power spectrum density and sampling rates

[rom9]

Hi

I am having trouble analysing Power Spectrum and Power Spectrum Density. I am measuring signals from a channel from which I need to get the sq.Vrms values which is obtained by intergrating the area under the curve of a Lorentzian Peak from the Power Spectrum. I tried using both Power Spectrum and Power Spectrum density from the Express VIs. However as I increase the sampling rate from say 5000 samples to 20000 samples at 50KHz, the area under the resonance peak at roughly 15kHz keeps decreasing leading to a different value everytime I change the sampling rate. Is there an explanation to this ? I kept all the other factors like number of averages etc constant.

Would really appreciate the help

Thanks 

[James_McN]

Hi Rom,

Off the top of my head the biggest difference between the two different power spectra that you have is that the number of samples you acquire changes the resolution of your spectra e.g. more samples gives a small df on the waveform cluster.  This should not give radically different values although might explain some small variation.  I shall however have a bit more of a look into this and confirm this suspicion or come up with a different explaination for you.

Regards,

[rom9]

Hi James

Thanks on your input. 

With a very high number of samples, the curve fitting (in Origin and Igor pro) is also not so good which I thought previously was the problem but the area under the curve changes by order of magnitude when the samples change from 5000 to 20000. That what I cant figure out why it was so large. I am trying to get something to do the curve fitting in labview itself. Would be great to get more thoughts on this.

Thanks

Rohit 

[James_McN]

Hi Rohit,

There is a curve fitting VI available in LabVIEW.  If you are using express VIs it is available under signal analysis.  Otherwise there are also VIs under mathematics>>fitting>>advanced curve fitting that can do this for you.

Regards,

[James_McN]

Hi Rohit,

Just wanted to check whether you had managed to get this working as you want now with the curve fitting VIs.

Regards,

[rom9]

Hi James

I was actually hoping u reply back since I have not been able to resolve the issue. Curve fitting looks fine, but I still get different values on changing the sample numbers. It seems as if the VI is trying to normalize the value of the plot somehow in relation to the number of samples.

Rohit 

[Jeremy_T]

Hi Rohit,

James is out-of-office today so I'll be taking a look at your issue. Could you possibly send a screenshot of the relevant part of code or even attach a copy of your VI? This may help me to get a better image of what exactly is going on with your code. If you could do this I'd be very grateful. Looking forward to hearing back from you,

Best regards

[Jeremy_T]

Hi again,

Also would you mind clarifying exactly what you mean by 'as I increase the sampling rate from say 5000 samples to 20000 samples at 50KHz'? Is it that you are increasing the sample rate from 5000 to 20000 samples per second? Or are you sampling at 50000 samples per second, changing from 5000 to 20000 samples in total? Hope to hear from you soon,

Best regards

[rom9]

Hi Jeremy

Thanks for getting in touch.

I have attached the file herewith. It has the VI and also the data files I gathered for my measurements in ASCII format . Also there are origin files  for some measurements showing how the area under the peak is made for three different number of samples, which  if u want I can try to send them as well.

The data files are in the format  ///sampling frequency_samples_no of averages_.lvm///. Also if u enter 20000 samples in the VI it makes 10000 samples only, so please put double the values of the required number of samples.

I hope this is clear. Hope to hear from u.

Thx a mil  *PLEASE CHANGE THE EXTENSION OF THE DATA FILES FROM "txt" to "lvm" to import in ASCII format.

Rohit

[rom9]

other files