Hi all,

Learning the ways of the oscilliscope, I am trying to capture the process of charging a capacitor with a long RC time constant (tau ~ 2 sec). I thought "single shot" would work great, but my scope never actually displays the waveform.

I have a Rigol 1000E series, and I followed the instructions in the manual (set the trigger threshold, use upward slope, etc). If I look at something on the scale of milliseconds like switch bounce, it captures the wave form great, so I think I am performing the setup more or less correctly.

Can anybody comment? Is it something with the scope capturing for a long period?

Interesting (to me): If I use the "rolling" view, I can watch my slow process and stop when I want to look at the waveform. Rolling is useless with quick things like switch bounce but maybe good for stuff like this.

 

Try DC coupling on your trigger source.

 

Try DC coupling on your trigger source.

It is DC coupled.

Like I say, I have it working with shorter time scale things.

 

I have the Ds 1054Z, and the problem you describe is most likely related to the scan delay/averaging that the scope does. It is the one thing about that scope that makes me turn to an analog or older TEK TDS210 on occasion. It is probably true about all DSO's but to a much less extent for high-end ones like Agilent/Keysight. Dave's review of the apparent noise/thickness of the trace for DSO scopes (see: EEVblog) discusses it too. In that review he compares 4 different DSO's, includeing a very expensive one.

Here are some things to try:
1) In the "Acquire" menu select "Peak." You can also try the "High Res" setting.
2) Decrease the memory depth. I use 3 kB
3) Shorten the scan time -- you may not capture the entire curve, but after you catch it, you can increase the scan time one or two clicks.
4) Set up a repetitive signal on another channel and trigger the scan on that.

What time constant are you trying to capture?

John

 

Try triggering from your 2 sec tau circuit on faster time scales and then see how slow you can go before it doesn't work.

 

Presumably, your circuit looks something like a battery, a switch, a resistor and capactor in series. If you put the switch between V+ and the resistor then the capacitor then ground, you can trigger off the other channel right after the switch.

For a 2sec capture you can also trigger it manually just before throwing the switch.

Hope this helps.

Bob