Can I set the number of "Frequency Lines" in my FFT?

[BeCoolYolanda]

Hello,

I am using the "SVFA Power Spectrum" VI to process mic pressure data. I want to show my result in FFT format with dB as my units, my question is how can I make this data look more presentable by lowering the amount of data points displayed without loosing the frequency range. My questions are:

i) is it possible to control the frequency resolution without changing frequency range / sampling rate?

or

ii) Can I somehow input a block size?

or

iii) Can I control the resolution band width also without changing the sampling rate and in turn the frequency range.

[Wild_man]

Nick,

If I understand correctly you are looking at the "power spectrum [dB EU]" output of the "SVFA Power Spectrum" VI. This is a cluster of 3 elements (f0, df, and magnitude.)

Your hope is to increase df while maintaining the full frequency spectrum so that you have fewer data points in the magnitude array. Looking at the VI I see no straightforward way to do this through the VI input, but you could pretty simply decimate the resulting magnitude array in such a manner that it only shows the data you would like to represent.

This would have the added benefit of maintaining the finer frequency resolution if you need it for analysis purposes while being able to display the low resolution data for a presentable look.

Thanks,

[BeCoolYolanda]

Well, the data that I would like to represent will be across the whole spectrum, I don’t want to subset anything. Another way of explaining it is that I would like to have magnitude resultants like the one displayed in the 1/3 octave, except I do not want the bandwidths to double as the frequency increases in the spectrum. I want smaller bandwidths with a constant frequency accession.

[BeCoolYolanda]

I wanted to set the number of "Frequency Lines" in my FFT so my graph looks presentable. This is usually a common FFT option but I can't seem to find it in Labview. Any help would be greatly appreciated.

[jamiva]

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@nick159 wrote:

I wanted to set the number of "Frequency Lines" in my FFT so my graph looks presentable. This is usually a common FFT option but I can't seem to find it in Labview. Any help would be greatly appreciated.

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I'm assuming the "number of Frequency Lines" is the number of BINs in your graph. Bin width is related to the total number of points in the FFT. In other words if you are using a 2048 pts FFT, try using 1024 pts. That will effectively double your BIN width (1/2 the number pf frequency points) without changing the Nyquist frequency.

[BeCoolYolanda]

Yes the number of frequency lines and the number of bins is the same thing. Your solution seems right but how do I set the pts?? I dont want to change my sampling rate because I dont want to shorten my frequency range.