| View previous topic :: View next topic |
| Author |
Message |
John Sigurdson
Joined: 03 Mar 2005
Posts: 8
Location: San Diego CA
|
 Posted: Mon Aug 22, 2005 11:20 pm Post subject: FFT resolution and frequency spectrum |
 |
|
David,
Starting with a pressure waveform in psi with 30 ms of data recorded at 1 MHz (30,000 data points). I lable the X axis as microseconds and Y axis as psi.
When I do an FFT of this signal, it produces a FFT plot of magnitudes from 0 to 500 KHz with one point (magnitude) about every 30 Hz ( n(1) = 16,385 ).
Q: Is there any way to increase the number of points so as to get 1 Hz resolution for waveforms sampled at 1 MHz? That is, n(1) = 500K. That would allow calculation of a standard 1 Hz frequency spectrum and, with added calculations, subsequent energy spectral densities etc. Or, is there a way to finesse that with existing settings?
John Sigurdson
_________________
John Sigurdson |
|
| Back to top |
|
 |
DPlotAdmin
Site Admin
Joined: 24 Jun 2003
Posts: 2310
Location: Vicksburg, Mississippi
|
| Posted: Tue Aug 23, 2005 9:48 am Post subject: |
|
|
| Quote: | | Q: Is there any way to increase the number of points so as to get 1 Hz resolution for waveforms sampled at 1 MHz? That is, n(1) = 500K. That would allow calculation of a standard 1 Hz frequency spectrum and, with added calculations, subsequent energy spectral densities etc. Or, is there a way to finesse that with existing settings? |
Only by increasing the number of points in your input. This isn't an arbitrary decision made by DPlot; it's how an FFT works. The frequency interval in the FFT will always be 1/(2*dt*(Nfft-1)), where Nfft = number of points in the FFT, always half the number of points in your input plus 1. For dt=1 microsecond and 30,000 input values, the frequency interval is 1/0.03 Hz = 33.33 Hz.
 And after typing all of that I realized it was an overly complicated way of saying that to get a 1Hz interval, you need 1 second's worth of input. 1/(2 * 1E-6 * 5E+5) = 1.
_________________
Visualize Your Data
support@dplot.com |
|
| Back to top |
|
|
John Sigurdson
Joined: 03 Mar 2005
Posts: 8
Location: San Diego CA
|
| Posted: Tue Aug 23, 2005 12:17 pm Post subject: |
|
|
Thanks, again, David
I should have known...
_________________
John Sigurdson |
|
| Back to top |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
FAQ
Search
Memberlist
Usergroups
Profile
Log in to check your private messages
Log in
