Fractional N frequency synthesizer with modulation compensation

1. A frequency synthesiser of the fractional N type comprising a primary voltage controlled oscillator for producing an output signal which is afforded to a phase detector via a primary divider to provide a control signal for the primary voltage controlled oscillator in the presence of a phase difference between a reference signal from a reference source and the signal afforded thereto from the primary divider, the division ratio of which may be varied in a fractional N mode to vary the frequency of the output signal in frequency steps smaller than the frequency of the reference signal and wherein a ripple signal tending thereby to be produced and causing frequency modulation of the output signal is compensated by a phase modulator, arranged to couple the primary divider and the phase detector and to receive a drive signal wherein the synthesiser further comprises a subsidiary phase lock loop arranged to provide an enhanced representation of the ripple signal in dependence upon the signal afforded to the phase detector from the phase modulator, and a digital to analogue converter arranged to provide the drive signal for the phase modulator in dependence upon the enhanced representation of the ripple signal.

2. A synthesiser according to claim 1 wherein the subsidiary phase lock loop comprises a subsidiary voltage controlled oscillator having a sensitivity lower than that of the primary voltage controlled oscillator, a subsidiary divider, and a subsidiary phase detector arranged to receive as a reference signal for the subsidiary phase lock loop a signal dependent upon the signal afforded to the phase detector from the phase modulator.

3. A synthesiser according to claim 2 wherein the subsidiary voltage controlled oscillator comprises a voltage tuned crystal oscillator.

4. A synthesiser according to one of claims 1, 2 or 3 comprising a sense detecting circuit, responsive to the enhanced respresentation of the ripple signal for providing a signal indicative of the sense of any error in the level of the drive signal to the phase modulator.

5. A synthesiser according to claim 4 comprising a voltage generator circuit arranged to provide a signal dependent upon the mean division ratio of the primary divider, and wherein the digital to analogue converter comprises a multiplying digital to analogue converter arranged to receive a reference signal dependent upon the combination of the signal provided by the voltage generator circuit and the signal provided by the sense detecting circuit and wherein the digital to analogue converter is arranged to provide the drive signal for the phase modulator in dependence upon the reference signal afforded thereto.

6. A synthesiser according to claim 4 wherein the sense detecting circuit comprises a first limiter arranged to receive a signal dependent upon the output signal from the digital to analogue converter and to provide a limited output signal therefrom, a further limiter arranged to receive a signal dependent upon the enhanced representation of the ripple signal provided by the subsidiary phase lock loop and to provide a limited output signal therefrom, and an exclusive OR gate arranged to receive output signals provided by the first and further limiters and to provide therefrom a signal indicative of the sense of the ripple signal.

7. A synthesiser according to claim 6 wherein the sense detecting circuit comprises a low pass filter for filtering the signal dependent upon the output signal from the digital to analogue converter.