Patent ID: 7005925

Claim:
A method of synthesising a low noise sinusoidal signal at a desired output frequency comprising using a first variable frequency source and providing a feedback control loop around the first variable frequency source to generate a feedback control signal in successive frequency comparison steps using second and third frequency reference signals, each of the second and third frequency reference signals being derived from a first frequency reference signal, the second frequency reference signal being compared with the output frequency of the first variable frequency source to generate a frequency difference signal, and said frequency difference signal being compared in frequency with the third frequency reference signal to generate said feedback control signal for the first variable frequency source, the frequency of the third frequency reference signal being equal to the difference of the frequency of the second frequency reference signal and the desired output frequency, the second and third frequency reference signals being generated by respective second and third frequency reference sources, characterised in that the first variable frequency sources comprises a first fractional N frequency synthesiser and the second frequency reference source comprises a second variable frequency oscillator the frequency of which is maintained in a limited range around its optimum natural frequency by a second fraction N frequency synthesizer to give optimum performance in producing a low noise output from the first variable frequency source.