1.6. AGC自动增益控制

这里整理一下GNURadio的自动增益控制是如何实现的。自动增益控制模块是在analog大类里实现，并一共定义了三种自动增益控制: agc，agc2，agc3。agc是最普通的自动增益控制,agc，agc2增加了attack和decay控制。attack指的是agc可以多快的响应功率迅速增加的信号，decay指的是agc可以多快的响应功率迅速减小的信号。这两个时间决定了AGC的带宽。通常来说，我们要求AGC的带宽要小于信号的最小频率，这样才不会影响信号的解调。

class ANALOG_API agc_cc
{
public:
    /*!
    * Construct a complex value AGC loop implementation object.
    *
    * \param rate the update rate of the loop.
    * \param reference reference value to adjust signal power to.
    * \param gain initial gain value.
    * \param max_gain maximum gain value (0 for unlimited).
    */
    agc_cc(float rate = 1e-4,
        float reference = 1.0,
        float gain = 1.0,
        float max_gain = 0.0)
        : _rate(rate), _reference(reference), _gain(gain), _max_gain(max_gain){};

    virtual ~agc_cc(){};

    float rate() const { return _rate; }
    float reference() const { return _reference; }
    float gain() const { return _gain; }
    float max_gain() const { return _max_gain; }

    void set_rate(float rate) { _rate = rate; }
    void set_reference(float reference) { _reference = reference; }
    void set_gain(float gain) { _gain = gain; }
    void set_max_gain(float max_gain) { _max_gain = max_gain; }

    gr_complex scale(gr_complex input)
    {
        gr_complex output = input * _gain;

        _gain += _rate * (_reference - std::sqrt(output.real() * output.real() +
                                                output.imag() * output.imag()));
        if (_max_gain > 0.0 && _gain > _max_gain) {
            _gain = _max_gain;
        }
        return output;
    }

    void scaleN(gr_complex output[], const gr_complex input[], unsigned n)
    {
        for (unsigned i = 0; i < n; i++) {
            output[i] = scale(input[i]);
        }
    }

protected:
    float _rate;      // adjustment rate
    float _reference; // reference value
    float _gain;      // current gain
    float _max_gain;  // max allowable gain
};