Simple digital test approach for embedded charge-pump phase-locked loops - Electronics Letters

system the time constant of which is mainly related to the dumping resistance of the RLC series branch in the equivalent circuit. A second pole is introduced by the rectifier [6], so the whole system could be approximated by a two pole transfer function. Note that the PWM modulator moves the negative slope edge of the input voltage signal changing the duty cycle and consequently the amplitude of the fundamental harmonic. Positive slope edges remain fixed, therefore the switching period imposed by the PFD circuit does not change and resonance operation is maintained.