Vibration compensated interferometric noise suppressed oscillator (INSO)

Abstract

Vibration compensation is provided for Interferometric Noise Suppressed Oscillators (INSOs). In an INSO the error signal at the mixer output responds linearly to changes in carrier frequency. A vibration compensation signal is summed with the error signal at the input to the feedback amplifier to provide the control signal to the loop phase shifter to suppress close-in phase noise near the carrier frequency and to reduce the effects of mechanical vibrations on oscillator phase noise. The addition of the vibration compensation signal does degrade carrier suppression, hence increases the flicker noise contributed by the INSO's LNA but does so without degrading overall oscillator phase noise. In a frequency tuned configuration, the vibration compensation signal reduces the effects of mechanical vibrations on oscillator phase noise independent of the tuning voltage applied to the phase shifter.

Claims

1. A vibration compensated interferometric noise suppressed oscillator (INSO), comprising: an amplifier, a loop phase shifter responsive to a control signal, and a resonator arranged in a loop to oscillate and produce a first signal at a carrier frequency, a second signal produced by reflection of a portion of the first signal by the resonator; a carrier suppression circuit responsive to the first and second signals to produce a carrier suppressed signal, a mixer responsive to the carrier suppressed signal and the first signal to produce an error signal corresponding to the phase difference between the first and second signals, wherein the error signal responds linearly to changes in the carrier frequency; a control signal generator including a feedback amplifier that amplifies the error signal to provide the control signal to the loop phase shifter to suppress close-in phase noise near the carrier frequency; and a vibration sense circuit configured to produce a vibration compensation signal indicative of mechanical vibrations, said vibration compensation signal summed with the error signal and fed to the control signal generator to reduce the effects of mechanical vibrations on oscillator phase noise.

2. The INSO of claim 1, further comprising a low noise amplifier (LNA) that amplifies the carrier suppressed signal, wherein summing the vibration compensation signal to the error signal degrades carrier suppression and increases flicker noise contributed by the LNA without degrading oscillator phase noise.

3. The INSO of claim 2, wherein the carrier suppression circuit includes a reference arm having an attenuator and a phase shifter, wherein at least one of the attenuator, phase shifter and mixer have noise contributions that is greater than a noise floor of the LNA, wherein a level of noise due to degraded carrier suppression is greater than the noise floor but less than the noise contribution of the at least one of the attenuator, phase shifter and mixer.

4. The INSO of claim 1, wherein the vibration compensation signal is G.Math.k.sub.a.Math.a where G is gain, a is the acceleration measured in gs, and k.sub.a is an accelerometer sensitivity in V.Math.g.sup.1, wherein the carrier frequency responds to the vibration compensation signal according to a tuning slope K.sub.VCO2 to provide the term G.Math.k.sub.a.Math.k.sub.VCO2.Math.a to reduce the effects of mechanical vibrations on oscillator phase noise.

5. The INSO of claim 1, further comprising an integrator configured to receive the error signal and generate an integrated output signal that is summed into either the feedback amplifier or the control signal to remove a DC offset.

6. The INSO of claim 1, wherein carrier suppression circuit includes a phase shifter responsive to a tuning voltage to adjust the carrier frequency, wherein the vibration sense circuit reduces the effects of mechanical vibrations on oscillator phase noise independent of the tuning voltage applied to the phase shifter.

7. The INSO of claim 6, wherein the carrier frequency responds to the tuning voltage according to a first tuning slope K.sub.VCO1 that varies with the applied tuning voltage over a tuning range at the phase shifter, wherein the carrier frequency responds to the vibration compensation signal according to a second tuning slope K.sub.VCO2 that is constant over the tuning range.

8. The INSO of claim 6, wherein the tuning voltage is a phase locked loop (PLL) voltage V.sub.PLL that locks the carrier frequency to an external reference frequency.

9. The INSO of claim 6, wherein a tuning range of the tuning voltage is at least 1,000 a tuning range of the vibration compensation signal.

10. The INSO of claim 6, wherein the suppression of phase noise due to the effects of mechanical vibrations is at least 20 db over a tuning range of the tuning voltage.

11. An interferometric noise suppressed oscillator (INSO), comprising: an amplifier, a loop phase shifter responsive to a control signal, and a resonator arranged in a loop to oscillate and produce a first signal at a carrier frequency, a second signal produced by reflection of a portion of the first signal by the resonator; a carrier suppression circuit responsive to the first and second signals to produce a carrier suppressed signal, a mixer responsive to the carrier suppressed signal and the first signal to produce an error signal corresponding to the phase difference between the first and second signals, wherein the error signal responds linearly to changes in the carrier frequency, said carrier suppression circuit including a phase shifter responsive to a tuning voltage to adjust the carrier frequency; a control signal generator including a feedback amplifier that amplifies the error signal to provide the control signal to the loop phase shifter to suppress close-in phase noise near the carrier frequency; and a vibration sense circuit configured to produce a vibration compensation signal indicative of mechanical vibrations, the vibration compensation signal being summed with the error signal that is fed to the control signal generator to reduce the effects of mechanical vibrations on oscillator phase noise independent of the tuning voltage applied to the phase shifter.

12. The INSO of claim 11, wherein the carrier frequency responds to the tuning voltage according to a first tuning slope K.sub.VCO1 that varies with the applied tuning voltage over a tuning range at the phase shifter, wherein the carrier frequency responds to the vibration compensation signal according to a second tuning slope K.sub.VCO2 that is constant over the tuning range.

13. The INSO of claim 11, wherein the suppression of phase noise due to the effects of mechanical vibrations is at least 20 db over a tuning range of the tuning voltage.

14. The INSO of claim 11, further comprising a low noise amplifier (LNA) that amplifies the carrier suppressed signal, wherein summing the vibration compensation signal to the error signal degrades carrier suppression and increases flicker noise contributed by the LNA without degrading oscillator phase noise.

15. The INSO of claim 14, wherein the carrier suppression circuit includes a reference arm having an attenuator and a phase shifter, wherein at least one of the attenuator, phase shifter and mixer have noise contributions that is greater than a noise floor of the LNA, wherein a level of noise due to degraded carrier suppression is greater than the noise floor but less than the noise contribution of the at least one of the attenuator, phase shifter and mixer.

16. A method of suppressing effects on oscillator phase noise in an interferometric noise suppressed oscillator (INSO) due to external vibrations, wherein an interferometric noise suppressed oscillator (INSO), the method comprising: sensing one or more accelerations of the INSO; producing a vibration compensation signal responsive to the one or more accelerations; summing the vibration compensation signal with an error signal at an output of a mixer in the INSO, wherein the error signal responds linearly to changes in a carrier frequency; and applying the vibration compensation and error signals to a feedback amplifier in the INSO to provide a control signal to a loop phase shifter in the INSO to suppress close-in phase noise near the carrier frequency and to reduce the effects of mechanical vibrations on oscillator phase noise.

17. The method of claim 16, wherein summing the vibration compensation signal to the error signal degrades carrier suppression and increases flicker noise contributed by a low noise amplifier (LNA) in the INSO without degrading oscillator phase noise.

18. The method of claim 16, further comprising: applying a tuning voltage to a phase shifter to adjust the carrier frequency, wherein summing the vibration compensation signal with the error signal reduces the effects of mechanical vibrations on oscillator phase noise independent of the tuning voltage applied to the phase shifter.

19. The method of claim 18, wherein the carrier frequency responds to the tuning voltage according to a first tuning slope K.sub.VCO1 that varies with the applied tuning voltage over a tuning range at the phase shifter, wherein the carrier frequency responds to the vibration compensation signal according to a second tuning slope K.sub.VCO2 that is constant over the tuning range.

20. The method of claim 18, wherein the suppression of phase noise due to the effects of mechanical vibrations is at least 20 db over a tuning range of the tuning voltage.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1, as described above, is a plot of oscillator phase noise under static and vibration conditions;

(2) FIG. 2, as described above, is a diagram of a voltage controlled oscillator (VCO) responsive to a vibration compensation signal to suppression the effects of vibration on oscillator phase noise;

(3) FIG. 3, as described above, is a diagram of the VCO of FIG. 2 in which the VCO is phase-locked to external reference signal;

(4) FIG. 4, as described above, is a diagram of a loop oscillator in which the oscillation frequency is adjusted by application of a tuning voltage such as the vibration compensation signal to the voltage control phase shifter (VCP) in the loop;

(5) FIGS. 5A-5B, as described above, are plots of the loop oscillator's tuning sensitivity and tuning slope K.sub.VCO (the slope of the tuning sensitivity);

(6) FIG. 6, as described above, is a plot of oscillator phase noise over the tuning range of the VCP;

(7) FIG. 7 is a simplified diagram of a vibration compensated phase-locked INSO in which the vibration compensation signal is added to the frequency error signal at the input of the feedback amplifier to suppress the effects of external vibrations on oscillator phase noise over the tuning range;

(8) FIG. 8 is a plot of the tuning slopes K.sub.VCO1 and K.sub.VCO2 at the inputs to the phase shifter and the feedback amplifier, respectively;

(9) FIG. 9 is a plot of comparing the effects of vibration compensation on oscillator phase noise with the vibration compensation signal applied at the inputs to the phase shifter and the feedback amplifier, respectively; and

(10) FIG. 10 is a plot of noise contributions for components in the phase detector relative to the LNA white noise floor.

DETAILED DESCRIPTION

(11) Although an INSO provides much higher fidelity (i.e., much lower phase noise) than a standard loop oscillator and exhibits very low vibration sensitivity because of the combination of noise suppression and rigid, high-Q resonators (e.g., Sapphire resonators) that are typically used, certain applications may require even greater immunity to mechanical vibrations. One technique is to add active vibration compensation to reduce the effects of external vibrations on oscillator phase noise.

(12) The vibration compensation signal cannot be input to loop phase shifter as in FIG. 3 because the noise-degeneration circuitry treats any signal added to the loop phase shifter control point as noise to be removed from the loop and reduces its effect by the gain of the noise-degeneration circuitry, typically 60 dB or more.

(13) The vibration compensation signal could be input to the phase shifter 42 in the carrier suppression block of phase detector 28 of the oscillator as shown in FIG. 1 of U.S. Pat. No. 5,841,322. However, phase shifter 42 has a non-linear tuning sensitivity that produces a tuning slope K.sub.VCO that varies over the tuning range of phase shifter 42. Vibration compensation is an open-loop compensation system. The overall gain must be determined experimentally and be stable over the tuning range to provide vibration compensation that reduces the effects of vibration on oscillator phase noise over that tuning range. The variation of K.sub.VCO over the tuning range of the vibration compensation signal will degrade the phase noise. Because the of the already low vibrations sensitivity, the vibration compensation may be understood to apply V.sub.Comp of a magnitude that would make very small changes in oscillator frequency. For example, in a 10 GHz oscillator with =10.sup.10 g.sup.1 undergoing a 1 g acceleration, V.sub.Comp.Math.k.sub.VCO would be equivalent to only 1 Hz. Addition of the vibration compensation voltage, V.sub.Comp to V.sub.Tune, therefore doesn't change V.sub.Tune significantly in an oscillator that might typically have a full tuning range of several kHz. However, any non-linearity in the tuning curve of the oscillator will result in generation of unwanted harmonic effects.

(14) In a frequency-tuned INSO, a tuning voltage such as V.sub.PLL is also applied to the phase shifter 42. The tuning range of the tuning voltage is much larger than that of the vibration compensation signal. For example, the tuning voltage may need to induce a 1 kHz change in the carrier frequency to compensate for thermal drifts. As such the magnitude of variations in Vtune may be 1,000 that of the variations in V.sub.Comp. Hence problems associated with non-linearity are of paramount importance in applications requiring the INSO to be tuned such as when phase locked using a PLL and the open-loop vibration compensation system will fail to suppress the effects on phase noise caused by external vibrations across that tuning range as shown by curve 600 in FIG. 6.

(15) More specifically, for the purposes of discussion, with reference to FIG. 1 of U.S. Pat. No. 5,841,322 two arms of the interferometer are identified: the first comprising the attenuator 44 and phase shifter 42 referred to as the reference arm, and the second comprising the connection between the circulator 22 and combiner 46 as the reflection arm. To maintain balance in the interferometer, the signals from both reference and reflection arms must arrive at the combiner 46 with the same amplitude but out of phase so that cancellation occurs. Hence, any phase shift in the reference arm must be matched by an equal and opposite phase shift in the reflection arm signal. In response to a phase shift in the reference arm and operation of the feedback circuit, the equal and opposite phase shift in the reflection arm comes about by virtue of a change in oscillation frequency and the reflection phase characteristics of the resonator. Again, the phase shift in reflection versus frequency is quite linear over a significant fraction of the resonator bandwidth. However, just as the non-linear voltage to phase relationship of the VCP in FIG. 4 produces a non-linear tuning curve, the same is true in the INSO, leading to a very similar tuning curve to that shown in FIG. 5A. Thus, the adaptation of applying the vibration compensation signal, alone or summed with a tuning voltage, to phase shifter 42 in the carrier suppression block of phase detector 28 suffers from the disadvantages just discussed.

(16) In accordance with present disclosure, a vibration compensation signal is summed with the error signal at the output of mixer 32 in FIG. 1 of U.S. Pat. No. 5,841,322 at the input to amplifier 64 of the control signal generator. The error signal responds linearly to changes in carrier frequency and thus the K.sub.VCO at this point in the oscillator is constant. Because vibration compensation operates open-loop this greatly increases the utility of the vibration compensation as it allows the oscillator to be tuned independently by the phase shifter while maintaining the desired constant value of k.sub.VCO relevant to the vibration compensation signal. However, inserting an additional signal into the oscillator loop at this point goes against long-established practice taught by U.S. Pat. No. 5,841,322 because doing so degrades carrier suppression, increasing the flicker noise contribution of amplifier 48.

(17) A simplified schematic of a vibration-compensated phase-locked INSO 700 is shown in FIG. 7. The basic oscillator comprises a sustaining amplifier 702, a loop phase shifter 704 and a resonator 706, such as a sapphire-loaded cavity, configured as a simple loop oscillator. One skilled in the art will understand that other components not directly relevant to the discussion have been omitted for simplicity. A phase detector or interferometric bridge 708 is used to detect phase fluctuations in the oscillator loop and generate an error signal 710 at mixer output 712 being a measure of frequency fluctuations from the nominal oscillation frequency. This error signal 710 is amplified (and possibly filtered) by a feedback amplifier 714 within a control signal generator before being fed back into the loop phase shifter 704 to cancel the phase noise in the loop and suppress close-in phase noise near the carrier frequency.

(18) Phase detector 708 includes a carrier suppression circuit 716 responsive to a first signal 718 at a carrier frequency that propagates around the loop and a second signal 720 that is a reflection of first signal 718 off of resonator 706 to produce a carrier suppressed signal 722, a mixer 724 responsive to the carrier suppressed signal 722 and the first signal 718 to produce the error signal 710 corresponding to the phase difference between the first and second signals.

(19) Carrier suppression circuit 716 includes a reference arm and a reflection arm. The reference arm includes a reference arm coupler 728 to couple a portion of first signal 718 out of the loop, a variable attenuator 730 and a phase shifter 732. The reflection arm includes a connection between circulator 734 that couples second signal 720 out of the loop. To maintain balance in the interferometer, the signals from both reference and reflection arms must arrive at a combiner 736 with the same amplitude but out of phase so that cancellation occurs. Hence, any phase shift in the reference arm is matched by an equal and opposite phase shift in the reflection arm signal due to the action of the feedback. Combiner 736 produces carrier suppressed signal 722 that is fed to a low noise amplifier 738 that amplifies the carrier suppressed signal 722. A mixer local oscillator coupler 740 couples a portion of first signal 718 from the loop that is fed through a mixer local oscillator phase shifter 744 to mixer 724.

(20) To phase-lock the INSO, PLL circuitry 750 is responsive to an output signal 752 extracted from the oscillator loop via coupler 754 and a reference signal 756 to produce a PLL tuning signal, V.sub.PLL that is applied to the tuning port of phase shifter 732 in the reference arm of the carrier suppression circuit. Although the tuning slope K.sub.VCO1 associated with phase shifter 732 is not constant over the tuning range, the closed-loop characteristics of the PLL are insensitive to variations in K.sub.VCO1 and the oscillator frequency will be locked to the reference signal frequency.

(21) To provide active vibration compensation for the INSO 700, a vibration sense circuit 760 including one or more accelerometers 762 (e.g., one or more of the orthogonal X, Y, Z axes summed together) and gain G 764 is configured to produce a vibration compensation signal V.sub.Comp 766 (e.g., G.Math.k.sub.a.Math.a) indicative of mechanical vibrations and as a function of an oscillator tuning slope K.sub.vco2 that is summed with the error signal 710 by summer 768 and fed to the feedback amplifier 714. The linearity of the error signal 710 to changes in oscillator frequency at the output of the mixer maintains a constant value of oscillator tuning slope K.sub.vco2 to suppress the effects of mechanical vibrations on oscillator phase noise over a tuning range of the oscillator.

(22) An accelerometer aligned with the earth's gravitational field will produce a constant DC output in addition to the response to vibrational accelerations. This will produce a constant degradation of the flicker noise in amplifier 738. By integrating the mixer error signal 712 via integrator 770 and summing it via summer 772 into either the feedback amplifier input or the loop phase shifter input (shown here), this DC offset can be removed on time scales longer than the integrator time constant.

(23) To understand the operation of this arrangement, consider the voltage 710 generated at 712 in the absence of the feedback being applied between amplifier 714 and phase shifter 704. At the nominal oscillating frequency, f.sub.0, at or near the center of resonance, the signal in the reference arm, P.sub.REF (Bold used to indicate a phasor or vector quantity having magnitude and phase) and the reflection arm, P.sub.RFL, are equal and opposite, P.sub.REF=P.sub.RFL and cancel at the combiner 736 leading to zero carrier power at the output of LNA 738 and zero voltage at the mixer output 712. However, if the frequency of oscillation varies by an amount much less than the bandwidth of the resonator, f<<BW.sub.RES, (for example because of a small fluctuation in loop phase caused by noise) then the power in the reflection arm becomes P.sub.RFL (1+f.Math.k.sub.RflPh) where k.sub.RflPh=dS.sub.11</df is the slope of the resonator reflection phase with respect to frequency. Hence the input power at LNA 738 becomes P.sub.REFP.sub.RFL=P.sub.RFL f.Math.k.sub.RflPh and correct adjustment of the LO Phase shifter 742 results in a voltage at the mixer output 712 directly proportional to f.Math.k.sub.RflPh. In typical applications the tuning range of the oscillator will be of order kHz while the BW of the resonator will be 100 kHz and in such cases k.sub.RflPh remains linear over the tuning range and the error signal voltage 710 at 712 is linearly related to a frequency error in the oscillator.

(24) The normal action of the noise degeneration circuit when the feedback between amplifier 714 and phase shifter 704 is connected, is to reduce the error signal 710 at mixer output 712 (the input to amplifier 714) to very close to zero, the reduction depending on the gain around the feedback loop. Adding a voltage Vcomp 766 means the circuit will instead produce an equal and opposite voltage at mixer output 712, and a proportional change in oscillator frequency depending principally on k.sub.RflPh and being independent of any phase shifter characteristic. Thus, an alternative means of tuning the oscillator frequency is available, one that is highly linear in voltage. This has the effect of holding tuning slope K.sub.VCO2 at this point in oscillator constant.

(25) In a frequency-tuned (e.g., phase-locked) INSO configuration, summing the vibration compensation signal to the error signal reduces the effects of mechanical vibrations on oscillator phase noise independent of the tuning voltage applied to the phase shifter 732. Even though K.sub.VCO1 at reference arm phase shifter 732 varies by perhaps 2 over the full tuning range this has no effect on the vibration compensation. K.sub.VCO2 characterizes a tuning slope relating the carrier frequency to the summed voltage of the error signal and the vibration compensation signal. Because the error signal responds linearly to changes frequency under open-loop conditions, K.sub.VCO2 remains constant. This is true independent of fixed or variable tuning voltage applied to reference arm phase shifter 732.

(26) Referring now to FIG. 8, tuning slopes K.sub.VCO1 800 at phase shifter 732 in the reference arm of the carrier suppression circuit and K.sub.VCO2 802 at mixer output 712 characterize the non-linear and linear tuning sensitivities at those locations in the oscillator. Application of the PLL tuning voltage V.sub.PLL to the tuning port of phase shifter 732 works to phase-lock the oscillator frequency to the reference signal frequency in large part because phase-locking is closed loop. By comparison, vibration compensation is open-loop. The effective gain applied to the acceleration measurements must be highly stable across the tuning range of the oscillator to effectively reduce the effects on oscillator phase noise due to external vibration. Tuning slope K.sub.VCO2 is constant as defined by a variation of less than 1.2 and preferably less than 1.1 over the oscillator tuning range.

(27) Referring now to FIG. 9, the effect on oscillator phase noise of applying the vibration compensation signal at the tuning port of phase shifter 732 and at the mixer output 712 are shown in noise suppression curves 900 and 902, respectively. For application of V.sub.comp to phase shifter 732, the PLL voltage range would need to be restricted to a small fraction of the oscillator tuning port voltage range to achieve at least a nominal 20 dB decrease in vibration sensitivity (corresponding to a reduction in apparent F by a factor of 10). By comparison, for application of V.sub.comp to mixer output 712, the PLL voltage range can span the full oscillator tuning port voltage range to achieve at minimum 20 DB decrease in vibration sensitivity and suitably at least 40 dB decrease over the full span of tuning voltage at the phase shifter.

(28) Although the application of V.sub.comp to mixer output 712 instead of the tuning port of phase shifter 732 achieves a marked improvement in phase noise in the presence of external vibration, the insertion of an additional signal into the oscillator loop at this point goes against long-established practice taught by U.S. Pat. No. 5,841,322 because doing so degrades carrier suppression, increasing the flicker noise contribution of LNA 738.

(29) However, with many suitable choices of LNA 738 the effectiveness of the carrier suppression results in a flicker noise contribution from the LNA 738 of 10 dB, or more, lower than the noise generated by practical choices for other components such as attenuator 730, phase shifter 732, and mixer 734. For example, FIG. 10 shows these noise contributions 900, 902 and 904 from the attenuator, phase shifter and mixer, respectively, relative to the white noise floor 906 of LNA 738. As carrier suppression is degraded, the flicker or 1/f, noise in the amplifier increase. Line 908 shows a level of noise due to degraded carrier suppression that is still somewhat lower than the mixer noise floor 904. The line 910 shows a further degraded level of carrier suppression where the amplifier noise exceeds the mixer noise but is still below the noise generated by both the attenuator and phase shifter. Since the final phase noise of the oscillator, while the noise degeneration circuit is operative, depends on the sum of these noise contributors, there is margin to degrade the level of carrier suppression without degrading the oscillator phase noise.

(30) While several illustrative embodiments of the disclosure have been shown and described, numerous variations and alternate embodiments will occur to those skilled in the art. Such variations and alternate embodiments are contemplated, and can be made without departing from the spirit and scope of the disclosure as defined in the appended claims.