问题
I have an issue regard to the unwrapping of phases over time.
I have a radar which monitors a slow periodic moving object. From the received signal, i want to see the phase of the object over time. The phase-over-time signal is extracted from FFTs (at the same FFT bin for all the FFTs). Here is the result i got:
Because the phases are wrapped, so I need to do an unwrapping for the phases. I use the Matlab command "unwrap" to to this. So I got:
The issue is marked at the red circle. Here I expect the signal at this time-instant to be unwrapped. However it wasn't, and the reason is:
- From time sample NO. 42 -> 44, the phases take two time samples to being
wrapped, instead of only one (i.e between two consecutive time samples).
Because of this the phase-over-time signal are not unwrapped correctly.
I also tried to used another phase-unwrapping method (Adaptive numerical integration), however the result is the same as using "unwrap" command from Matlab.
Here is the phase-over-time signal I expected to see (I did the unwrapping manually):
What is the problem mentioned above (Is it a well-known problem or has any name for it)? And of course what is the solution for this?
I would really appreciate any help from you! Thanks alot.
回答1:
Matlab's unwrap function tries to avoid any jumps between consecutive values larger than tol. By default, tol is pi. Your jumps from -1.644 to -0.7 to 1.55 are not larger than pi, so unwrap doesn't find it necessary to adjust your data. Note, even if the -0.7 wasn't in there, unwrap still wouldn't adjust your data as the jump would still be less than pi.
The tolerance is an optional parameter in unwrap, you can set:
unwrap(x,tol)
I would suggest setting your tolerance to pi/2 or 3*pi/4, depending on your data.
The unwrap documentation probably has more information
http://uk.mathworks.com/help/matlab/ref/unwrap.html?refresh=true
回答2:
It seems that your input signal is between -pi and pi an your desired output between 0 and 2pi, so why dont'you simply add 2pi to the negative values ? Here is a try:
I = s<0;
s(I) = s(I) + 2*pi;
That should provide the desired output, in a more simpler way.
Best,
来源：https://stackoverflow.com/questions/32442396/phase-unwrap-issue-the-unwrapping-of-the-phases-is-not-correcly