Rigol MSO5074: Why it triggers only on one type of edge

[listerreg]

Hi everyone,

I've just bought this oscilloscope and I'm trying to understand why when I set the edge trigger to "either" it usually catches constantly only the rising or only the falling edge in the Run mode.

I've got a simple sine wave generated by a DAC with the frequency of about 4Hz. All the oscilloscope settings are default besides the edge triggering is set to either slope and the trigger level is in the middle of the signal envelope.

When I set the scale so only quarter of the period is captured I have this somewhat straight line on the screen. The thing is that it is usually not changing from rising to falling repeatedly but only one "type" is displayed.

I can actually change which part of the wave is displayed (the rising or the falling slope) with some irrelevant actions like pressing a menu button.

If I want all the occurrences of the rising and falling slopes in the wave to be displayed repeatedly I had to "zoom" the scale so only 1/10th of the period is captured.

Why with a such low frequency signal the oscilloscope is not triggering on the actual next slope?

Thanks

[tooki]

Maybe you have a hold off time set?

[listerreg]

The hold off is at its 8ns default setting.

[Fungus]

Hi everyone,

I've just bought this oscilloscope and I'm trying to understand why when I set the edge trigger to "either" it usually catches constantly only the rising or only the falling edge in the Run mode.

Maybe the time between triggers is very close to the frequency of your signal so it just happens that way. Coincidence.

[T3sl4co1l]

Zoom in.  I'm betting there's a transient, either coincident with DAC update, or possibly other ambient noise (but the latter is likely to be uncorrelated, so the waveform may jump around visibly).

Tim

[newbrain]

Maybe the time between triggers is very close to the frequency of your signal so it just happens that way. Coincidence.

I think this is the right explanation, one thing to try is a different setting for the memory depth of the capture - this will affect the time it takes to fill the buffer, and possibly decouple the trigger period from the input signal one.
Shorter memory lengths should  work better.

[TurboTom]

If your trigger mode is set to "Auto", change it to "Normal". At least on Rigol's older scopes, auto mode won't very well work with slow signals. Maybe it helps on the 5k series as well.

[listerreg]

In my signal conditions are met once every +100ms and my buffer holds 50ms worth of samples (500 MSa/s, 25 Mpts).