| 摘要 |
An output having reduced additive phase-noise in at least one defined frequency band is obtained from a voltage-controlled oscillator (5) by applying a frequency-modulating signal omega mt to the oscillator to produce a sideband within the defined band, the signal having a frequency much higher than the highest multiplicative phase-noise frequency (referred to the unmodulated oscillator frequency) in that band and producing a modulation index DELTA P much greater than the modulation index DELTA Pn of the total phase-noise (i.e. additive plus multiplicative) in that band. The oscillator output is then phase-modulated with a signal at omega mt selected from the output of a phase-shift frequency-discriminator (9) having an appropriate delay (d) which receives the oscillator (5) output either after the phase-modulation thereof by modulator (8), or before the phase-modulation thereof by modulator (20), or both, in a sense and at an amplitude which tends to cancel the additive phase-noise modulation in the oscillator output. In a preferred form the discriminator (9) receives the oscillator (5) output before its phase-modulation by modulator (20), and a frequency-modulating pilot-tone ( omega m + omega q)t of modulation index DELTA Pq is also applied to the oscillator, where DELTA P> DELTA Pq>Pn. A feedback loop (29-34) detects variations in the phase and amplitude of the pilot-tone content of the final output and, via multiplier (27), controls the modulating input to modulator (20) in a sense to minimise the pilot-tone content and hence also the additive phase-noise. |
