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 Converting FFT into dB		  New Topic  Reply to Topic
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woodster

UK
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Posted - 09/11/2004 :  09:41:46 AM  Show Profile  Edit Topic  Reply with Quote  View user's IP address  Delete Topic
Origin Version (Select Help-->About Origin): 7
Operating System: XP

I was wondering how to convert a forward Amplitude FFT into decibels, so that I can plot my S21 transmission.

Cheers

Edited by - woodster on 09/11/2004 12:38:15 PM

Mike Buess

USA
3037 Posts
Posted - 09/11/2004 :  1:54:51 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
The standard expression is dB = 20*log(amplitude/reference). If you choose a reference amplitude of 1 then you can enter the expression 20*log(col(r)) in the Set Column Values dialog for column r.

Mike Buess
Origin WebRing Member
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woodster

UK
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Posted - 09/11/2004 :  2:55:06 PM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Are you sure 20*log(col(r)) is right? because that gives me an insertion loss of about 100dB for my filter, which is just wrong... (hopefully!!). Do i have to normalise col(r) first?

Thanks for the help
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Mike Buess

USA
3037 Posts
Posted - 09/12/2004 :  07:06:20 AM  Show Profile  Edit Reply  Reply with Quote  View user's IP address  Delete Reply
Yes, that expression assumes a reference amplitude, or normalization factor, of 1. If you know the reference level use 20*log(col(r)/ref). If the col(r) values represent Vout then ref=Vin.

...You might also need to take into account any normalization done by the FFT operation. I don't remember exactly what Origin does, but FFT values are typically multiplied or divided by N or sqrt(N), where N is number of points.

Mike Buess
Origin WebRing Member

Edited by - Mike Buess on 09/12/2004 10:48:22 AM
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