Question about single shot

[ksat44]

Say you want to measure the startup activity of an unknown power supply, but you don't have a clue what the amplitude or frequency of that signal is going to be. You want to use single shot to do the capture but, since the signal is unknown, you're not sure what to set your volts and time/division values to so that the waveform displays properly. Your scope also has a limited memory buffer, which makes post-capture adjustments somewhat difficult.

What would you do in this situation? Would you keep re-taking the capture, adjusting the scope's settings each time until you get something you can adjust properly? I guess there's no auto triggering-like feature when using single shot, so I'm just curious what approach one would take in this situation.

[ataradov]

You can use "Normal" mode, which would automatically retrigger and hold the waveform until another trigger arrives. So, it will auto-update every time you power cycle the power supply.

But generally, yes, for unknown signals, just set parameters you think are close and then adjust as needed.

[DavidAlfa]

First set V/div to slightly over 0, time/div to a large value, try between 10 to 100ms/div, this should capture plenty of time for the startup preview, this should give you an idea of what to do next.
After this adjust time/div, v/div and trigger level to capture whatever you saw earlier with best detail.
Of course this will need several tries, but it's nothing complicated.

[BillyO]

What memory depth does your scope have?

Following DavidAlfa's suggestion of setting V/div to slightly over 0 (or under 0 if the PS is negative) and time/div to 100ms/div should enough, then just zoom in on the captured trace by deceasing the time/div until you see what you want.   Then further captures can be done with the new setting.

[ksat44]

Thank you for the tips. The scope is one of those SigPeak/Zeeweeii 2 channel "jobbies," as Dave would say. The manual claims memory depth is 128k points. I've been messing around with it since I got it a week ago and that's when the question about single shot came up. I'm not actually trying to measure SMPS startup activity, rather just used it as an example of a transient signal I might want to capture where the amplitude/frequency were unknowns.

[Doctorandus_P]

In general you make a "best guess" first and then attempt to refine the options. Quite often you can also run the scope in "auto" or un triggered mode to get some idea of the voltage and frequency content on the scope input.

If it is a rare event you want to capture, then spending more brain energy in the intial guess may pay off. Even relatively affordable scopes have a quite deep memory these days, and often you can zoom in a lot more then you can zoom out. More and better trigger capabilities are one of the main distinguishing marks of higher end scopes.