problem with third octave band frequency range

[npai]

Hi,

I am trying to see the third octave band of a signal. The VI which I build is giving me the third octave band, but it is giving me from 20 Hz and not from 10Hz, which is necessary for my analysis. I tried to change the low band frequency to 10 Hz but still it is showing from 20Hz. So if anybody can help me in solving this poblem. Please take a look at the program which I made and let me know the some changes which will give me the third octave from 10 Hz. The sampling frequency is 24000Hz. I am attaching the Vi and the file.

Thanks,
Nitin

[Jarrod_S.]

What VIs are you using for the octave analysis? You didn't include one of your subVIs in the zip file (OctaveAnaliysis.vi).

[npai]

I resolved that problem, It had to do with some change in one of the sub vi which was converting the freq range to band number. I changed that sub vi and now the program runs fine.

Thank you very much

Nitin

[MeCoOp]

I realize this post is about a year old but I've run into the same problem, only my problem is with the high end.  I need to be able to read from 50 - 10000 Hz.  I can only retrieve data up to 8000 Hz regardless of the bandwidth selected.  I'm using the SVT Fractional Octave Analysis vi.

Thanks.

[Neal M]

Hello MeCoOp,

There are a couple of things that are a little unclear or may affect the issue you have.  First of all, what is your sampling rate?  If it is less than 20k, then nyquist theory shows that you will be unable to get as high of a bandwidth as you need. 

Secondly, are you selecting the bandwidth with the bandwidth input or by your frequency range?  I realize this sounds like an odd question, but the bandwidth input is actually to select the divisions of the bins in your octave analysis (e.g 1/3 octave would be 3 bins per octave), where as the frequency range sets the limits of the analysis.

If you could post back with the answers to these questions, along with possibly posting the code you have (or screenshots of the pertinent sections),  I can look into your problem a little more.

[MeCoOp]

Hi Neal, thanks for replying.  To answer your questions:

    Number of Samples per Channel - 20K

    Frequency Range - 50 - 10K

    Bandwidth - 1/3 Octave

With this setup I can only get readings from 50 - 8K.  I've even tried increasing the frequency range but the results remain the same.  I've attached a screen shot of my block diagram.

Thanks.

[Neal M]

So it appears that you are using a task that is already configured in Measurement and Automation Explorer.  The configuration window of the task is where the sampling rate would be set (see attached .jpg).  The Number of Samples per Channel you specify on your vi is how many samples the DAQmx Read pulls out of the buffer each time it is called. It would be greatly helpful if you could provide me the task settings.

Also, since you are using a bandwidth of 1/3 octave, I would suggest trying substituting the Fractional Octave Analysis vi for the Third Octave Analysis vi, just to narrow down the issue a little more.

[johnsold]

I have not looked at the details of the third octave VIs, but I wonder if the specified frequencies are the CENTERs of the bands. If so, then part of the band would be above the Nyquist limit. Perhaps the VI is smart enough to prohibit that? DO you get any errors when you try to set the limit to 10 kHz?

Lynn

[MeCoOp]

Lynn, that did it.  I set my sampling frequency to 25k and was able to get 10k.  I was getting error code 1963, poor programming technique is to blame.  If I would have set my program up to report errors I may have solved the problem on my own.  Thanks for all the help everyone!

Bobby