Thanks a lot Tom for your help. I now understand how Costas loop operates. After all the tests that I did following your suggestions I was able to find the answer for a lot of my unanswered questions.
Thanks again, Ali On Wed, May 27, 2009 at 10:42 AM, Tom Lutz <[email protected]> wrote: > > I am using Flex-2400 boards and the received signal is ideally at > baseband > > which in fact is not possible because, of various factors. Now, we have a > > signal that has a center frequency 'fc' which is not 0Hz. Assuming the > > costas to lock at this carrier I can achieve my goal. Isn't it right? > > > > The Costas loop can lock to fc, and will output the original signal > mixed with fc (i.e. your signal at baseband), so yes is the answer. > It is worthwhile to look close at the secondary output of the Costas > loop. This output is the frequency the loop is locked to, in radians > per sample. If you take the real part of this output (the imaginary > is just 0), and multiply it by sampling_frequency/(2*pi) and send it > to the oscilloscope or other graphical sink, you can see what > frequency, in Hertz, the loop is locked to. Nominally, you can try > alpha=0.01 and beta=alpha^2/4 for the phase and frequency gains, > respectively. Try things out in simulation first. > > > Can you also tell me how fmax and fmin are calculated? For example, > dbpsk.py > > has fmax=0.1 and fmin=-0.1. How do we get these? > > > > Frequency (in radians per sample) = 2*pi*(frequency) / (sample rate), > so if your carrier were 2MHz at a sampling rate of 6MHz, it would be > 2*pi*2000000/6000000~=2.09 > > fmax=0.1 and fmin=-0.1 means the loop is set to lock to 0Hz (DC), with > a tolerance of +/- 0.1Radians/Sample. > > Pick some margin above and below based on expected drift (say, for > example, 1.95MHz and 2.05MHz), and calculate similarly to get your > fmin and fmax. Given that dbpsk.py has fmax=0.1 and fmin=-0.1, I'm > lead to believe that the costas loop can work at DC, in which case the > frequency would be roughly 0 and the phase would adjust to compensate > for error. > > I don't consider myself an authoritative source for this stuff, as I > just started playing with it myself, but this should get you going at > least. > > Tom > > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >
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