Chirp detection for synchronization

[nolispe]

Hello,

I am currently doing an internship on marine acoustics and I am facing some difficulties.
The transmitted signal is composed of 3 tones (1sec) then 2 chirp signals (or LFM) (50ms each)then the data I am supposed to process. So I need to find where the chirps are on my received signal to be able to demodulate the data. I know some techniques : matched-filters using correlation function in Matlab but it didn't work. FYI I know the Bandwidth (4kHz) and the time duration (50ms) of the chirp signal.

Thank you in advance for your help.

 

[MacGyverS2000]

It's an internship, you're there to work as well as learn from the more experienced members... what does your supervisor or your coworkers have to say about your question?

Dan - Owner

http://www.Hi-TecDesigns.com

 

[nolispe]

Actually I talked about my difficulties to him but the only answer he gave me is "read books about it"! But my problem is a technical one. I am trying to do the correlation with Matlab but the problem is the chirp duration is only 50ms when the signal is 13s long. So matlab tells me it cannot correlate two signals of different length. So I duplicate periodically my signal but somehow it didn't work... And when I use the Matlab chirp function, the frequency response is weird (triangular periodic signal)

 

[MacGyverS2000]

Then the obvious solution should be correlate with similar-sized signals, no? Create a window the same size as the signal you wish to correlate with and run it down the main signal stream. At some point in sliding the window, you will get a strong correlation, which means you've found the signal of interest.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[nolispe]

Yes it should. The only problem is I am working with Matlab and I'm not sure it is possible to create a window (if it is I have no idea how to create one).
I figured out why the frequency response with the matlab chirp function is weird : it just duplicates the chirp signal periodically hence the triangular signal (duplication of lines = triangular signal)

 

[GregLocock]

Matlab is entirely capable of solving this (well frankly so is Excel).

The cross correlation between signals called JustAChirpAndABunchOfZeros and TheSignalTheyWantMeToAnalyse will tell you where to find the chirp in the latter.

Of course there are about a zillion other ways of solving this, but I think it is just a 3 or 4 liner in matlab.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[MacGyverS2000]

I have to agree with your boss at this point, then... if you're unable to handle creating a simple window function in MatLab, you have a lot of reading ahead of you.

Good luck!


Dan - Owner

http://www.Hi-TecDesigns.com

 

[nolispe]

Yes. I think you're right... Thank you all for your help

 

[rhodie]

Engineers: a professional group composed of folks with the God-given ability to make you feel like a bastard for asking them a question.

 

[MacGyverS2000]

rhodie,

You have a problem with the discussion here?

These fora are for practicing engineers, not students. Internships are a bridge between the two, but in the end it's a way to get on-the-job experience while learning as a student. It quickly became obvious the OP did not have a good grasp of the fundamental concepts necessary to perform his main line of work, and in those cases it solves nothing to provide a canned answer... the student learns little. In those cases, it's the mentor's job to help guide him, not ours.

Doing otherwise leads to more and more students flooding the forum looking for an easy answer without taking the time to understand what they're asking.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[zappedagain]

Well stated, Dan.

John D

 

[rhodie]

You're right. Now on to the next thread where we complain about how the younger engineers today just can't cut the mustard compared with our generation.

 

[MacGyverS2000]

rhodie,

Did you ever stop to think the next generation can't cut the mustard because some members of our generation keep giving them the easy way out instead of helping them learn the underlying theory?

If you want to spend a great deal of your time helping what will be an exponentially growing group of students coming to your help site, knock yourself out... just don't do it here.

Dan - Owner

http://www.Hi-TecDesigns.com