I'm using the function generator on my 5444B to make square-wave output at 16 MHz. I find that the actual waveform jumps between two states, each with different on-state duration. I see this both when using the PicoScope to measure the signal, and when using a separate stand-alone scope.
I've included a screengrab below [1] of persistence mode to show the two state problem. Notice (e.g.) the second pulse has a single rising edge, but two falling edges (one rarer than the other), so the on-state duration is different in the two cases. There's a periodicity of ~1 second to the cycling between the two states. I can record and post a screencast showing this if that would be useful.
Also, at a lower frequency (e.g. 10 MHz), I do not see this problem [2].
Thanks for your help.
[1] Persistence mode at 16 MHz showing the two-state problem
[2] Persistence mode at 10 MHz showing normal behaviour.
Can you post a psdata file, File->Save and save the current waveform, as this will give all your settings.
A periodicity of ~1 second to the cycling suggests that you have trigger mode set to auto. Auto means that after a period of 1 second the trace will be drawn even if the trigger condition has not been met.
I'm using repeat for triggering. The periodicity is actually shorter than 1 sec. Either way, I don't follow the argument. Why would an auto trigger affect the width of the waveform? Also, I see the same problem when viewing on a separate scope in "normal" mode.
This is a standard effect of Direct Digital Synthesis (DDS) waveform generation.
The Generator DAC has a fixed update rate, in our case 200MS/s, so the output signal can only change every 5ns. If you are generating a square wave then the DAC can only output an edge on the 5ns boundaries, so if your signal period is not a multiple of 5ns the edge will jitter around by one sample.