode.
I can assure you this is a big issue for everyone. Rigol's High Res mode should be avoided. It will create problems if used.
High-res acquisition mode operates in oversampling only
Rigol doesn't do this, so, although it is high-res, it's not an acquisition mode.
See that the sample rate is 1Gsa/s in my screenshot, which should leave plenty of effective sample rate for a 100kHz signal.@Galaxyrise: Interestingly, the reason the 100kHz signal is so attenuated in the image you posted is that it falls almost directly in the first null point of the stopband @ 1ms/div (see image above which shows the null points of an averaging filter). Compare it to this image using the same settings as you, but with a 150kHz sine wave - which falls beyond the first null point (although still attenuated by -12dB):
Playing around with sending sweeps to the Rigol while in High Res mode reveal the nulls that exist in the stopband at each time base setting (while using 14MB/AUTO mem depth). These null points are at the averaging frequency (sample rate/number of samples averaged) and its harmonics.
Bandwidths @ different time base settings in High Res mode on DS2000
Time base Bandwidth (-3db) First null in stopband 10ms/div ~4.3kHz ~10kHz 5ms/div ~8.6kHz ~20kHz 2ms/div ~21.6kHz ~50kHz 1ms/div ~43.3kHz ~100kHz 500us/div ~86.6kHz ~200kHz 200us/div ~173.2kHz ~400kHz 100us/div ~346.4kHz ~800kHz 50us/div ~692.8kHz ~1.6MHz 20us/div ~1.38MHz ~3.2MHz 10us/div ~2.77MHz ~6.4MHz 5us/div ~5.54MHz ~12.8MHz
Bandwidths @ different time base settings in High Res mode on DS2000
Time base Bandwidth (-3db) First null in stopband 10ms/div ~4.3kHz ~10kHz 5ms/div ~8.6kHz ~20kHz 2ms/div ~21.6kHz ~50kHz 1ms/div ~43.3kHz ~100kHz 500us/div ~86.6kHz ~200kHz 200us/div ~173.2kHz ~400kHz 100us/div ~346.4kHz ~800kHz 50us/div ~692.8kHz ~1.6MHz 20us/div ~1.38MHz ~3.2MHz 10us/div ~2.77MHz ~6.4MHz 5us/div ~5.54MHz ~12.8MHz
Thanks for this table, BTW. That's very useful info to have, and not provided by any of the vendors, to my knowledge.
By limiting the BW seems to be designed for audio applications, or something like that. For now I never used this mode.
I am more interested in the ANTI-ALIASING option, I would like to know how RIGOL implemented it, what sampling method used etc...