PHASE NOISE DETECTOR

Abstract: A phase noise detector for a signal having a carrier frequency, comprising a frequency discriminator producing a dispersed signal from the signal and a phase detector means (28) responsive to the carrier frequency in the signal and to a carrier suppressed signal to produce an output signal corresponding to the noise close to the carrier frequency in the signal, the carrier suppressed signal being produced by a carrier suppression means (30) responsive to two input signals, the input signals corresponding to the signal, one of the input signals being the dispersed signal.

Description: TITLE PHASE NOISE DETECTOR TECHNICAL FIFI n The present invention relates to phase noise detectors, and particularly to phase noise detectors in microwave oscillators having a high Q resonator as a frequency in which the phase noise detector is used to suppress noise close to the carrier frequency in the microwave oscillator. BACKGROUND ART Phase noise detectors have uses in a variety of applications . One such application is microwave oscillators, where phase noise detectors are used to servo noise close to the carrier frequency from the oscillator. Microwave oscillators are well known and are used in applications where a high frequency signal source is needed. Two examples of such applications are radar and telecommunications systems, which typically require a signal source with good spectral purity. Loop oscillators are well known in the art. Loop oscillators typically include a microwave amplifier, a resonator and a phase shifter arranged in a loop . There are limitations to the performance of loop oscillators, such as the limitations imposed by flicker noise in the microwave amplifier. Flicker noise adds noise close to the carrier frequency in the microwave oscillator, which is undesirable in applications requiring a stable frequency source. Another type of oscillator known in the art is a cavity stabilised oscillator, wherein a frequency source such as an external oscillator or a voltage controlled oscillator, which produce a comparatively noisy signal, is input to a high Q resonator used as a frequency discriminator. The resonator acts to filter some of the noise from the frequency source, and the resonator and a detector, such as a phase detector, are used to measure the noise in the frequency source and, based upon that measurement, servo some of the noise from the frequency source. Both of these types of oscillator have limitations on their performance imposed by the non-ideal nature of the components. There are ways of compensating for these limitations by incorporating a servo circuit to suppress some of the noise. A paper presented by D.P. Tsarapkin at the 1994 IEEE International Frequency Control Symposium entitled "Low phase noise sapphire disk dielectric resonator oscillator with combined stabilisation" discusses known configurations for suppressing noise in microwave oscillators, some of which have been known for over 20 years. United States Patent No 4,555,678 in the name of Galani et al discloses a microwave loop oscillator in which a phase noise detector is used to suppress noise close to the carrier frequency in the oscillator. The oscillator disclosed in Galani provides increased performance characteristics in terms of noise close to the carrier frequency in the loop of the oscillator. There are limitations to the amount of noise suppression that can be achieved using the disclosure in Galani, however. In practice, the amount of noise suppression is fundamentally limited by the noise in the phase noise detector, the gain in the servo circuit used to suppress the noise and by noise inherent in the servo circuit. At present, a significant amount of noise is added by the mixer in phase noise detectors used in noise suppression circuits. Flicker noise added by the mixer has therefore placed restrictions on the performance of oscillators because of the limitations imposed in suppressing noise. This limitation applies to all types of oscillators incorporating a noise suppression circuit, including loop oscillators and cavity stabilised oscillators. SUMMARY OF THE INVENTION Accordingly, the invention resides in a phase noise detector for a signal having a carrier frequency, comprising a frequency discriminator producing a dispersed signal from the signal and a phase detector means responsive to the carrier frequency in the signal and to a carrier suppressed signal to produce an output signal corresponding to the noise close to the carrier frequency in the signal, the carrier suppressed signal being produced by a carrier suppression means responsive to two input signals, the input signals corresponding to the signal, one of the input signals being the dispersed signal. According to a preferred feature of the invention, the carrier suppression means comprises a power combiner, a phase shift means and an amplitude matching means, the phase shift means and the amplitude matching means arranged to be operative on the two input signals before being input to the power combiner such that the power combiner produces the carrier suppressed signal from the two signals input to the power combiner. According to a preferred feature of the invention, the dispersed signal is a signal reflected from the frequency discriminator. According to an alternative preferred feature of the invention, the dispersed signal is a portion of a signal output from the frequency discriminator. According to a preferred feature of the invention, the frequency discriminator comprises a resonator. According to a further preferred feature of the invention, the resonator is close to critically coupled. Accordingly, the invention also resides in a microwave oscillator including a phase noise detector according to any one of the preceding statements, wherein the signal is a microwave signal in the microwave oscillator, the signal output from the phase detector means being fed back to suppress noise close to the carrier frequency in the microwave oscillator. According to a preferred feature of the invention, the frequency discriminator forms part of the microwave oscillator. According to a preferred feature of the invention, the microwave oscillator is a loop oscillator including a microwave amplifier and a phase shift means responsive to a control signal, the microwave amplifier, the phase shift means and the frequency discriminator being arranged in a loop, the signal output from the phase noise detector being input to an electronic circuit which produces the control signal for the phase shift means such that the phase shift means suppresses noise close to the carrier frequency in the loop oscillator. According to an alternative preferred feature of the invention, the microwave oscillator includes a frequency source responsive to a control signal, the frequency source producing the signal, wherein the signal output from the phase noise detector is input to an electronic circuit which produces the control signal for the frequency source, the control signal adjusting the frequency of the frequency source to suppress noise close to the carrier frequency in the frequency source. Accordingly, the invention also resides in a method of detecting phase noise in a signal having a carrier frequency, comprising the steps of: producing from the signal a dispersed signal using a frequency discriminator; adjusting the phase difference between the signal and the dispersed signal such that, at the carrier frequency, the signal and the dispersed signal have a predetermined phase difference; adjusting the amplitudes of the signal and the dispersed signal such that, at the carrier frequency, the signal and the dispersed signal have substantially equal amplitude; combining the signal and the dispersed signal to produce a carrier suppressed signal; and demodulating the carrier suppressed signal to produce an output signal corresponding to phase noise close to the carrier frequency in the signal. According to a preferred feature of the invention, the dispersed signal is a signal reflected from the frequency discriminator. According to an alternative preferred feature of the invention, the dispersed signal is a portion of a signal output from the frequency discriminator. According to a preferred feature of the invention, the frequency discriminator comprises a resonator. According to a further preferred feature of the invention, the resonator is close to critically coupled. According to a preferred feature of the invention, the phase difference between the signal and the dispersed signal is adjusted by passing at least one of the signal and the dispersed signal through a phase shifter to alter the phase of that signal by a predetermined amount. According to a preferred feature of the invention, the amplitudes of the signal and the dispersed signal are adjusted by passing at least one of the signal and the dispersed signal through an attenuator to attenuate the amplitude of that signal by a predetermined amount. According to a preferred feature of the invention, the signal and the dispersed signal are combined by passing the signal and the dispersed signal through a power combiner, the power combiner producing the carrier suppressed signal from the signal and the dispersed signal. According to a preferred feature of the invention, the carrier suppressed signal is demodulated by passing the carrier suppressed signal and the carrier frequency into a double balanced mixer. BRIEF DESCRIPTION OF THE DRAWINGS Five embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic drawing of a loop oscillator according to the first embodiment; Figure 2 is a schematic drawing of a cavity stabilised oscillator according to the second embodiment; Figure 3 is a schematic drawing of an alternative loop oscillator according to the third embodiment; Figure 4 is a schematic drawing of a further alternative loop oscillator according to the fourth embodiment; and Figure 5 is a schematic drawing of a Pound stabilised loop oscillator according to the fifth embodiment. DESCRIPTION OF THE BEST MODE OF THE INVENTION The embodiments are directed towards microwave oscillators which use phase noise detectors to suppress noise in the microwave oscillator. It should be appreciated, however, that the present invention has application other than in microwave oscillators. In Figure 1 of the drawings there is shown a microwave loop oscillator 10 comprising a microwave amplifier 12, a resonator 14, phase shifters 16 and 18, isolators 20, a circulator 22, an attenuator 24 and a band pass filter 26 arranged in a loop. The arrow A indicates the direction of flow of microwaves around the oscillator 10. The band pass filter 26 is used to prohibit the oscillator 10 from operating on spurious modes of the resonator 14. The attenuator 24 is used to ensure the small signal gain margin is sufficient to ensure oscillation without damaging the microwave amplifier 12 or inducing excess noise in the microwave amplifier 12. The phase shifters 16 and 18 are used to ensure the phase shift around the loop is a multiple of 2π, satisfying the conditions for oscillation. The phase shifter 18 is controlled by a control signal, for example the phase shifter 18 may comprise a voltage controlled phase shifter. The resonator 14 has an input coupling close to critical to increase the power dissipated in the resonator 14, which is necessary to improve the frequency discriminator noise floor. The critical coupling causes the majority of a signal input to the resonator 14 will be transmitted into the resonator 14 and a small portion of the signal will be reflected from the input. If the carrier frequency of the signal input to the resonator 14 is not equal to a resonant frequency of the resonator 14, the reflected signal will have been phase shifted by the resonator 14 by an amount corresponding to the difference between the resonant frequency of the resonator 14 and the carrier frequency of the input signal. The oscillator 10 further comprises a phase noise detector 28 including a carrier suppression means 30, a double balanced mixer 32 and a phase shifter 34. The carrier suppression means 30 comprises a phase shifter 42, an attenuator 44, a power combiner in the form of a 3dB 90° hybrid 46, an microwave amplifier 48, a power limiter 50 and isolators 52. The 3dB 90° hybrid 46 has input ports 54 and 56, and output ports 58 and 60. The oscillator 10 still further comprises couplers 36, 38 and 40. The coupler 36 produces an output signal from the loop. The coupler 36 can be positioned anywhere in the loop. The couplers 38 and 40 produce signals which are portions of the signal in the loop prior to the signal being incident upon the resonator 14. The signal produced by the coupler 38 passes through the phase shifter 34 and into the mixer 32. The circulator 22 is preferably located at the input to the resonator 14, such that the reflected signal is directed from the loop into the input port 54 of the 3dB 90° hybrid 46. The signal produced by the coupler 40 passes through the phase shifter 42, the attenuator 44 and into input port 56 of the 3dB 90° hybrid 46. The phase shifter 42 and the attenuator 44 are arranged such that, at the centre frequency of the resonator 14, the signals appearing at the input ports 54 and 56 of the 3dB 90° hybrid have equal amplitude and are in quadrature. The 3dB 90° hybrid 46 acts to split the signals present at each of the input ports 54 and 56 equally and directs the split signals to each of the output ports 58 and 60. In doing so, the 3dB 90° hybrid 46 adds a 90 degree phase shift to the split signal appearing at the output port 60 from the input port 54. Similarly, the 3dB 90° hybrid 46 adds a 90 degree phase shift to the split signal appearing at the output port 58 from the input port 56. Thus, the split signals from the input ports 54 and 56 are 180 degrees out of phase at the output port 58 and are in phase at the output port 60. It should be appreciated that other forms of power combiner can be used. For example, a 3dB 180° hybrid could be used, in which case the two signals appearing at the input ports of the hybrid would need to be in phase. If a 6dB coupler was used, the two signals input to the coupler would need to be 180° out of phase. The form of power combiner used determines the requirements on the phase difference needed between the two signals input to the power combiner. The important feature is that the input signals have the correct phase difference to enable carrier suppression. Consequently, the carrier appears at the output port 60 whilst the carrier is suppressed at the output port 58. Carrier suppression of upto 80dB has been achieved using this circuit. Preferably, the carrier suppression is at least twice the gain of the microwave amplifier 48, so that the microwave amplifier 48 operates in small signal mode to minimise noise produced by the microwave amplifier 48. Since the signal appearing at the output port 58 consists substantially of only the difference between the signal appearing at the input ports 54 and 56, the signal represents the noise in the loop. The signal appearing at the output port 58 is amplified by the microwave amplifier 48, passed through the power limiter 50 and input to the mixer 32. The power limiter 50 is provided to protect the mixer 32 from being damaged by high output power from the microwave amplifier 48 when conditions of carrier suppression are not met. It is anticipated that in some applications the amplifier 48 may be omitted. Without carrier suppression, the signal appearing at the mixer 32 from the circulator 22 would have been dominated by the carrier, with the noise component of the signal having an amplitude considerably smaller than that of the carrier. The presence of the carrier results in the microwave amplifier 48 and the mixer 32 operating in large signal mode. Large signal mode operation of the microwave amplifier 48 and the mixer 32 causes the microwave amplifier 48 and the mixer 32 to add flicker noise. The flicker noise so added would be detrimental to the operation of the oscillator. Carrier suppression of the signal ensures small signal operation of the microwave amplifier 48, which satisfies the conditions of low noise operation of the mixer 32. The signal appearing at the output port 60 of the 3dB 90° hybrid 46 contains predominantly the carrier in the signal. The output port 60 can be coupled to a balanced load to absorb the signal appearing at the output port 60. Alternatively, the signal appearing at the output port 60 can be used to monitor the amplitude of the signal in the loop and control the same through an amplitude stabilisation circuit. The phase shifter 34 adjusts the phase of the signal coupled from the loop by the coupler 38 such that, at the carrier frequency of the oscillator 10, the signals appearing at the inputs of the mixer 32 are in quadrature. The signal output from the mixer 32 is input to a electronic circuit 62 comprising a low noise amplifier 64 and a low pass filter 66. The signal output from the electronic circuit 62 is used as the control signal for the phase shifter 18. The phase shifter 18 induces a phase shift in the loop in response to the control signal such that noise close to the carrier frequency in the loop is suppressed. Typically, noise is introduced into the signal in the loop by the microwave amplifier 12. When the signal is incident upon the resonator 14 the presence of the noise results in a dispersed signal in the form of a phase shifted reflected signal being directed by the circulator 22 into the carrier suppression means 30. The phase shift in the reflected signal results in the signals at the input ports 54 and 56 of the 3dB 90° hybrid 46 not being in exact quadrature. Thus the signal present at the output port 58 is representative of the noise in the loop. Further, the signals appearing at the inputs to the mixer 32 will similarly not be in quadrature, resulting a signal output from the mixer 32 corresponding to the noise in the loop. From the signal output from the mixer 32 the electronic circuit 62 produces the control signal for the phase shifter 18 such that the phase shifter 18 suppresses the noise close to the carrier in the loop. The embodiment shown in Figure 2 is directed toward a cavity stabilised oscillator, with like reference numerals denoting like parts to those shown in Figure 1. The oscillator 100 shown in Figure 2 includes a frequency source 102 which is input to the resonator 14. The signal output from the resonator 14 is used as an output. In effect, the frequency source 102 is locked to the resonator 14. The frequency source 102 is typically a frequency multiplied SAW oscillator, a dielectric resonator oscillator, a VCO, a YIG oscillator or the like. The frequency source 102 is controllable by a control signal. The phase detecting means 28 operates in the same manner as described in the first embodiment, in this embodiment producing a signal corresponding to the noise in the frequency source 102. The signal output from the mixer 32 is input to the electronic circuit 62 which produces the control signal for the frequency source 102. Thus, in this embodiment, the phase noise detector 28 is used to suppress noise in the frequency source 102 which is locked to the resonator 14. Alternatively, the signal output from the mixer 32 could be used to measure the performance of the frequency source 102, since it represents the noise in the frequency source 102. Thus the present invention can be used to monitor phase noise in a signal or frequency source. A third embodiment of the invention is shown in Figure 3, with like reference numerals denoting like parts to those shown in Figure 1. The embodiment shown in Figure 3 is also a loop oscillator. The loop oscillator 200 shown in Figure 3 differs from the loop oscillator 10 shown in Figure 1 in the position of the couplers 38 and 40 in the loop. In Figure 3, the couplers 38 and 40 are positioned at the output of the resonator 14. Further, the circulator 22 in the first embodiment has been replaced by a coupler 202. Since couplers are directional, that is they couple signals traveling in a single direction only, the coupler 202 is arranged such that it couples the reflected signal from the resonator 14. The couplers 38 and 40 may be positioned at the output of the resonator 14 as shown in Figure 3 without substantially affecting the performance of the loop oscillator. The coupler 38 can be positioned anywhere in the loop. The couplers 40 and 202 must be positioned such that there is a difference between the signals coupled from the loop, which difference is a result of dispersion by the resonator 14. Thus in the embodiment shown in Figure 1 , the coupler 40 produces a signal undispersed by the resonator 14 while the circulator 22 directs the reflected signal dispersed by the resonator 14 from the loop. In the embodiment shown in Figure 3, the coupler 202 couples the reflected signal dispersed by the resonator 14 and the coupler 40 couples the signal output from the resonator 14. While the signal output from the resonator 14 has also been dispersed by the resonator 14, it has been dispersed to a different extent to the reflected signal. Consequently, the embodiment shown in Figure 3 still provides improved noise performance. However, if the couplers 202 and 40 were both positioned at the output of the resonator 14, the signals coupled by the couplers 202 and 40 would be the same, and the phase detector means 28 would not function. A fourth embodiment of the invention is shown in Figure 4, with like reference numerals denoting like parts to those shown in Figure 1. The embodiment shown in Figure 4 is also a loop oscillator. The loop oscillator 300 shown in Figure 4 differs from the loop oscillator 10 shown in Figure 1 in that the signal present at the output port 60 is passed through the phase shifter 34 and input to the mixer 32. Since the signal present at the output port 60 is predominantly the carrier frequency in the loop, it is suitable to be used in the LO input of the mixer 32. Thus in this embodiment, the need for the coupler 38 in the first embodiment has been eliminated, resulting in a more compact design. A fifth embodiment is shown in Figure 5 with like reference numerals denoting like parts to those shown in Figure 1. The fifth embodiment is a Pound stabilised loop oscillator 400, in which an external frequency source 402 is used to modulate the carrier frequency of the oscillator 400. A summer 404 and a detector diode 406 are also provided. The output from the microwave amplifier 48 is input to the detector diode 406, and the demodulated signal output from the detector diode 406 is input to the mixer 32. The frequency source 402 is also input to the mixer 32. The mixer 32 produces an output signal which is passed through the low pass filter 66 and input to the summer 404. The frequency source 402 is also input to the summer 404. There is also provision for a bias Uo to be input to the summer 404. The output from the summer 404 is the sum of the signals input to the summer 404. The output from the summer 404 is used as the control signal for the phase shifter 18. Modifications and variations such as would be apparent to a skilled addressee are deemed within the scope of the present invention. In particular, the power combiner can take forms other than a 3dB 90° hybrid, such as a Wilkinson power combiner, or a 6dB or 10dB coupler. If the microwave oscillator was manufactured in microstrip form, an impedance transformer can be used as the power combiner.

Claims: CLAIMS 1. A phase noise detector for a signal having a carrier frequency, comprising a frequency discriminator producing a dispersed signal from the signal and a phase detector means responsive to the carrier frequency in the signal and to a carrier suppressed signal to produce an output signal corresponding to the noise close to the carrier frequency in the signal, the carrier suppressed signal being produced by a carrier suppression means responsive to two input signals, the input signals corresponding to the signal, one of the input signals being the dispersed signal. 2. A phase noise detector as claimed in claim 1 , wherein the carrier suppression means comprises a power combiner, a phase shift means and an amplitude matching means, the phase shift means and the amplitude matching means arranged to be operative on the two input signals before being input to the power combiner such that the power combiner produces the carrier suppressed signal from the two signals input to the power combiner. 3. A phase noise detector as claimed in claim 2, wherein the carrier suppressed signal produced by the power combiner is amplified before being input to the phase detector means. 4. A phase noise detector as claimed in any one of claims 2 or 3, wherein the amplutude matching means comprises an attenuator. 5. A phase noise detector as claimed in any one of the preceding claims, wherein the dispersed signal is a signal reflected from the frequency discriminator. 6. A phase noise detector as claimed in any one of claims 1 to 4, wherein the dispersed signal is a portion of a signal output from the frequency discriminator. 7. A phase noise detector as claimed in any one of claims 2 to 6, wherein the power combiner comprises a 3dB 90° hybrid. 8. A phase noise detector as claimed in any one of the preceding claims, wherein the phase detector means comprises a double balanced mixer. 9. A phase noise detector as claimed in any one of the preceding claims, wherein the frequency discriminator comprises a resonator. 10.A phase noise detector as claimed in claim 9, wherein the resonator is close to critically coupled. 11.A microwave oscillator including a phase noise detector as claimed in any one of the preceding claims, wherein the signal is a microwave signal in the microwave oscillator, the signal output from the phase detector means being fed back to suppress noise close to the carrier frequency in the microwave oscillator. 12. A microwave oscillator as claimed in claim 11 , wherein the frequency discriminator forms part of the microwave oscillator. 13. A microwave oscillator as claimed in any one of claims 11 or 12, wherein the microwave oscillator is a loop oscillator including a microwave amplifier and a phase shift means responsive to a control signal, the microwave amplifier, the phase shift means and the frequency discriminator being arranged in a loop, the signal output from the phase noise detector being input to an electronic circuit which produces the control signal for the phase shift means such that the phase shift means suppresses noise close to the carrier frequency in the loop oscillator. 14.A microwave oscillator as claimed in any one of claims 11 or 12, wherein the microwave oscillator includes a frequency source responsive to a control signal, the frequency source producing the signal, wherein the signal output from the phase noise detector is input to an electronic circuit which produces the control signal for the frequency source, the control signal adjusting the frequency of the frequency source to suppress noise close to the carrier frequency in the frequency source. 15.A method of detecting phase noise in a signal having a carrier frequency, comprising the steps of: producing from the signal a dispersed signal using a frequency discriminator; adjusting the phase difference between the signal and the dispersed signal such that, at the carrier frequency, the signal and the dispersed signal have a predetermined phase difference; adjusting the amplitudes of the signal and the dispersed signal such that, at the carrier frequency, the signal and the dispersed signal have substantially equal amplitude; combining the signal and the dispersed signal to produce a carrier suppressed signal; and demodulating the carrier suppressed signal to produce an output signal corresponding to phase noise close to the carrier frequency in the signal. 16. A method of detecting phase noise as claimed in claim 15, wherein the dispersed signal is a signal reflected from the frequency discriminator. 17. A method of detecting phase noise as claimed in claim 15, wherein the dispersed signal is a portion of a signal output from the frequency discriminator. 18.A method of detecting phase noise as claimed in any one of claims 15 to 17, wherein the frequency discriminator comprises a resonator. 19. A method of detecting phase noise as claimed in claim 18, wherein the resonator is close to critically coupled. 20.A method of detecting phase noise as claimed in any one of claims 15 to 19, wherein the phase difference between the signal and the dispersed signal is adjusted by passing at least one of the signal and the dispersed signal through a phase shifter to alter the phase of that signal by a predetermined amount. 21. A method of detecting phase noise as claimed in any one of claims 15 to 20, wherein the amplitudes of the signal and the dispersed signal are adjusted by passing at least one of the signal and the dispersed signal through an attenuator to attenuate the amplitude of that signal by a predetermined amount. 22.A method of detecting phase noise as claimed in any one of claims 15 to 21 , wherein the signal and the dispersed signal are combined by passing the signal and the dispersed signal through a power combiner, the power combiner producing the carrier suppressed signal from the signal and the dispersed signal. 23.A method of detecting phase noise as claimed in claim 22, wherein the power combiner comprises a 3dB 90° hybrid. 24.A method of detecting phase noise as claimed in any one of claims 15 to 23, wherein the carrier suppressed signal is demodulated by passing the carrier suppressed signal and the carrier frequency into a double balanced mixer.

Inventors: IVANOV EUGENE NIKOLAY, TOBAR MICHAEL EDMUND, WOODE RICHARD ALEC, IVANOV, EUGENE, NIKOLAY, TOBAR, MICHAEL, EDMUND, WOODE, RICHARD, ALEC
Applicants: IVANOV EUGENE NIKOLAY, POSEIDON SCIENT INSTR PTY LTD, TOBAR MICHAEL EDMUND, UNIV WESTERN AUSTRALIA, WOODE RICHARD ALEC, IVANOV, EUGENE, NIKOLAY, POSEIDON SCIENTIFIC INSTRUMENTS PTY. LTD., THE UNIVERSITY OF WESTERN AUSTRALIA, TOBAR, MICHAEL, EDMUND, WOODE, RICHARD, ALEC
Application year: 1995
Main classification: G01R29
IPC codes: G01R29, G01R31, H03B1, H03B5, H03L7, H04B15