| T O P I C R E V I E W |
| woodster |
Posted - 09/11/2004 : 09:41:46 AM
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 |
| 3 L A T E S T R E P L I E S (Newest First) |
| Mike Buess |
Posted - 09/12/2004 : 07:06:20 AM
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 |
| woodster |
Posted - 09/11/2004 : 2:55:06 PM
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 |
| Mike Buess |
Posted - 09/11/2004 : 1:54:51 PM
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 |
