Hi guys

I have been trying to process my data with FFT to analyze the natural frequency
of it. I try to use a high pass filter of 1 or 2 HZ because the source of the
signal is a DC and i found that the result is still weird. It is a DC motor
attach to a load and run it and will last for approximately 2 sec only.

So i would like to ask the best DSP method to analyze this as if i just use FFT
i would see a dominant 0Hz.

Cheers
Penixilin

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Penixilin-

> I have been trying to process my data with FFT to analyze
> the natural frequency of it. I try to use a high pass filter
> of 1 or 2 HZ because the source of the signal is a DC and i
> found that the result is still weird. It is a DC motor
> attach to a load and run it and will last for approximately
> 2 sec only.

Your HPF would have to be really sharp to remove just a few Hz.  What is your
sampling rate?  What type of filter are
you using?  How many taps (FIR) or what order (IIR)?

> So i would like to ask the best DSP method to analyze this
> as if i just use FFT i would see a dominant 0Hz.

If you use a large enough FFT size you should be able to separate dc from 1 Hz. 
Your Hz/bin should be at least 0.5 or
less (i.e. sampling rate/FFT size).

-Jeff

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