GB2221110A - FM oscillator - Google Patents

Abstract

An FM oscillator having an output of low phase-noise content, primarily for sub-carrier generation, is obtained by providing an FM oscillator 1 and a CW reference oscillator 2, the latter having an output frequency close to the centre frequency of the former but having a substantially lower total noise content close to its output frequency. The two outputs are multiplied together in a multiplier 3, and by means of filter 5, 6, there is selected from the product a component comprising the zero-order Bessel Function of the modulation index of the FM oscillator multiplied by a function of the difference between the total noise in the two oscillator outputs. This component is combined with the FM oscillator output to produce an output having the centre frequency of the CW oscillator, which is frequency-modulated at the modulating frequency of the FM oscillator and which has the total noise content of the CW oscillator close to the centre frequency. Preferably the centre frequency of the FM oscillator is converted to the output frequency of the CW oscillator in a phase-lock loop 1, 3, 8, before multiplication with the CW oscillator output and the above combination is effected by applying the above selected component to phase-modulate, 10, the thus-converted FM oscillator output. The phase 10 can be replaced by a single side band generator (7, Fig 1). <IMAGE>

Description

Improvements in or relating to oscillator noise reduction This invention relates to methods and apparatus for providing an oscillator having an output of low phase-noise content, arising directly, or indirectly via subsequent modulation processes, from additive and multiplicative noises, and has one application in frequency-modulated (FM) oscillators used to produce a sub-carrier structure.

Oscillator noise can be classified into two main forms.

"Additive" noise is generated in a resistive element within an oscillator and occurs initially at a high frequency comparable with wo, the oscillator centre frequency. "Multiplicative" noise is generated by other elements within an oscillator and can be initially of relatively low frequency xn eg less than 10 kHz, but is translated by non-linearities within these elements to the higher frequencies of w0 w n Both forms of noise phase-modulate and amplitude-modulate the oscillator output. In their higher-frequency forms both additive and multiplicative phase-noises are modified, as regards their spectral characteristics, by the loop transfer characteristics of the oscillator.They can be further modified, in these high-frequency forms, by inherent non-linearities within the loop, eg by an amplitude-limiter designed to reduce all amplitude modulation.

The foregoing paragraph relates to continuous-wave (CW) oscillators. When the oscillator is deliberately FM modulated over a relatively wide band, eg by VCO (voltage-controlled oscillator) action, the multiplicative noise close to the carrier or centre frequency is reproduced about each sideband so that the sideband noise spectra are in effect correlated, ie contain the same noise signal. The additive noise signals about each sideband are not thus correlated but are random as between sidebands, apart from components, around the centre frequency, which are modified by the above loop non-linearities and in effect act as if they arose from the aforesaid translation process.

The very mechanism of frequency modulation which produces the sidebands tends to degrade frequency stabilities, particularly that of the centre frequency, and thus to introduce spurious phase-noise and amplitude-noise. Known methods of reducing the phase-noise have a limited effect, as discussed in our copending Patent Co-operation Treaty (PCT) Application claiming priority from GB Appln No. 8716458, which discloses improved methods and apparatus for further reducing only the random component of additive phase-noise.

Multiplicative phase-noise, and the equivalent correlated component of additive phase-noise, cannot be detected about each modulation sideband by means of frequency discrimination and therefore cannot be reduced by the techniques therein disclosed.

The present invention provides an FM oscillator in which the total phase-noise content of its output (additive and multiplicative phase-noise, plus any amplitude-noise), all close to its carrier or centre frequency, is reduced to the level of the combined additive and multiplicative phase-noise in a low-noise CW reference oscillator. (In practice amplitude noise in both oscillators is of little importance since it is easily removed by limiting). The CW reference oscillator may be a crystal, atomic or other type, of frequency similar to the centre frequency of the FM oscillator but having lower total phase-noise close to its output frequency than does a conventional wideband FM oscillator.

The present invention does not operate over the whole frequency band of the oscillator, but only over regions surrounding its centre frequency and each sideband of the FM waveform, provided that the phase-noise in these sidebands arises from the frequency/phase modulation of the entire FM waveform by the additive and multiplicative noises close to the centre frequency; ie the reduction of sideband noise is restricted to that noise which is correlated as between sidebands and originates from the aforesaid noises about the centre frequency. Two further limitations are that the invention affects only frequency modulation by a sinusoidal waveform (or waveforms) and improves only relatively short-term phase-noise. In a preferred form long-term frequency-stabilisation is provided in addition.For the purposes of this Application, phase-noise is, in effect, a short-term form of frequency instability.

According to the present invention a method of providing a frequency-modulated (FM) oscillator whose output has a total phase-noise content equal to that in the output of a reference continuous-wave (CW) oscillator comprises: providing an FM oscillator and a CW oscillator, the latter being selected to have an output frequency close to the centre frequency of the FM oscillator but having a substantially lower total noise content close to its output frequency; multiplying together the outputs of the two oscillators and selecting by filter action from the product thereof a component comprising the zero-order Bessel Function of the modulation index of the FM oscillator multiplied by a function of the difference between the total noise in the two oscillator outputs;; and combining said component with the FM oscillator output to produce an output which has the centre frequency of the CW oscillator, which is frequency-modulated at the modulating frequency of the FM oscillator, and which has the total noise content of the CW oscillator close to said centre frequency.

Said combining may be effected in single-sideband generating action including a tor/2 phase-shifting of said component and of said FM oscillator output.

The centre frequency of the FM oscillator may be converted to the output frequency of the CW oscillator by phase-lock loop action prior to multiplication with the output of the CW oscillator, and said combining may be effected by applying said component to phase-modulate the thus-converted FM oscillator output.

The invention also provides apparatus for use in a method as aforesaid.

The invention will now be described, by way of example, with reference to the accompanying drawings wherein: Fig 1 is a schematic diagram of an oscillator circuit embodying one form of the present invention; Fig 2 is a similar diagram embodying another form of the present invention.

The following notation will be used in the subsequent description of the invention: FM FM oscillator carrier or centre angular frequency, eg at 200 MHz Xm e FM oscillator modulating angular frequency, eg at 200 kHz #r - CW reference oscillator angular frequency, eg at approx 200 MHz AP a modulation index of #m' eg 20 radians #Po - multiplicative and additive phase-noise modulation of FM oscillator, both based around #o #Pr - multiplicative and additive phase-noise modulation of CW reference oscillator, both based around #r # = phase-difference between #r and centre frequency of FM oscillator after phase-locking Jo,J1 etc = Bessel Functions of appropriate arguments Referring now to Fig 1, a wide-band voltage-controlled oscillator (VCO) 1 has an output expressed in the above notation by sin (#ot+#P cos #mt + #Po) m 0 where AP is of relatively low frequency.

The above expression for the VCO output implies the presence of an amplitude-limiter either internal or external to the VCO for eliminating/reducing all amplitude-modulation components.

A CW reference oscillator 2, eg a crystal oscillator having inherently low total noise close to its output frequency, has an output expressed in the above notation by sin (#rt + APr) where again all amplitude modulation has been removed.

These two outputs are fed to a multiplier 3. If necessary any noise (total) sidebands remote from #r which could beat with the sidebands of the FM oscillator 2 and thus give spurious outputs, can be removed by including a bandpass filter 4 centred on #r and of bandwidth less than 2#m The above two signals are multiplied in circuit 3 and the product of frequency (O - ur) is selected by the filter 5 which rejects (#o + #r). This product is cos (#o - rt + #Po - APr + AP cos U t) or cos (#ot-#rt + #Po - #Pr) cos (#P cos #mt) - sin (#ot - #rt + #Por - #Pr) shin (#P cos #mt) The cos (#P cos #mt) and sin (#P cos #mt) terms can be expanded in a known manner to produce the terms Jo(#P), J1(P) cos #mt, etc.Thus after filtering the product in filter 6 to remove the #m terms the output becomes m Jo(#P) cos (#ot - #rt + #Po - #Pr) Provided (O - ur) is greater than the frequencies contained in O and AP (thus also passed by filter 5), the 0 r quadrature term Jo(#P) sin (#ot - #rt+ #Po - #Pr) can be derived.

These two terms, together with the signal from VCO1 and its quadrature version, can thus be combined in a conventional single-sideband generator 7 (which includes quadrature phase-shifters 11, 12) to produce an output signal Jo (#P) sin (#ot + #P cos #mt + #Po - #ot + #rt - #Po + - sin (wrt + AP cos mmt + APr) X Jo (#P) (1) Expression (1) represents a wideband FM output with its additive and multiplicative noises close to #r determined solely by the reference noise #Pr. The amplitude of this output is determined by the factor Jo(#P). As is known, the value of J (#P) falls to zero at some values of AP, and hence this factor must be kept finite by suitable choice of AP.

It will be seen that the low multiplicative-noise FM output now has a carrier frequency of #r instead of the original #o. However these two frequencies can be made fairly close and their values chosen to give the final required carrier output frequency.

The above-described arrangement has two disadvantages.

Firstly, a conventional single-sideband generator does not necessarily eliminate the second sideband; a reduction of about 20 dB is usually achieved. Leakage of the second sideband is undesirable as it will increase the noise content.

If a pilot tone having a frequency in the band occupied by AP 0 is added to the original FM modulating signal wm, an arrangement using the pilot-tone monitoring principle of Fig 5 of the aforesaid POT Application can be used to improve the reduction of unwanted #Po via reduction of the pilot tone.

The other disadvantage is that (#o - #r) must be greater than the absolute frequencies of AP and Ap as already mentioned, 0 r and must also be less than wm. These latter restrictions, In together with the added complexity of the pilot-tone technique, makes the alternative arrangement of Fig 2 preferable.

The arrangement of Fig 2 is based upon deliberately making o become equal to wr. As in Fig 1, the output of the CW r reference oscillator 2 is fed to a multiplier 3, if necessary via filter 4. However, the output of multiplier 3 is fed via a low-pass (virtually passing only DC) filter 8 to the voltage-control terminal of the VCO1. Thus VCO1, multiplier 3 and filter 8 act as a conventional phase-lock loop, with the normal result that w becomes wr, ie from the long-term aspect, 0 excluding phase-noise.The output of VCO1 can now be expressed as sin (a)rt + AP cos w t + AP ) ---. (2) m 0 A second output from multiplier 3 is taken to a low-pass filter 51 which rejects 2w and passes the frequencies o 0 contained in AP and AP and, as before, thence to a low-pass 0 r filter 6 to remove the wm components. The (#o wr) term is now a DO term which is desirably removed, eg by a series capacitor, to avoid adversely affecting the subsequent amplifier 9 (see below).The thus-filtered multiplier output is now Jo(#P) cos (# + #Po - #Pr) If # is made equal to #/2 (# tends to #/2 if #o before locking is approximately equal to w ), then the above output becomes JO(AP) sin (#Po - APr) # Jo(#P) x (#Po - #Pr) ..... (3) This output is amplified in amplifier 9 to increase the amplitude JO(AP) to unity and is applied as a modulating signal to a low-deviation phase-modulator 10 where it modulates the phase-locked signal from oscillator 1.The resultant phase-modulated FM output signal bcomes cos (#rt + #P cos #mt + #Po - #Po + #Pr) - cos (#rt + #P cos #mt + #Pr) ..... (4) ie a signal having its noise content defined by the noise around #r as in equation (1).

The phase-modulator 10 can be of the kind shown in Fig 6 of the aforesaid POT Application. Instead of the simple phase-lock loop shown in Fig 2, a loop of kind shown in Figure 4 of our copending GB Application No 8705428 (Publication No. 2,187,907A) can be used to make - #/2 automatically.

Otherwise it may be necessary to adjust w manually to make 0 and hence # - #/2.

Since the multiplier output depends on the factor Jo (#P) as in Fig 1, AP must again be chosen to produce a finite output. In practice, AP is chosen to give a maximum, or near maximum, value for J0 (AP). In effect JO(AP) represents a potential sensitivity reduction, in detecting the phase-noise of equation (2). The larger the deviation AP, the greater the sensitivity reduction even after maximising the Bessel Function. Such a reduction, however, will be much less than that likely from a frequency-discriminator which could be used to detect AP or AP directly from equations (3) and (4). The 0 r latter detection could be used subsequently, after processing, to reduce the noise in a manner similar to that described in the aforesaid POT Application, but with the disadvantages described therein.

The limitation regarding JO(AP) means that the present invention is normally restricted to applications in which the modulation wm is used to generate a sub-carrier structure, In where AP can be of a selected fixed value. However, reduced noise around the centre (basic carrier) frequency, as achieved by the present invention, is valuable even in a sub-carrier structure, because of its importance in some subsequent signal-processing techniques.

Claims (5)

Claims

1. A method of providing a frequency-modulated (FM) oscillator whose output has a total phase-noise content equal to that in the output of a reference continuous-wave (CW) oscillator comprising: providing an FM oscillator and a CW oscillator, the latter being selected to have an output frequency close to the centre frequency of the FM oscillator but having a substantially lower total noise content close to its output frequency; multiplying together the outputs of the two oscillators and selecting by filter action from the product thereof a component comprising the zero-order Bessel Function of the modulation index of the FM oscillator multiplied by a function of the difference between the total noise in the two oscillator outputs;; and combining said component with the FM oscillator output to produce an output which has the centre frequency of the CW oscillator, which is frequency-modulated at the modulating frequency of the FM oscillator, and which has the total noise content of the CW oscillator close to said centre frequency.

2. A method as claimed in claim 1 wherein said combining is effected in single-sideband generating action including a w/2 phase-shifting of said component and of said FM oscillator output.

3. A method as claimed in claim 1 wherein the centre frequency of the FM oscillator is converted to the output frequency of the CW oscillator by phase-lock loop action prior to multiplication with the output of the CW oscillator, and wherein said combining is effected by applying said component to phase-modulate the thus-converted FM oscillator output.

4. Apparatus for use in a method as claimed in any of claims 1 to 3.

5. A method or apparatus for providing an FM oscillator whose output has a total noise content equal to that in the output of a reference CW oscillator having a substantially lower total noise content close to its output frequency substantially as hereinbefore described with reference to the accompanying drawings.