Hi, I have another question regarding this. I sunk the output from the Costas onto FFTsink and got this. http://img172.imageshack.us/img172/9956/costasfft.jpg Are the peaks that appear on this the lower and upper sidebands of the data signal?
Thanks, Ali On Thu, May 28, 2009 at 5:38 AM, Mir M. Ali <mirmurt...@gmail.com> wrote: > 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 <tommyl...@gmail.com> 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 >> Discuss-gnuradio@gnu.org >> http://lists.gnu.org/mailman/listinfo/discuss-gnuradio >> > >
_______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org http://lists.gnu.org/mailman/listinfo/discuss-gnuradio
