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 Relation between FFT amplitude and signal amplitud
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cosy

Germany
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Posted - 05/30/2006 :  05:11:03 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Origin Version (Select Help-->About Origin): Pro7
Operating System:xp
Hi,
Please verify if my assumptions about the relationship between the Normalised FFT amplitude and the signal amplitude is right:

Signal Amplitude = Normalised FFT Amplitude/2.

FFT Power = (Normalised FFT Amplitude)^2/Number of points

Regards,
COSY.

Mike Buess

USA
3037 Posts

Posted - 05/30/2006 :  07:04:43 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Hi COSY,

There are more tha one FFT normalization methods but I assume you are talking about Analysis > FFT. According to the Origin Reference...
quote:
Normalization in a Forward FFT divides the Real results by N/2 (both AC and DC components). Normalization in a Backward FFT divides the Real results by 2, except for the DC component which is left unchanged.
and
quote:
The Power is derived from the Real^2/N, where N is the number of points
The first normalization occurs only when the Normalize Amplitude FFT setting is checked. The power normalization is independent of that setting.

Mike Buess
Origin WebRing Member

Edited by - Mike Buess on 05/30/2006 07:43:41 AM
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6jal2

Canada
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Posted - 01/24/2007 :  8:29:31 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
1. When doing Analyze-->FFT--->Normalized, is output amplitude from the FFT analysis the actual amplitude or the amplitude/2?

2. Why might I get such a significant difference in peak amplitude for my fundamental when using the rectangular (2.59) and Hanning (1.37) windows?

The actual peak of my data is 2.8 and the curve is a nearly perfect sinusoid.
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Mike Buess

USA
3037 Posts

Posted - 01/24/2007 :  9:49:10 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
2. The peak area will be the same. For a perfect sinusoid the peak is a delta function with area under one point. Apodization (windowing) broadens the peak so peak height must decrease to maintain same area.

Mike Buess
Origin WebRing Member

Edited by - Mike Buess on 01/24/2007 9:50:07 PM
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