Re: [oc] New! FFT core, transform gain issue

[sradio@opencores.org]

----- Original Message ----- 
From: h.larsen@r...  
To: cores@o...  
Date: Tue, 19 Nov 2002 16:56:37 +0100 
Subject: [oc] New! FFT core, transform gain issue 

> 
> 
> Hi, 
> 
> I have tested the cfft core, but found that the gain is 29.4 both 
> in N=1024 
> FFT and IFFT, whereas the documentation says it should be 0.0287 
in 
> FFT 
> mode, and 29.4 in IFFT mode. 

0.0287=29.4/1024,

In Matlab, the ifft pocess divide N. (:

> 
> I simply supply a real ampliude of a sine wave to the input of the 
> cfft1024X12: Something like-> 
> Iin=A*sin(2pi*F*t) + DC; 
> Qin=0; 
> 
> and measure the output amplitude (FFT or IFFT, no difference, 
> except to 
> phase of output). It gives a peak at bin 0 of DC*29.4 and two peaks 
> at the 
> proper bins with an amplitude of 14.7*A each, which makes sense 
> only if the 
> gain is 29.4=14.7*2 (two bins). 
> 
> Anybody who have tried similar things? 
> 
> In particular a comment from the briliant designer ZHAO Ming would 
> be very 
> welcome. 
> 
> regards 
> henning larsen 
> Electronics des. engineer 
> Risoe national laboratory 
> Denmark 
> 
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