FFT Frequency Resolution vs. Graph Update Rate of a Vibration Signal

Hello,
First, what I know:
The frequency resolution (Hz) of an fft is 1/acquisitionTime (e.g. if 2 seconds of data are acquired, the frequency resolution is 1/2s = 0.5Hz).
What I'm Wondering:
Let's say I'm displaying an acquired signal on a graph and freq res = 0.5Hz. The graph update rate is therefore approx 2sec.  This may appear to be an application with a problem from a user's perspective.
Can I increase the update rate?
It seems that with vibration data (i.e. mostly non-periodic) I could create a buffer that is 2sec long.  The first data acquisition sequence will fil the buffer with 2sec of data. Then I could just acquire something shorter than 2sec of data (let's say 1/4sec). If I am acquiring data continuously and I push the 1/4sec of data into the front of my buffer then perform the fft again, I have effectively increased my graph update rate to 1/4sec and my users think I'm great.
Can anyone shed any light on whether or not (why/why not) this will work?
Thanks for your time,
Chris

The best tool NI provides for your application is the Zoom FFT. This allows you to specify the span (frequency range of the FFT) and the zoom (the delta f such as 0.5hz) in the FFT. The Zoom FFT has a built in buffer, called overlap and allows you to set the overlap so that it updates at the appropriate rate, event though you may need many seconds of data to get the delta-f in the FFT you are looking for. We package all this and manage units to display the correct units for power density, power, linear, magnitude, rms, 0 to peak, peak to peak, etc. We even provide a express VI for the zoom FFT and it is also part of the Sound and Vibration Assistant, requiring no programming at all.
http://digital.ni.com/public.nsf/allkb/F70DB3441DC5AB9986256E97005BA825
http://zone.ni.com/devzone/cda/tut/p/id/6107
http://zone.ni.com/devzone/cda/epd/p/id/5505
http://sine.ni.com/nips/cds/view/p/lang/en/nid/12152
Preston Johnson
Principal Sales Engineer
Condition Monitoring Systems
Vibration Analyst III - www.vibinst.org, www.mobiusinstitute.com
National Instruments
[email protected]
www.ni.com/mcm
www.ni.com/soundandvibration
www.ni.com/biganalogdata
512-683-5444

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Dear Sir
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Attachments:
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I set the dt value to 0.002. For this I set sampling rate 2000hz and no.of samples 1000000. In this way I achieve frequency span of 250hz. But resolution is still 100hz (as shown in attach 1). My question is how can I set resolution to 10hz? I increase the number of samples but from doing this only the frequency span changes and resolution remain same as 100hz. Resolution become 10hz only when I set total frequency span to 50 hz (but My frequency span of intrest is 0~500hz or 0~250hz not below thos).
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fft graph2.JPG ‏39 KB
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Attachments:
Final DAQ.vi ‏107 KB

Hi How81,
   I won't comment on your code, I'm unable to see it (I have LV 71.1). So I'll discuss a little bit the DSP.
   If you want such a low frequency resolution, you have to sample a little longer. If your top-frequency is 25 Hz, I don't see the need for sampling at 1 kS/s, that gives a Nyquist freq. of 500 Hz!!! You'll only have a large amount of data, not of your interest!
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Picmanova 2
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T +385 1 6622230
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Attachments:
AI Acquire Waveform (waveform).vi ‏14 KB

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FFT Frequency Resolution vs. Graph Update Rate of a Vibration Signal

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