Re: frequency spacing

ilter <x@xxxxxxxx> wrote in news:1146582612618-359556@xxxxxxxxxxxxxxx:

Dear All,
I am acquiring analog signal and I am using 'filtering' , 'tone
measurements' , 'fft' express vi's.I realized that fft found a peak
frequency of a value which is 1000 times lower than the actual
frequency. In my vi sampling rate is 1000 Hz, number of samples to be
acquired is 1000. Looking at&nbsp;the
attached&nbsp;example&nbsp;&nbsp;I figured out that I need the
frequency spacing information. Using 'build cluster' I merged FFT
output with sampling rate and resulting plot looked correct.But this
does not solve my problem, since I need to have the dynamic data
stream to be 'correct'. I need to find the frequency of a different
signal using 'tone measurements' express vi which currently is giving
a 1000 times lower value . I also want to use filtering vi, which does
work properly under this condition. Any suggestion will be more than
appreciated. Regards,
ilter


fft.JPG:
http://forums.ni.com/attachments/ni/170/182552/1/fft.JPG



Since you sample rate is 1K, and you're getting a max at a frequency 1000X
less than you would think, you must be getting a max at .3Hz or less, I
would imagine.

Subtract the mean from your signal before taking the fft. Better yet, fit
the whole data set to a line, and then subtract out that line, and take the
fft.
--
Scott
Reverse name to reply
.

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