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SoftFM
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Finished pilot PLL (not tested).

This commit is contained in:
Joris van Rantwijk 2013-12-30 11:22:21 +01:00
parent f0d58eac10
commit f371d1bef3
1 changed files with 22 additions and 10 deletions
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32
FmDecode.cc
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@ -84,8 +84,17 @@ void PhaseDiscriminator::process(const IQSampleVector& samples_in,
// Construct phase-locked loop.
PilotPhaseLock::PilotPhaseLock(double freq, double bandwidth, double minsignal)
{
// This is a type-2, 4th order phase-locked loop.
// I don't understand what I'm doing; hopefully it just works.
/*
* This is a type-2, 4th order phase-locked loop.
*
* Open-loop transfer function:
* G(z) = K * (z - Qz) / ((z - Qp) * (z - Qp) * (z - 1) * (z - 1))
* K = 3.125 * (bandwidth * 2 * PI)**3
* Qz = exp(-0.2 * bandwidth * 2*PI)
* Qp = exp(-2.5 * bandwidth * 2*PI)
*
* I don't understand what I'm doing; hopefully it just works.
*/
// Set min/max locking frequencies.
m_minfreq = (freq - bandwidth) * 2.0 * M_PI;
@ -101,13 +110,16 @@ PilotPhaseLock::PilotPhaseLock(double freq, double bandwidth, double minsignal)
double t = exp(-2.5 * bandwidth * 2.0 * M_PI);
m_phasor_a1 = -2.0 * t;
m_phasor_a2 = t * t;
m_phasor_b0 = m_phasor_a1 + m_phasor_a2;
m_phasor_b0 = 1 + m_phasor_a1 + m_phasor_a2;
// Create loop filter to stabilize the loop.
// Zero at z = exp(-0.2 * bandwidth * 2*PI),
m_loopfilter_b0 = 1.0;
m_loopfilter_b1 = - exp(-0.2 * bandwidth * 2.0 * M_PI);
// TODO : loop gain
// Filter has one Zero at z = exp(-0.2 * bandwidth * 2*PI).
m_loopfilter_b0 = 0.5 * bandwidth * 2.0 * M_PI;
m_loopfilter_b1 = - m_loopfilter_b0 * exp(-0.2 * bandwidth * 2.0 * M_PI);
// After the loop filter, the phase error is integrated to produce
// the frequency. Then the frequency is integrated to produce the phase.
// These two integrators form the two remaining poles, both at z = 1.
// Reset frequency and phase.
m_freq = freq * 2.0 * M_PI;
@ -159,10 +171,10 @@ void PilotPhaseLock::process(const SampleVector& samples_in,
// Use simple linear approximation of arctan.
phase_err = phasor_q / phasor_i;
} else if (phasor_q > 0) {
// We are lagging more than 45 degrees behind the input.
// We are more than 45 degrees ahead of the input.
phase_err = 1;
} else {
// We are more than 45 degrees ahead of the input.
// We are lagging more than 45 degrees behind the input.
phase_err = -1;
}
@ -174,7 +186,7 @@ void PilotPhaseLock::process(const SampleVector& samples_in,
}
// Run phase error through loop filter and update frequency estimate.
m_freq += m_loopfilter_b0 * phase_err
m_freq -= m_loopfilter_b0 * phase_err
+ m_loopfilter_b1 * m_loopfilter_x1;
m_loopfilter_x1 = phase_err;