Oscillator phase-noise reduction

Details Oscillator phase-noise reduction Oscillator phase-noise reduction

1990-02-15

1991-02-19

Mis, David

Modulators

Frequency modulator

Including stabilization or alternatively distortion, noise...

455 63, H03C 300, H03C 309

Patent

active

049947672

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to methods and apparatus for reducing the phase-noise in the output from an oscillator, in particular the additive phase-noise. It has one application in frequency-modulated (FM) oscillators and can also provide a non-modulated (CW) output having reduced additive phase-noise.
2. Discussion of Prior Art
Phase-noise can be classified into two main sources. The first is "additive" noise, in which the thermal noise of the oscillator, within the latter's spectral band, is added to its spectrum after amplification in accordance with the loop transfer function of the oscillator. This noise is subsequently converted indirectly to phase-modulation sidebands of the instantaneous oscillator frequency by the removal of amplitude-modulation (AM) components as a result of amplitude-limiting processes within the oscillator. This noise is thermal over the entire band, and is thus uncorrelated with itself over this band (except for a component produced by the above limiting processes, which is unaffected by the present invention).
The second source of phase-noise is "multiplicative" noise, which involves low-frequency noise (typically flicker noise) well outside the spectral band of the loop transfer function but generated within the oscillator. This low-frequency noise is converted upwards to lie within the loop band, as a result of non-linearities within the oscillator. Since, in effect, this noise modulates the oscillator, it produced sidebands corresponding to direct frequency/phase modulation of the oscillator. If the oscillator is itself deliberately modulated, e.g. by VCO (voltage-controlled oscillator) action, these noise sidebands are produced about each deliberately produced modulation sideband. These separate noise sidebands are thus correlated with each other unlike the sidebands from any separate additive noise which, as described above, are uncorrelated over the entire band.
In the case of FM oscillators, the very mechanism of frequency modulation which produces the sidebands tends to degrade frequency stabilities and thus to introduce spurious phase and amplitude noise arising from both additive and multiplicative sources. As will be shown, existing methods of reducing such phase-noise have a limited effect, and it is an object of the present invention to produce a further reduction. This reduction is effected in the additive phase-noise only, not in the multiplicative phase-noise (nor, as stated, in the unaffected additive component mentioned earlier, which effectively becomes multiplicative phase-noise).


SUMMARY OF THE INVENTION

The present invention does not operate over the whole frequency band of the oscillator, but only over a region surrounding each sideband. However, this is sufficient where the information and noise bandwidth is small compared with the modulation frequency, as is usually the case. Two further limitations are that the invention affects only frequency modulation by a sinusoidal waveform (or waveforms) and improves only relatively short-term additive phase-noise. Long-term frequency-stabilisation can be provided in addition, using known arrangements for this purpose. For the purpose of the Application, phase-noise is, in effect, a short-term form of frequency instability, as will be shown.
According to the present invention a method of reducing the additive phase-noise in the output from an oscillator at a given frequency, in at least one defined frequency band, comprises: frequency-modulation sideband of said given frequency within said band, said 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 much greater than the modulation index of the total phase-noise (i.e. additive plus multiplicative) in than band; modulating frequency selected from the output of a frequency-discriminator having an appropriate delay which receives an amplitude-limited version of the oscillato

REFERENCES:
patent: 3137816 (1964-06-01), McLin et al.
patent: 3546703 (1970-12-01), Kurth
patent: 4543542 (1985-09-01), Owen
patent: 4755774 (1988-07-01), Heck

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