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Glitchy AWG output on 5444B

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Glitchy AWG output on 5444B

Postby wellesley » Thu Jan 08, 2015 4:27 am

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
persistence_16MHz.png


[2] Persistence mode at 10 MHz showing normal behaviour.
persistence_10MHz.png
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Re: Glitchy AWG output on 5444B

Postby Martyn » Thu Jan 08, 2015 8:33 am

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.
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Re: Glitchy AWG output on 5444B

Postby wellesley » Thu Jan 08, 2015 4:05 pm

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.

Here's the psdata file:
glitchy.psdata
(24.86 KiB) Downloaded 47 times


Also attached are screencasts showing the phenomenon at 16 MHz. The waveform is stable at 10 MHz.
Glitchy at 16 MHz
http://youtu.be/BLgapdHh2HI

Stable at 10 MHz
http://youtu.be/MF_fnJGGglw

I see this effect at a range of frequencies. e.g. 3, 6, 7, 8, 9 MHz, but not at 1, 2, 4, 5, 10 MHz.
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Re: Glitchy AWG output on 5444B

Postby Martyn » Fri Jan 09, 2015 10:34 am

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.
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