Acquition of 20KHz Pulse Waveform with PXI 6031E

[Sarang Jaiswal]

Hey all,

I want to acquire 20KHz Pulse Waveform in LabVIEW applied from the function/waveform generator. I am using PXI 6031E for acquition. The waveform must be acquired continuously.

I want to know what should be the value of

sample per channel                     --->  of DAQmx Create Channel

Rate                                                --->  of DAQmx Create Channel

Number of sample per channel  --->  of DAQmx Read

If I put the values as

sample per channel                     =  3000   

Rate                                             =  30000

Number of sample per channel   = 3000

then the detected frequency in LabVIEW is approx around 10K though I am providing 20 K ..... why is this happening ??

I hope you all have got my question. In case of any more clarification please let me know.

Thank You.

[AnalogKid2DigitalMan]

Sounds like aliasing (you ae sampling your waveform too slow), please read section on it here:

https://www.ni.com/en/shop/data-acquisition/measurement-fundamentals/analog-fundamentals/acquiring-a...

Try bumping your rate up to at least 40,000 to satisfy minimum Nyquist criteria

[Sarang Jaiswal]

hey thanks for replying,
I completely agree with you that it shoild be greater than Nyquist Rate but if we increase the Rate the the acqution becomes very slow. I attached  an Oscilloscope to the function generator and saw the waveform generated. the updation in LabVIEW is very slow as compared to the oscilloscope .......also if we increase the rate the updation in chart become slower and slower......

I want instant updation, because of the delay by charts i am losing valuable data.

i hope you got my problem. i tried many combinations but could'nt get one please let me know if there in any formula for this

Thank you. waiting for your reply

[Dennis_Knutson]

The formula is Nyquist. There's no getting around it. If your scope was set to a sampling rate less than the source frequency, it wouldn't be correct either. How exactly is your program written? Do you create the channel and start the task outside a while loop with only a DAQmx read inside the loop? Are you using the DAQ Assistant? Have you tried one of the shipping examples like Cont Acq&Graph Voltage-Int Clk?

[Sarang Jaiswal]

Hey Dennis,
Well lets consider the example which is shiped with the LabVIEW (i.e. Cont Acq&Graph Voltage-Int Clk). I am using PXI 6031E. The Sampling Rate is 100kS/s  and  I am using only one channel. Now if I give 40000 or more as Rate then also I am not getting the result which I wanted. I dont want to loose a single data (i know its not possible but i want to avoid it as far as i could)

I want the waveform to be shown similar to as shown in oscilloscope.

[Dennis_Knutson]

The Nyquist frequency is the  minimum that you should set your sampling rate to. Of course it's not going to look like the scope if the scope is sampling at 100 Ms/sec. To avoid missing any information, set the DAQ board to the highest possible rate that it supports. Have you actually done this? If you haven't, why not? You seem to have conflicting requirements and I'm not sure why. You want the DAQ capture to be of high resolution but want to use a low sample rate.

[Sarang Jaiswal]

Hey Dennis,
I have tried using that too. See, I am using PXI 6031E which has 100kS/s so I think maximum I can sampling value will be 100 x 1000 = 100000.(if i am not wrong)
But if i give this value then obviously the reconstruction of the wave is much better but as i said earlier the update on the chart become's slow. I mean i cant see updates quickly if i reduce the frequency. I dont know the reason for this.....

That's why I want to keep  Rate's value a little low so that the update's are fast on charts.....

Thank you.

[Dennis_Knutson]

Well, like I asked earlier, how is your program structured? It sounds like that's where the problem is. How many samples are you requesting? In the example I mentioned, is that refresh fast enough for you? If your VI is slower, then post your vI so someone can look at it instead of guessing.

[Sarang Jaiswal]

hmmm.....yaa even i think there is some problem in my program, i will post it tommorow (actually it in some other system and now the Lab is also closed  )

well thanks a lot. please look at my program tommorow.

[McKala[DE]]

Continued in the PXI Forum: https://forums.ni.com/t5/PXI/Query-for-PXI-6031E/m-p/273463