AnsweredAssumed Answered

Cross correlation time traces are not symmetric

Question asked by samuel on Dec 11, 2012
Latest reply on Dec 13, 2012 by samuel
We would like to use the VSA software to measure the cross correlations between some signals and found something really strange when using the cross correlation trace : in the following exemple, we are using 2 baseband inputs on the same MXA, but we had the same problem when grouping 2 MXA in one measurement.

On screenshot 1, I am sending a sine at the center frequency on channel 1 and a sine slightly detuned on ch2 (as you can see on the main Time traces on the left). As expected, the Cross Correlation time trace is oscillating at the difference frequency over time, as seen on the lower right channel.

Now, I simply invert the 2 cables going to channel 1 and channel 2, such that channel 2 is excited at the center frequency and channel 1 is slightly detuned. While everything is consistent on the other panels, the lower right one shows a signal that looks completely different from the previous one: the Cross correlation trace is now a flat line that oscillates up and down from trace to trace with an amplitude of plus/minus one square (same as the amlpitude of the sine in the previous test). I don't understand at all this behaviour: I would have expected the cross correlation vector to rotate in the phase space with exactly the same rotation speed, but opposite direction.

What is going on there ? I saw on that thread that the cross correlation signal was not completely symmetric with respect to the 2 time traces involved due to a square filtering window. But I don't see how multiplication by a squared window that's only twice as small as the original timetrace could produce such an effect. It looks like demodulation is not done with respect to the same frequency in both cases or something like that. I keep turning things in my head, but I don't see what is exactly going on there?

I hope the problem is explained clearly enough and someone can help.
Thanks in advance,