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Driving me nuts!
Posted:
Sep 21, 1996 3:33 PM


Here's a question that has probably been asked before:
I'm trying to view a timedomain plot of some frequency domain data I have (From an HP network analyzer). All I need to do is figure out how to derive the time axis. Here's the code
 points = 4096; frequency = input('Frequency (MHz)? '); frequency = frequency * 1e6; lowfreq = 200E6; highfreq = 400E6;
BW = highfreqlowfreq; FreqStep = BW./(points1);
%create the frequency axis f = [lowfreq:FreqStep:highfreq];
%create the time axis TimeStep = 1./(2*BW); t = [0:TimeStep:TimeStep.*(points1)];
%Set up an impulse signal at the desired frequency %Figure out the center bin for the selected frequency deltafreq = frequencylowfreq; deltapoints = deltafreq/FreqStep; %frequency better be within BW deltapoints = round(deltapoints); %Has to be an integer
%create a signal within the band with impulse at deltapoints yf = [zeros(1,deltapoints1) ones(1,1) zeros(1,pointsdeltapoints)];
%plot the signal figure(1) plot(f,yf); fftaxis = axis; xlabel('freq (Hz)'); ylabel('amplitude'); title('Freq domain');
%create the fft y = ifft(yf);
%plot the time domain figure(2) plot(t,real(y)); V = axis; %axis([0 max(t) V(3) V(4)]); xlabel('Time (s)'); ylabel(''); title('Time domain');
%create the fft yf1 = fft(y);
%plot the ifft figure(3) plot(f,yf1); axis(fftaxis); xlabel('Freq'); ylabel('amplitude'); title('Reconstructed freq domain');
 What happens is that the freq domain and the reconstructed freq domain are perfect. The only problem is the scaling of the time domain axis. As far as I know, the time resolution is simply
1/(2*bandwidth)
and the entire time axis is
(1/(2*bandwidth) * numpoints)
But that doesn't seem to work. Does anyone have the real story on obtaining the proper time resolution and time span for a bandwidth limited discrete frequency domain data set? It's been driving me nuts!
Robert
  An EE guy who just lives and breaths off road motorcycles. 



