On Mon, Mar 3, 2014 at 8:32 PM, zhenhua han <hzhua...@gmail.com> wrote: > I'm reading the code of Airprobe and I found this function. > > double gsm_receiver_cf::compute_freq_offset(const gr_complex * input, > unsigned first_sample, unsigned last_sample) > ~ > > It calculates the frequency offset with "freq_offset = phase_offset * > 1625000.0 / (12.0 * M_PI);" I guess the (1625000.0/12*pi) equals > (1625000/6)/(2pi) and 1625/6 kbit/s is the modulation rate of GSM. > > So, what is the principle of this calculation? How to calculate the > freq_offset with phase_offset. I tried some derivation but failed. Can > anyone help me to find out the derivation ?
The offset is derived assuming the signal is a constant tone. The code is averaging the phase shift between subsequent samples in a burst. Frequency is the first derivative of phase with respect to time, so the phase shift multiplied by the sample rate will give you the frequency of the tone. -TT _______________________________________________ Discuss-gnuradio mailing list Discuss-gnuradio@gnu.org https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
