Sine.vi clicking when used for realtime output with DAQ Assisstant.

Question

What I'm Trying to Do: Stepped sine analysis. The program outputs from the DAQ device each of the frequencies entered into the array constant on the left (see VI) in order, taking a measurement of the magnitude of the signal input to the DAQ device for each frequency. The program only takes a measurment and moves onto the next frequency once it has decided that the magnitude of the input signal is stable. Stability is defined by the stablility upper and lower limits labelled in the lower double nested loop (see VI). The program measures the inputs magnitude 10 times as second, and calculates the difference between the current magnitude and the last magnitude each time. The difference must be within the stability limits 20 times in a row for a measurement to be saved and the frequency changed.
The Problem: phase of sine.vi is resetting each time round the loop for some (not all) frequencies, even though it's set not to, causing clicks (number of clicks per seconds depends on block size). This doesn't happen if the lower nested for loop (the one that checks for stabliity, labelled on the attached VI) is removed. I thought that maybe the two DAQ Assistants (one for output and one for input) were interfering with eachother, but the problem occurs even when the DAQ Assistant Input is changed to a signal generator.
Thanks in advance for any help,
Christian
Message Edited by crnbaker on 12-08-2009 05:00 AM
Attachments:
steppedsine_forum.vi ‏116 KB
steppedsine.JPG ‏74 KB

Answer 1

Hi Christian,
I have been looking into this problem though I must warn you that since I don't not have your exact hardware configuration I can not exactly recreate your problem. I have also carried out some research to determine whether there are any known instances of this problem, and unfortunately there are not.
The fact that the VI runs OK when the lower loop is deleted suggests to me a problem in this lower loop. Have you tried initialising the shift register. An uninitialised shift register could explain such inconsistent behavior. Symptomatic of this kind of problem would be the VI running correctly the first time you run it and then producing unexpected behavior subsequently.
Best Regards,
Christian Hartshorne
Applications Engineer
National Instruments