Display jitter

[cafe_liegeois]

I'm trying to decide between purchasing RP or another solution running under Mac OS with normal audio interfaces. My main problem with the Mac solution is some strange kind of periodic display jitter in the scope and FFT analyzer with external signals or internally generated signals that are fed back with a certain amount of delay.

So, I've looked at RP as a possible alternative and found this video:
https://www.youtube.com/watch?v=Qvv29N0Td-4

She's using one RP to generate some sort of square wave signal and a second RP to display it. And, lo and behold, at 2:35 into the video there is exactly the same horizontal jitter I'm having on my Mac solution. And not only that: the FFT display (3:00 into the video) shows the same rhythmic fluctuations that I'm seeing in the demo version of my Mac software.

Is this some general phenomenon that is inherent to this kind of measuring system or what am I missing?

Ralf

[pavel]

Hi Ralf,

I think that this kind of jitter could be observed with any oscilloscope. Basically, the simple level trigger fires at slightly different moments because of the noise.

You can find a detailed discussion of this problem in the following video:

https://www.youtube.com/watch?v=FwCrS7jCMpM

Best regards,

Pavel

[cafe_liegeois]

If it were caused by noise the jitter would be random, wouldn't it? Here, it's quite regular.

The funny thing is that it doesn't happen when I feed the signal of the internal generator back into the measurement input either directly or via an external DUT with very little delay. In both cases I get a perfectly stable display. Happens only with externally generated or delayed signals.

Ralf