PXI-5406 voltage dropout (function generator)

[charlesjay]

Hi, I have a PXI-5406 that is experiencing voltage dropout problems.  I generate a signal of a constant voltage.  The frequency changes at up to 10x per second, but is mostly constant.  I've hooked up a scope to the signal and can see the voltage drop to 0 at times of rapid change e.g. the frequency will change from 800Hz to 1.5kHz in 20ms.  I've swapped the card with no change.  Is what I'm trying to do within the card's capabilities?  It seems like it should be.  I'm not sure where to go from here.  I'm about to post the code that accesses the card to initialize it and update the frequency.

[SamuRei]

Hey charlesjay,

I think it would be difficult to determine what the issue is here without seeing how you are coding this. It would make sense to me that we would see issues / gaps in a case where we were changing the waveform very frequently; samples of the waveform (let's call it a sine wave) need to be generated and written to the 5406's buffer before being sent out. I could be wrong here, but I don't think changing the frequency is just as simple as appending new information to the buffer of your device.

One thing to consider would be making sure that you only write one period of the waveform in question at a time. This would help to minimize the time that it takes to clear and write in a period of a different frequency wave. 

But a lot of this is just ideas based on how I think your code is running. Posting it for others to look at would be helpful. 

Best, 

[A_Donfigulation_Error_Has]

Samurei, the NI 5406 works a bit differently than an arb, this white paper explains it more in detail: http://download.ni.com/evaluation/pxi/Direct_Digital_Synthesis.pdf . Changing the frequency is a matter of changing the "tuning word" which determines which sample from the LUT is going to be generated next. 

Charlesjay, as samurei has mentioned, I think you will need to post your code. You say that you are generating a constant voltage (DC Signal?) and then you say you are trying to change its frequency, which doesn't make sense. Posting a screenshot of the scope output would be useful as well.

[kirsch]

What you're doing should be within the device capabilities. Frequency changes on 5406 are phase continuous. Post example code that reproduces the issue so we can take a look. Thanks. 

[charlesjay]

hi folks, thank you for the responses.  I'm back to this issue and need to resolve it.  My code is attached.  The Initialize action happens and then Write is called frequently (our main loop runs at 120Hz).  Does anyone see a problem?

[charlesjay · Accepted solution]

SOLVED.  I replaced the configure call in the write step with a property node to update the frequency and the dropout ceased to occur.  There must be something that happens in that configure call that resets the card somehow.  The examples installed with LabVIEW/FGEN use a property node, so follow them 🙂