FFT of the portion of a signal

[franky_parrez]

Hi dudes

When trying to get the FFT (with the spectral measurements VI) of a portion of a signal, I get the following error: "The waveform dt parameter is <= 0".

I attach a simple program where I split the input signal in two and try to get the FFT of both parts.

Thank you for your attention!!

[johnsold]

I do not have the DAQ stuff so it is hard to tell.  Can you run the VI and save some typical data as default on the Input Signal graph?

Which VI is generating the error?  What is the exact error message and error code?

Lynn

[franky_parrez]

Hi Lynn,

I attach an excel file with the data of a typical input signal. The VI failing is the Spectral Measurements Express. The exact text of the error is (it appears just after I run the program):

"Error -20315 occurred at TEST.vi
  (waveform index 0 of 1)

Possible reason(s):

Analysis:  The waveform dt parameter is <= 0."

Regards and thank you for replying.

Jose

[Stephan_D]

Hey Jose,

please post your VI in a way so that everybode can run your program without the hardware and see the same error like you. It would be nice if the people doesn't need to rework your code so that they can load the data from the Excel file.

Also, include a Screenshot of the error. This is the only way the people in this forum can help you without to spend more time than necessary.

Stephan

[franky_parrez]

Hi Stephan,

You are right. I have simplified the program but I was unable to build the input waveform from the excel data (I have been trying the last hour surfing the forum; I have to say that I am rookie in Labview).

Regards,

Jose

[Stephan_D]

Hi Jose,

I tried your simplified VI with a simulated DAQ card. I can run the VI without a error message or something else. How does the error look at your PC?

Stephan

[DFGray]

Your original VI runs without error on my machine with a simulated DAQ device.  I suspect your hardware is stopping the acquisition before you get over 2500 points, so your second section has no data, giving you the error.  Use probes to look at the data on your wires when you are generating the error (right click the wire and select Probe, then run).  If this is the problem, then fix your program logic.

[franky_parrez]

Hi guys,

I made a mistake in the simplified VI I posted before. The inclusion of a trigger is essential, and I think the problem is with the trigger VI. I attach also a screenshot of the error. The purpose of the trigger is to catch the sound pulse produced by an ultrasonic whistle, and then I want to split the signal in different segments in order to analyze the frequency spectrum in each of them.

Kind regards, and I really appreciate that you worry about this.

Jose