LOW NOISE SYNTHESISER

摘要

A low noise sinusoidal signal at a desired output frequency is synchronised using a first variable frequency oscillator (1) and providing a feedback control loop around the first oscillator (1) to generate a feedback control signal in successive frequency comparison steps using second and third frequency reference signals (44, 42). Each of the second and third frequency reference signals are derived from a first frequency reference signal, the second frequency reference signal (44) being compared with the output frequency of the first oscillator (1) to generate a frequency difference signal (43), and said frequency difference signal (43) being compared in frequency with the third frequency reference signal (42) to generate said feedback control signal for the first oscillator (1), the frequency of the third frequency reference signal (42) being equal to the difference of the frequency of the second frequency reference signal (44) and the desired output frequency. The second and third frequency reference signals (44, 42) are generated by respective second and third frequency reference sources, the second frequency reference source comprising a second variable frequency oscillator (38) the frequency of which is maintained in a limited range around its optimum natural frequency to give optimum performance in producing a low noise output from the first oscillator (1).