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Altera_Forum
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11 years ago

Costas Loop oscillates in ModelSim while locks in MATLAB!

Hello again, I have designed a Costas Loop in MATLAB and it simulates very well... locking and recovering the carrier. Here are the results: http://www.alteraforum.com/forum/attachment.php?...
Altera_Forum's avatar
Altera_Forum
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11 years ago

We hope error becomes zero... what about loop filter's output? do we hope it goes back to zero too?

If we do, this is impossible because inside the loop filter there is register that is summed with the incoming error, once error is zero it accumulates with its self.

If we don't, then NCO word will be updated with the steady value of LF's output (if we suppose it settles... but it actually does not).

This figure should depict what I mean:

http://www.alteraforum.com/forum/attachment.php?attachmentid=10476&stc=1

I believe we should hope for the loop filter's output to be zero or so too. Because as you mentioned we are not like DSP guys and our NCO is different.