Frequency modulated phase locked loop with fractional divider and jitter

Abstract

A fractional-N type frequency synthesizer has a voltage controlled oscillator controlled in a phase-locked loop by a divide by N divider and a phase comparator responsive to the divided frequency and to a reference frequency Fr. An accumulator is responsive to the desired fractional part of the N and is clocked by Fr to produce carry signals for producing the required periodic variations in N. A second accumulator produces periodic equal and opposite further variations in N to reduce the magnitude of the error waveform which would be given to the phase-detector output by the variations in n caused by the first accumulator. A digital to analog converter and a differentiating circuit produce a jitter correction signal for reducing residual jitter. A coherent detector detects for the presence of any residual jitter at the control input of the VCO and resulting from the fractional-N control circuit. Any such residual jitter produces a control signal which adjusts the value of the jitter correction signal accordingly. Two-port frequency modulation is produced by an in-band circuit incorporating an integrator and a full band circuit. With no FM input, a counter detects any divergence in output frequency from the desired value and caused by spurious input at the integrator. A resultant control signal offsets any such spurious inputs. The coherent detector detects at the control input of the VCO any component which is coherent with the in band modulation signal and adjusts the full band modulation signal to eliminate this, so that correct FM is produced.