inverse Fourier transform of a signal

Hello all, I got a real signal from network analyzer and this network analyzer can transform the signal for me. However, when I used the command ifft, I couldn't get the signal that I saw before. Could you please suggest how to do it? The stimulus in the txt file is the frequency (the range is from 0.1 Ghz to 0.3 Ghz).

if true
clear all
close all
clc
load('cable.mat');
filename = 's11.txt';
M = csvread(filename,9,0);
f=M(:,1);
s11=complex(M(:,2),M(:,3));
y = ifft(s11);
plot(abs(y))
end

[Star Strider]

You cannot take the ifft of an incomplete fft such as yours. You need all the frequencies from 0 to 0.3 GHz, or preferably the original time-domain signal that you can then filter to isolate the frequencies of interest to you.

[Shan Chu]

Hi, is it ok if I apply zeros for the range of frequency from 0 to 0.1 Ghz? Then using the ifft

Answer 1

[John BG]

Shan

try the following:

1.- don't care about the file extension:

A=csvread('s11.s2p',9,0)

or

A=csvread('s11.txt',9,0)

or

A=csvread('s11.dat',9,0)

or

A=csvread('s11.csv',9,0)

It does not matter, as long as the file extension is .s2p .txt .dat .csv the input is exactly the same.

2.- get plot-able data

f=A(:,1)
s11re=A(:,2)
s11im=A(:,3)
s11mod=(s11re.^2+s11im.^2).^5
s11ang=atan(s11im./s11re)

3.- now try plotting s11:

plot(s11mod)

or

s11=s11re+i*s11im
plot(s11.*conj(s11))

or

plot(s11mod.^2)

or

plot(10*log10(s11mod.^2))

5.- do you really need s11 in time domain? s11 is already frequency domain

6.- You may also find useful to build an rfckt object to replicate the 3D expensive real Network Analyzer plot onto the rftool Smith chart and polar diagram:

data=rfckt.datafile
data.AnalyzedResult.Freq=f

S_data=complex(zeros(2,2,1601))
s11re=A(:,2)
s11im=A(:,3)
s11=s11re+i*s11im
S_data(1,1,:)=s11
data.AnalyzedResult.S_Parameters=S_data

now the variable 'data' can be imported into rftool

launch rftool:

rftool

go to: File > Import from Workspace

choose: data

on the lower half of the rftool GUI tick only s11, all other s parameters empty

Smith Chart: Z Chart

now you should see a more or less static copy of what you got while cutting the coaxial.

If you find this answer of any help solving this question, please click on the thumbs-up vote link,

thanks in advance

John

[e-mail]

[Shan Chu]

What you did I already knew how to do. my only question is: how to convert s11 from frequency domain to time domain by using 'ifft'. If you know, please help. Thank you anyway

[Sagar Dutta]

Shan Chu, Did you find the solution to convert into time domain using ifft? I am also trying to do the same thing, need some help.

[Deepshikha Bhargava]

Facing the same problem. Has anyone found the solution?