| 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 frequ 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 ( 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 se 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 ).
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