insufficient frequency resolution error

[gchaw]

Hi,

I am using Labview SignalExpress to send a voltage and then receive another voltage. I'm using an example which I modified such that the physical channels match the ones on my device. The example I modified is the SVA_Amplitude Swept THD (DAQmxAO-AI).seproj

The trigger source is PFI4.

I'm using an NI USB-6218 card.

The error we keep getting is "insufficient frequency resolution at the "Tone Measurements" step.

The error states:"There is insufficient frequency resolution to perform this measurement. Improve the frequency resolution by increasing the number of samples used to compute the FFT and/or decreasing the sampling frequency of the acquisition."

I was wondering if anyone could help me fix this error.

Any help is appreciated. Thanks!

[gchaw]

Actually, the error comes from the THD measurement which cannot process due to another error occurring during the execution of the sweep. The first iteration of the sweep is correct. But for the following iterations, the real output signal is wrong, cut after 2 or 3 periods. The preview of DAQmx Generate is correct but the actual output is wrong.

[enderWiggin10]

Hi gchaw,

Do you have any information on the error number or description?  When you mention the output is wrong, is the signal clipped, attenuated, noisy, etc?

Best,

[titpoul]

Hello Adam,

there is no error signaled by SignalExpress. The output signal is too short, it lasts between 10 and 50 ms, whereas it should be longer, like 500ms or more.

Guillaume

[enderWiggin10]

Hi Guillame,

I ran your code and I changed the trigger on your Analog Input from PFI4 to the Analog Output Start Trigger and I was able to acquire all 500ms worth of data on my M-series card.  Try this out to see if it works as well. 

Best,

[titpoul]

Hello,

there is no AO Start Trigger for the Analog Input with the NI USB 6218, that's why we routed /Dev1/ao/StartTrigger on /Dev1/PFI4. Do you suggest another solution?

Best,

Guillaume

[enderWiggin10]

Hi Guillaume,

I was able to reproduce the same symptoms.  Unfortunately, the advanced trigger terminals (i.e. ai/sampleclock) are not available for USB M-series device in Signal Express.  They are however available in LabVIEW.  I will be passing this information on to R&D.  Is it possible just to use a software trigger in Signal Express for both of these, or not trigger at all?  Let us give this a shot to see if this is able to at least get the voltage output to be output in its entirety.

Best,

[titpoul]

Actually, the problem seems to be related to the output generation. I simplified the my example by keeping only the sweep, signal waveform simulation and the output generation. The first point of the sweep is ok, but next points are wrong, as if the loop did not wait for the end of the output generation.

Traduire

[titpoul]

Instead of using a trigger, I specified that the Analog Input Acquisition starts after the generation, which produces a correct sweep. I suppose that the synchronisation is not assured, but that it's enough for gain measurements.

However, if a distortion analyse is checked in the block "Tone measurements", I still have the error of insufficient frequency resolution.

Best,

Guillaume

[enderWiggin10]

Hi Guillaume,

I was able to run this with a USB-6218 after using your program and removing the trigger.  The project is attached.

Best,