Thanks for all the discussions. It has definitely given me clarity into what is happening. . I understand that the DC component of the transmitted signal, along with the frequency offset causes a strong tone at the receiver. I have linked two plots of the real part of the received baseband samples with and without RRC pulse shaping. http://www.ece.gatech.edu/~sriram/rrc.gif http://www.ece.gatech.edu/~sriram/norrc.gif
It is clear that there is a strong tone corresponding to the frequency offset and the data modulation can be seen although not clearly. Also, using the RRC pulse shaping seems to be better when I observe the real part of the received sample (although the power plots are similar with and without RRC). I am just wondering how I could demodulate this and how practical OOK receivers operate. Should this tone be filtered out using a high pass filter (difference of consecutive samples?) ? Since the data sample variation is faster than the frequency offset ( about 100 samples for one cycle of the offset frequency), it looks likely that a filter which passes the high frequency (Data) variations and filters out the low frequency (frequency offset) must be helpful? Am I correct in thinking this way? Feeding this to the costas loop does not help much. The costas loop (with default parameters that work for dBpsk) is unable to demodulate the bits correctly. I am wondering if tweaking its parameters to values different from the default ones might help. I currently use a -i 256 and -d 128, since my laptops don't keep up for lower values. I will experiment to see what happens if they are increased. Thanks, Sri On 9/30/08, Johnathan Corgan <[EMAIL PROTECTED]> wrote: > > On Mon, Sep 29, 2008 at 2:28 PM, Brian Padalino <[EMAIL PROTECTED]> > wrote: > > > Don't some of the daughterboards also have some AGC built in? I can > > see if the interpolation rate is not high enough, the signal power > > will not go down enough (especially after the RRC filtering) to really > > look like much of a difference if any due to the AGC circuitry and > > other transients that may occur on signals quickly coming on then off. > > > None of the daughterboards have AGC as far as I'm aware (Matt, > correct me if I'm wrong on this.) > > It's not that the filtering is preventing the envelope from going to > zero (though it might be; RRC's are intentionally designed to > introduce ISI in a very specific way). It's just that with the > waveform he's sending, there is a strong carrier component at passband > that shows up as a constant beat frequency in the receiver due to > frequency offset. > > > > Please correct me if I am wrong, but I think using a very large > > interpolation rate might help clarify the situation. > > > It would improve things. If the baseband "square wave" had it's > fundamental frequency near the RRC filter limit, or near the Nyquist > limit of the baseband sampling rate if the RRC wasn't in use, there > would be few to no harmonics that make it through the filtering and/or > interpolation. The transmitted waveform would be a carrier and two > sidebands, a classic AM waveform. I think that's what he is seeing. > > > -- > Johnathan Corgan > Corgan Enterprises LLC > http://corganenterprises.com/ >
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