stair steps on digital scope

[digitaladdictions]

I recently replaced my Rigol DS1102E with a Rigol DS2202A with all the software upgrades included. I bought a Siglent SDG2042X and hacked it to 120MHz at the same time.  I also have a Tektronix 2246 analog scope.

What I am curious about is the stair step appearance of a signal when a lot of information is displayed on the screen. For example a 1MHz sine wave with a horizontal setting between 1us to 5us where the screen has a lot of signal information to display. On the digital scope you can clearly see stair steps which are not shown on the analog scope. The memory depth is set to auto and is choosing around 56Kpts. Manually setting the memory depth has no effect on the stair steps either higher or lower.

If I look at just a few cycles 500ns / division or smaller I do not get the stair steps. Well if I look really closely I can see smaller stair steps but I am attributing that to the DDS function generator vs a real analog source. I am more curious about the large steps when more information is being displayed.

Is this due to the screen resolution?   

[digitaladdictions]

Actually, I think I may have just answered my own question. Since reducing the vertical scale increases the amount of stairs displayed and thus more accurately represents the signal I would tend to believe this is due to the display resolution of the screen. A shorter distance to travel from the positive peak to the negative peak in the same horizontal distance means fewer horizontal pixels are needed.

I know this is still sort of an entry level scope and is showing its age as well. It's a shame the screen is not a higher resolution though. The effect of the screen resolution was not even something that crossed my mind when checking out my options. I just assumed they were all good enough and the screen had more to do with physical size than resolution. Not really a deal breaker and it's still a real nice upgrade over the DS1102E. I just want to be able to speak authoritatively on why the signal looks the way it does.

[theatrus]

I assume you are capturing a new signal (normal/auto trigger) and not rescaling a single trigger?

Like most scopes in its class it has an 8 bit A/D, so single captures have at most 256 vertical steps. When displaying continuous signals the scope will use "digital phosphor", which is visual averaging/decay so stepping won't be prevalent.

[digitaladdictions]

Correct.  I am not talking about scaling a captured signal.  I am away from my lab right now but had the following screnshot on my desktop. The measurements displayed are all over the place since its a fairly wide selection of an FSK signal but they are irrelevant.  Each rising or falling edge of the signal is made up of at most 10 discrete steps. 

The much smaller steps that I blamed on the DDS generator very well may be the 8bit DAC of the scope.

[theatrus]

That appears more like display aliasing (fast signal, not much horizontal resolution).

[tooki]

Agreed. The limit here is the horizontal resolution, not the vertical. Use a faster time base and you'll see the steps disappear.

[David Hess]

Based on the record length and sampling rate, that is aliasing of the display.  That should not happen on an index graded display but cheap DSOs are cheap.  At best Rigol traded good display processing for update rate.