iFFT Scaling Problem

Hi,

I want the transform a return loss versus frequency data to time domain. I have a .txt file from a cable analyzer within frequency return loss and phase information.

I wrote the ifft code. At time axis there is no problem , but the return loss values in time domain didn't match with the cable analyzer result. So i have to multiply it with a value. The code i wrote is as below:

fid = fopen('10.txt');

values= fscanf(fid,'%f %f %f \n',[3,inf]); 

values = values';fclose(fid);

f = values(:,1);

rloss = values(:,2); 

degree = values(:,3); 

amp = 10.^(rloss/10); 

radian = degree*pi/180; 

complex = amp.*exp(i*radian); 

GAMMA_F = complex; 

%-------------IFFT-----------------------%

NFFT =517; 

f1 = 700e6;

f2 = 1000e6;

Fstep = (f2-f1)/(NFFT-1);

dt = 1/(Fstep*(NFFT-1)); 

t = dt* [0:NFFT-1]; 

GAMMA_T = 30*ifft(GAMMA_F,NFFT); 

RL = 10*log(abs(GAMMA_T));

In this code i have to multiply the ifft result with 30 in order to match with analyzer result. Why?

And if i change the source .txt file for a different experiment result. I have to multiply with a different factor?

I need to help to resolve this problem.

Thnaks in advance!

[Nina Nounou]

hi i have the same problem as you ,can you help me please

Answer 1

[Amit]

Hi, I too have the same problem. If you resolved it could you share your result with me. Thanks.

Answer 2

[Wayne King]

It's hard to say anything definitive without knowing what is actually in this code:

rloss = values(:,2); 
degree = values(:,3); 
amp = 10.^(rloss/10);

I don't think this is your issue, but I doubt you want:

RL = 10*log(abs(GAMMA_T));

I suspect you minimally want

RL = 10*log10(abs(GAMMA_T));

or

 RL = 20*log10(abs(Gamma_T));

[Dr. Seis]

I would have suspected that the lack of symmetry (i.e., lack of negative frequencies) in the frequency domain data would have been the culprit.

[Wayne King]

@Elige Grant that is also a very good point!