Avoiding jitter

[Markitus]

Hello,

I am trying to send 3 signals to an oscilloscope, but it turns out there is jitter. I can barely see it, but my supervisor says it is too much. How can I reduce/remove it?? I thought it was due to the program, but I tried with just two signals, a very basic/ program, and I am still getting jitter on the oscilloscope.

I really don't know too much of labview, so I have been using an  example to send these signals through daqmx. You will find attached the program.

thank you very much in advance

[Ben]

I suspect the name of your VI reveals the issue.

Try combining the two outputs into a single task that you open once use then close when the program ends.

Ben

[Markitus]

Hello,thanks for your response.

I did it, also with only one signal, but i still see that annoying (and for me barely detectable) jitter. Even with only one signal and one task.

cheers,

[Ben]

Then use a hardware timed continuous output. As long as you keep the output buffer filled the hardware will clock the output.

If that has jitter then crystal on the board has a problem.

Ben

[John_P1]

What frequency are you trying to generate and what sample clock rate are you using?  Can you elaborate more on exactly what you are seeing on the oscilloscope?

By "jitter", do you mean that the output waveform has inconsistent periodicity?  The DAQ card (you didn't mention which one) will update the output sample-by-sample.  If the sample rate isn't an even multiple of the desired frequency then you will have some inconsistency from period to period as the samples won't fall at the same phase for every period.  Keep in mind that the sample rate is an integer divide-down of the timebase available on your DAQ device (e.g. 100 MHz for X Series, 20 MHz for M Series) so you can't simply sample at a rate which goes evenly into your frequency for all frequencies.

If this is a problem, you can mitigate the behavior by outputting at a faster rate--be sure to increase the buffer size (roughly proportionally) as you increase the sample rate to avoid underflow errors.  I would probably recommend just outputting at the maximum update rate for your DAQ card.

Best Regards,

[Markitus]

Thanks for your reply John and Ben.

What I am having in my signal is a "vibration" in time and voltage. In the actual program, I have two signals through one task, and a third through another task. I have a 5V and 80 Hz sinusoidal signal, and a 800 square signal. I have used an example to make the daq-mx, and I believe they are hardware timed. Well, on the oscilloscope, i can see how the whole wave slightly oscillates, around 0.05V. The time jitter is very clear in the fast wave, and it is around 15micro s.

I have done as you told me, and now I am using 800000 as Sample clock rate, and 80000 as buffer size, but i still have this jitter.

My daq card is a PCi-6289.

I have attached the actual program with the 3 signals.

thank you very much for helping with my noob doubts, I haven't found very helpful the tutorials I have found on internet to solve my problems.

[MarcoMauri]

Try generating a single constant-amplitude sinusoidal wave (removing the math script node).

The "vibration" is still there?

Marco

[Markitus]

Hello, thanks for your response Marco.

I have done as you said, removing the mathscript and setting a constant amplitude, but I still have jitter.

Find attached a video of the oscilloscope

Thanks for your help,

marc

[Ben]

Dumb question (I can't watch videos here).

Is the "jitter" in time or amplitude?

Jitter generally applies to time. If it is amplitude your issue may be due to resolution.

Ben

[Markitus]

There is a clear jitter in time on the fast signal (800hz), and there is the voltage jitter, that i can't barely detect ( but my supervisor finds it obvious), and it affects the sinusoidal (the slow one, 80Hz) and the square signal ( 800hz)