diff options
| -rw-r--r-- | gnuradio-core/src/lib/general/gr_fll_band_edge_cc.h | 26 |
1 files changed, 25 insertions, 1 deletions
diff --git a/gnuradio-core/src/lib/general/gr_fll_band_edge_cc.h b/gnuradio-core/src/lib/general/gr_fll_band_edge_cc.h index 584f62610..09baf7fde 100644 --- a/gnuradio-core/src/lib/general/gr_fll_band_edge_cc.h +++ b/gnuradio-core/src/lib/general/gr_fll_band_edge_cc.h @@ -39,6 +39,26 @@ class gri_fft_complex; * \brief Frequency Lock Loop using band-edge filters * * \ingroup general + * + * The frequency lock loop derives a band-edge filter that covers the upper and lower bandwidths + * of a digitally-modulated signal. The bandwidth range is determined by the excess bandwidth + * (e.g., rolloff factor) of the modulated signal. The placement in frequency of the band-edges + * is determined by the oversampling ratio (number of samples per symbol) and the excess bandwidth. + * The size of the filters should be fairly large so as to average over a number of symbols. + * The FLL works by calculating the power in both the upper and lower bands and comparing them. The + * difference in power between the filters is proportional to the frequency offset. + * + * In theory, the band-edge filter is the derivative of the matched filter in frequency, + * (H_be(f) = \frac{H(f)}{df}. In practice, this comes down to a quarter sine wave at the point + * of the matched filter's rolloff (if it's a raised-cosine, the derivative of a cosine is a sine). + * Extend this sine by another quarter wave to make a half wave around the band-edges is equivalent + * in time to the sum of two sinc functions. The baseband filter fot the band edges is therefore + * derived from this sum of sincs. The band edge filters are then just the baseband signal + * modulated to the correct place in frequency. All of these calculations are done in the + * 'design_filter' function. + * + * Note: We use FIR filters here because the filters have to have a flat phase response over the + * entire frequency range to allow their comparisons to be valid. */ class gr_fll_band_edge_cc : public gr_sync_block @@ -46,7 +66,11 @@ class gr_fll_band_edge_cc : public gr_sync_block private: /*! * Build the FLL - * \param taps (vector/list of gr_complex) The taps of the band-edge filter + * \param samps_per_sym (float) Number of samples per symbol of signal + * \param rolloff (float) Rolloff factor of signal + * \param filter_size (int) Size (in taps) of the filter + * \param alpha (float) Loop gain 1 + * \param beta (float) Loop gain 2 */ friend gr_fll_band_edge_cc_sptr gr_make_fll_band_edge_cc (float samps_per_sym, float rolloff, int filter_size, float alpha, float beta); |