control_loop.h

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/* -*- c++ -*- */
/*
 * Copyright 2011,2013 Free Software Foundation, Inc.
 *
 * This file is part of GNU Radio
 *
 * GNU Radio is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 3, or (at your option)
 * any later version.
 *
 * GNU Radio is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with GNU Radio; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#ifndef GR_BLOCKS_CONTROL_LOOP
#define GR_BLOCKS_CONTROL_LOOP

#include <gnuradio/blocks/api.h>

namespace gr {
  namespace blocks {

    /*!
     * \brief A second-order control loop implementation class.
     *
     * \details
     * This class implements a second order control loop and is
     * inteded to act as a parent class to blocks which need a control
     * loop (e.g., gr::digital::costas_loop_cc,
     * gr::analog::pll_refout_cc, etc.). It takes in a loop bandwidth
     * as well as a max and min frequency and provides the functions
     * that control the update of the loop.
     *
     * The loop works of alpha and beta gains. These gains are
     * calculated using the input loop bandwidth and a pre-set damping
     * factor. The damping factor can be changed using the
     * #set_damping_factor after the block is
     * constructed. The alpha and beta values can be set using their
     * respective #set_alpha or #set_beta functions if very precise
     * control over these is required.
     *
     * The class tracks both phase and frequency of a signal based on
     * an error signal. The error calculation is unique for each
     * algorithm and is calculated externally and passed to the
     * advance_loop function, which uses this to update its phase and
     * frequency estimates.
     *
     * This class also provides the functions #phase_wrap and
     * #frequency_limit to easily keep the phase and frequency
     * estimates within our set bounds (phase_wrap keeps it within
     * +/-2pi).
     */
    class BLOCKS_API control_loop
    {
    protected:
      float d_phase, d_freq;
      float d_max_freq, d_min_freq;
      float d_damping, d_loop_bw;
      float d_alpha, d_beta;

    public:
      control_loop(void) {}
      control_loop(float loop_bw, float max_freq, float min_freq);
      virtual ~control_loop();

      /*! \brief Update the system gains from the loop bandwidth and damping factor.
       *
       * \details
       * This function updates the system gains based on the loop
       * bandwidth and damping factor of the system. These two
       * factors can be set separately through their own set
       * functions.
       */
      void update_gains();

      /*! \brief Advance the control loop based on the current gain
       *  settings and the inputted error signal.
       */
      void advance_loop(float error);

      /*! \brief Keep the phase between -2pi and 2pi.
       *
       * \details
       * This function keeps the phase between -2pi and 2pi. If the
       * phase is greater than 2pi by d, it wraps around to be -2pi+d;
       * similarly if it is less than -2pi by d, it wraps around to
       * 2pi-d.
       *
       * This function should be called after advance_loop to keep the
       * phase in a good operating region. It is set as a separate
       * method in case another way is desired as this is fairly
       * heavy-handed.
       */
      void phase_wrap();

      /*! \brief Keep the frequency between d_min_freq and d_max_freq.
       *
       * \details
       * This function keeps the frequency between d_min_freq and
       * d_max_freq. If the frequency is greater than d_max_freq, it
       * is set to d_max_freq.  If the frequency is less than
       * d_min_freq, it is set to d_min_freq.
       *
       * This function should be called after advance_loop to keep the
       * frequency in the specified region. It is set as a separate
       * method in case another way is desired as this is fairly
       * heavy-handed.
       */
      void frequency_limit();

      /*******************************************************************
       * SET FUNCTIONS
       *******************************************************************/

      /*!
       * \brief Set the loop bandwidth.
       *
       * \details
       * Set the loop filter's bandwidth to \p bw. This should be
       * between 2*pi/200 and 2*pi/100 (in rads/samp). It must also be
       * a positive number.
       *
       * When a new damping factor is set, the gains, alpha and beta,
       * of the loop are recalculated by a call to update_gains().
       *
       * \param bw    (float) new bandwidth
       */
      virtual void set_loop_bandwidth(float bw);

      /*!
       * \brief Set the loop damping factor.
       *
       * \details
       * Set the loop filter's damping factor to \p df. The damping
       * factor should be sqrt(2)/2.0 for critically damped systems.
       * Set it to anything else only if you know what you are
       * doing. It must be a number between 0 and 1.
       *
       * When a new damping factor is set, the gains, alpha and beta,
       * of the loop are recalculated by a call to update_gains().
       *
       * \param df    (float) new damping factor
       */
      void set_damping_factor(float df);

      /*!
       * \brief Set the loop gain alpha.
       *
       * \details
       * Sets the loop filter's alpha gain parameter.
       *
       * This value should really only be set by adjusting the loop
       * bandwidth and damping factor.
       *
       * \param alpha    (float) new alpha gain
       */
      void set_alpha(float alpha);

      /*!
       * \brief Set the loop gain beta.
       *
       * \details
       * Sets the loop filter's beta gain parameter.
       *
       * This value should really only be set by adjusting the loop
       * bandwidth and damping factor.
       *
       * \param beta    (float) new beta gain
       */
      void set_beta(float beta);

      /*!
       * \brief Set the control loop's frequency.
       *
       * \details
       * Sets the control loop's frequency. While this is normally
       * updated by the inner loop of the algorithm, it could be
       * useful to manually initialize, set, or reset this under
       * certain circumstances.
       *
       * \param freq    (float) new frequency
       */
      void set_frequency(float freq);

      /*!
       * \brief Set the control loop's phase.
       *
       * \details
       * Sets the control loop's phase. While this is normally
       * updated by the inner loop of the algorithm, it could be
       * useful to manually initialize, set, or reset this under
       * certain circumstances.
       *
       * \param phase    (float) new phase
       */
      void set_phase(float phase);

      /*!
       * \brief Set the control loop's maximum frequency.
       *
       * \details
       * Set the maximum frequency the control loop can track.
       *
       * \param freq    (float) new max frequency
       */
      void set_max_freq(float freq);

      /*!
       * \brief Set the control loop's minimum frequency.
       *
       * \details
       * Set the minimum frequency the control loop can track.
       *
       * \param freq    (float) new min frequency
       */
      void set_min_freq(float freq);

      /*******************************************************************
       * GET FUNCTIONS
       *******************************************************************/

      /*!
       * \brief Returns the loop bandwidth.
       */
      float get_loop_bandwidth() const;

      /*!
       * \brief Returns the loop damping factor.
       */
      float get_damping_factor() const;

      /*!
       * \brief Returns the loop gain alpha.
       */
      float get_alpha() const;

      /*!
       * \brief Returns the loop gain beta.
       */
      float get_beta() const;

      /*!
       * \brief Get the control loop's frequency estimate.
       */
      float get_frequency() const;

      /*!
       * \brief Get the control loop's phase estimate.
       */
      float get_phase() const;

      /*!
       * \brief Get the control loop's maximum frequency.
       */
      float get_max_freq() const;

      /*!
       * \brief Get the control loop's minimum frequency.
       */
      float get_min_freq() const;
    };

    // This is a table of tanh(x) for x in [-2, 2] used in tanh_lut.
    static float
    tanh_lut_table[256] = { -0.96402758, -0.96290241, -0.96174273, -0.96054753, -0.95931576,
                            -0.95804636, -0.95673822, -0.95539023, -0.95400122, -0.95257001,
                            -0.95109539, -0.9495761 , -0.94801087, -0.94639839, -0.94473732,
                            -0.94302627, -0.94126385, -0.93944862, -0.93757908, -0.93565374,
                            -0.93367104, -0.93162941, -0.92952723, -0.92736284, -0.92513456,
                            -0.92284066, -0.92047938, -0.91804891, -0.91554743, -0.91297305,
                            -0.91032388, -0.90759795, -0.9047933 , -0.90190789, -0.89893968,
                            -0.89588656, -0.89274642, -0.88951709, -0.88619637, -0.88278203,
                            -0.87927182, -0.87566342, -0.87195453, -0.86814278, -0.86422579,
                            -0.86020115, -0.85606642, -0.85181914, -0.84745683, -0.84297699,
                            -0.83837709, -0.83365461, -0.82880699, -0.82383167, -0.81872609,
                            -0.81348767, -0.80811385, -0.80260204, -0.7969497 , -0.79115425,
                            -0.78521317, -0.77912392, -0.772884  , -0.76649093, -0.75994227,
                            -0.75323562, -0.74636859, -0.73933889, -0.73214422, -0.7247824 ,
                            -0.71725127, -0.70954876, -0.70167287, -0.6936217 , -0.68539341,
                            -0.67698629, -0.66839871, -0.65962916, -0.65067625, -0.64153871,
                            -0.6322154 , -0.62270534, -0.61300768, -0.60312171, -0.59304692,
                            -0.58278295, -0.57232959, -0.56168685, -0.55085493, -0.53983419,
                            -0.52862523, -0.51722883, -0.50564601, -0.49387799, -0.48192623,
                            -0.46979241, -0.45747844, -0.44498647, -0.4323189 , -0.41947836,
                            -0.40646773, -0.39329014, -0.37994896, -0.36644782, -0.35279057,
                            -0.33898135, -0.32502449, -0.31092459, -0.2966865 , -0.28231527,
                            -0.26781621, -0.25319481, -0.23845682, -0.22360817, -0.208655  ,
                            -0.19360362, -0.17846056, -0.16323249, -0.14792623, -0.13254879,
                            -0.11710727, -0.10160892, -0.08606109, -0.07047123, -0.05484686,
                            -0.0391956 , -0.02352507, -0.00784298,  0.00784298,  0.02352507,
                             0.0391956 ,  0.05484686,  0.07047123,  0.08606109,  0.10160892,
                             0.11710727,  0.13254879,  0.14792623,  0.16323249,  0.17846056,
                             0.19360362,  0.208655  ,  0.22360817,  0.23845682,  0.25319481,
                             0.26781621,  0.28231527,  0.2966865 ,  0.31092459,  0.32502449,
                             0.33898135,  0.35279057,  0.36644782,  0.37994896,  0.39329014,
                             0.40646773,  0.41947836,  0.4323189 ,  0.44498647,  0.45747844,
                             0.46979241,  0.48192623,  0.49387799,  0.50564601,  0.51722883,
                             0.52862523,  0.53983419,  0.55085493,  0.56168685,  0.57232959,
                             0.58278295,  0.59304692,  0.60312171,  0.61300768,  0.62270534,
                             0.6322154 ,  0.64153871,  0.65067625,  0.65962916,  0.66839871,
                             0.67698629,  0.68539341,  0.6936217 ,  0.70167287,  0.70954876,
                             0.71725127,  0.7247824 ,  0.73214422,  0.73933889,  0.74636859,
                             0.75323562,  0.75994227,  0.76649093,  0.772884  ,  0.77912392,
                             0.78521317,  0.79115425,  0.7969497 ,  0.80260204,  0.80811385,
                             0.81348767,  0.81872609,  0.82383167,  0.82880699,  0.83365461,
                             0.83837709,  0.84297699,  0.84745683,  0.85181914,  0.85606642,
                             0.86020115,  0.86422579,  0.86814278,  0.87195453,  0.87566342,
                             0.87927182,  0.88278203,  0.88619637,  0.88951709,  0.89274642,
                             0.89588656,  0.89893968,  0.90190789,  0.9047933 ,  0.90759795,
                             0.91032388,  0.91297305,  0.91554743,  0.91804891,  0.92047938,
                             0.92284066,  0.92513456,  0.92736284,  0.92952723,  0.93162941,
                             0.93367104,  0.93565374,  0.93757908,  0.93944862,  0.94126385,
                             0.94302627,  0.94473732,  0.94639839,  0.94801087,  0.9495761 ,
                             0.95109539,  0.95257001,  0.95400122,  0.95539023,  0.95673822,
                             0.95804636,  0.95931576,  0.96054753,  0.96174273,  0.96290241,
                             0.96402758 };

    /*!
     * A look-up table (LUT) tanh calcuation. This function returns an
     * estimate to tanh(x) based on a 256-point LUT between -2 and
     * 2. If x < -2, it returns -1; if > 2, it retursn 1.
     *
     * This LUT form of the tanh is "hidden" in this code because it
     * is likely too coarse an estimate for any real uses of a
     * tanh. It is useful, however, in certain control loop
     * applications where the input is expected to be within these
     * bounds and the noise will be greater than the quanitzation of
     * this small LUT. For more accurate forms of tanh, see
     * volk_32f_tanh_32f.
     */
    static inline float
    tanhf_lut(float x)
    {
      if(x > 2)
        return 1;
      else if(x <= -2)
        return -1;
      else {
        int index = 128 + 64*x;
        return tanh_lut_table[index];
      }
    }

  } /* namespace blocks */
} /* namespace gr */

#endif /* GR_BLOCKS_CONTROL_LOOP */