APPARATUSES AND METHODOLOGIES FOR VIBRATION EFFECTS CORRECTION IN OSCILLATORS

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First Claim
1. A method for vibration correction in an oscillator, the method comprising:
 sensing vibrations along one or more axes via at least one accelerometer mounted on the oscillator;
determining corrective factors based on an acceleration signal received from the at least one accelerometer by referencing a lookup table; and
controlling the oscillator based on at least the corrective factors.
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Abstract
A method and system for vibration correction in an oscillator. The method includes sensing vibrations along one or more axes via at least one accelerometer mounted on the oscillator, determining corrective factors based on an acceleration signal received from the at least one accelerometer by referencing a lookup table; and controlling the oscillator based on at least the corrective factors.
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1 Claim
 1. A method for vibration correction in an oscillator, the method comprising:
sensing vibrations along one or more axes via at least one accelerometer mounted on the oscillator; determining corrective factors based on an acceleration signal received from the at least one accelerometer by referencing a lookup table; and controlling the oscillator based on at least the corrective factors.
1 Specification
This application is a continuation of U.S. application Ser. No. 15/363,191 filed Nov. 29, 2016, which claims the benefit of priority from U.S. Provisional Application No. 62/409,583 filed Oct. 18, 2016, the entire contents of each are incorporated herein by reference.
Vibration has devastating effects on communication and radar equipment as described in A. W. Warner and W. L. Smith, “Quartz crystal units and precision oscillators for operation in severe mechanical environments,” 14th Annu. Symp. Freq. Contr., 1960, pp. 200216. The impact and concern of vibration on electrical systems have been increasing as our population becomes more mobile, communications systems become more interconnected, and information demand increases. Recent trends in the satellite industry technology and the automobile industry push for automated highway with phase array/MIMO (MultipleInput, MultipleOutput) systems that require very precise phase control. The communication industry is striving to increase spectral efficiencies causing a push towards higher modulation waveform that is placing a higher emphasis on spectral purity of the RF (Radio Frequency).
The foregoing “Background” description is for the purpose of generally presenting the context of the disclosure. Work of the inventor, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
An aspect of the present disclosure includes a method for vibration correction in an oscillator. The method senses vibrations along one or more axes via at least one accelerometer mounted on the oscillator, determines corrective factors based on an acceleration signal received from the at least one accelerometer by referencing a lookup table; and controls the oscillator based on at least the corrective factors.
Another aspect of the present disclosure includes an oscillator circuit. The oscillator circuit includes a crystal oscillator providing an RF (Radio Frequency) output, an accelerometer mounted on the crystal oscillator for sensing vibrations and for providing an acceleration signal associated with the vibrations; and processing circuitry. The processing circuitry is configured to determine corrective factors as a function of the acceleration signal by referencing a lookup table, and control the crystal oscillator based on at least the corrective factors.
Another aspect of the present disclosure includes a communication system. The communication system includes at least one electronic device being clocked by an oscillator circuit, wherein the oscillator circuit includes a crystal oscillator providing an RF (Radio Frequency) output, an accelerometer mounted on the crystal oscillator for sensing vibrations and for providing an acceleration signal associated with the vibrations, and processing circuitry configured to determine corrective factors as a function of the acceleration signal by referencing a lookup table, and control the crystal oscillator based on at least the corrective factors.
The foregoing paragraphs have been provided by way of general introduction, and are not intended to limit the scope of the following claims. The described embodiments, together with further advantages, will be best understood by reference to the following detailed description taken in conjunction with the accompanying drawings.
A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout several views, the following description relates to apparatuses and associated methodologies for vibration correction.
Vibration cancellation methods may be categorized between active and passive corrective approaches. The passive corrective approaches can be broken down further into various mechanical isolation systems such as combining vibrational opposing crystals to reduce vibration affects or mechanical shock absorbers as described in C. Nelson, “Reducing phase noise degradation due to vibration of crystal oscillators,” (2010), Graduate Theses and Dissertations, Paper 11900. The passive corrective approaches may be problematic due to cost and size. With advent of new cost effective vibration sensing technology, the active approaches become attractive and easily implementable given common circuitry in most designs.
An analog implementation method is described in V. J. Rosati, “Suppression of vibration effects on piezoelectric crystal resonators,” U.S. Pat. No. 4,453,141 incorporated herein by reference in its entirety. A mixed signal implementation is described in M. E. Frerking, “Vibration compensated crystal oscillator,” U.S. Pat. No. 4,891,611 incorporated herein by reference in its entirety. Both known methods have limiting qualities preventing successful correction at higher vibration frequencies. The limiting factor is the ability to provide correction for modulation and gsensitivity frequency response of VCXO. In addition, calibration methods described in known methods do not provide exact correction.
The methods of the present disclosure enhance active compensation theory by determining an exact corrective factor. Both digital and analog implementation methods are described herein.
The instantaneous frequency of an oscillator undervibration can be expressed as
w(t)=w_{o}{1+γα_{vib }cos(w_{v}t)} (1)
where w_{v }is the vibration frequency, w_{o }is the fundamental frequency of resonators, γ is the magnitude component of the gsensitivity vector, and α_{vib }is the amplitude of acceleration as described in W. Warner and W. L. Smith, “Quartz crystal units and precision oscillators for operation in severe mechanical environments,” 14th Annu. Symp, Freq. Contr., 1960, pp. 200216 incorporated herein by reference in its entirety.
The instantaneous phase can be determined from integration of the instantaneous frequency response
and therefore the waveform of the oscillator can be expressed as
Assuming the modulation in index is small,
equation (3) can be expanded using Bessel functions and the spectrum can be described by the following
where L is the relative level dBc (Decibels relative to the carrier) for the first order sideband. The resultant spectrum equation (4) may be used in determining the gsensitivity vector using a signal or spectrum analyzer. The gsensitivity vector can be measured, for example, in laboratory via a spectrum analyzer and a controlled vibration table, by applying equation (4) to determine orthogonal axial components of the gsensitivity vector, and performing the RMS (Root Mean Square) sum of the axial components as expressed by
γ(f)=√{square root over (γ_{x}^{2}+γ_{y}^{2}+γ_{z}^{2})} (5)
Then, the spectral response may be expressed as
where {right arrow over (γ)} is the gsensitivity vector and {right arrow over (α)} is the acceleration vector.
The vibration effects on PSK (Phase Shift Keying) and QAM (Quadrature Amplitude Modulation) modulations can be devastating. An 8PSK signal can be described by the following equation
In a second exemplary communication system 202, the digital baseband signal is converted to analog by a DAC 220 driven by a clock/LO synthesizer 218 which is clocked by a vibrational sensitive oscillator 216. The analog baseband signal is upconverted to RF frequency using a modulator 222 and a BUC 238. The received analog modulated signal is demodulated using a demodulator 224 driven by a LO/clock synthesizer 226 which is being clocked by a vibrational sensitive oscillator 230. The demodulated analog signal is converted to a digital baseband signal using a ADC 228 and a LNB 240 driven by the LO/clock synthesizer 226 which is being clocked by the vibrational sensitive oscillator 230. The transmitters and the receivers in systems 200, 202 also include DSPs to process information received from the physical layer.
Comparing Equations (3) and (7), the vibrational modulated RF signal can be expressed as
The impacts on the modulation are depicted in
Similarly to the derivation of vibrational effects on a crystal, the modulation message of a FM carrier at frequency w_{o }is can be expressed as
m(t)=α_{m }cos(w_{m}t) (9)
where α_{m }is the amplitude of the modulation message and w_{m }is the frequency of the modulation message.
When applied to the input of a VCXO (Voltage Controlled Crystal Oscillator), the resultant output can be expressed as
where K_{VCO }is the tuning slope of VCXO as described in L. Ders, “Frequency Modulation (FM) Tutorial”.
Given that both equation (3) and equation (11) take the same form and assuming superposition, in theory, the vibrational effect can be completely canceled out when
Based on equation (12), an active corrective mechanism can be achieved given that the gsensitivity vector can be correctly determined. Note that equation (12) assumes no frequency dependence of gsensitivity vector or modulation bandwidth of the VCXO. In one embodiment, taken the effects of the frequency dependence of the gsensitivity vector and/or modulation bandwidth of the VCXO into account, equation (12) can be expressed as
where M(f) is the modulation response of the VCXO. The modulation frequency response can be described as the VCXO ability to induce frequency change on the output of the VCXO when excitation is applied to the VCXO control voltage. Flattening the frequency response of the VCXO can be accomplished through corrective compensation using analog/digital filters. The group delay through the VCXO increases near and past the bandwidth and thus inherently affect a real time corrective response.
As can be seen from equations (3), (11), and (12), the error in cancellation is caused by imperfections in amplitude, frequency, and phase of the corrective modulation. If coherence is assumed, only the phase and amplitude errors exist. Hence, the sum of the phase shifted sine waves may be expressed as
E_{sum}=√{square root over (E_{vib}^{2}+E_{cor}^{2}−2*E_{vib}E_{cor }cos(θ_{delta}))} (14)
where E_{vib }is the amplitude of a vibrational induced modulation, E_{cor }is the amplitude of the corrective FM modulation, and θ_{delta }is phase or time delay between the vibrational and corrective modulation.
Expressing the corrective amplitude as a ratio of error amplitude
E_{sum}=E_{vib}√{square root over (1+R^{2}−2R cos(θ_{delta}))} (15)
where R is the ratio of corrective to vibrational induced modulation.
The delta error then results in
Schematic 500 shows the correction improvement in dB versus the phase error in radians. Schematic 502 shows the correction improvement versus the amplitude error.
Thus, the gsensitivity vector and modulation bandwidth of the oscillator may be measured along the threeaxis up to a vibration frequency of interest. Another manufacturing interest is the calibration of MEMS sensors and gain parameters.
Therefore, the modulation response and any frequency dependency of the gsensitivity vector along each axis may be taken into account when designing a corrective vector.
The analog and mixed signal corrective response can be rewritten to include frequency response of external components
The following conditional statement
result in an ideal correction, where c_{x}, c_{y}, and c_{z }are spectrally flat corrective vectors up to the vibrational frequency of interest. Having independent corrective variables
L_{sum}(f)G(f) or L_{dig}(f)L_{an}(f)G(f) (21)
and
L_{x}(f),L_{y}(f), and L_{z}(f) (22)
lead for easier correction of frequency response with calibrated vector. The mixed signal response from MEMS data is sufficiently oversampled in order to minimize the penalty of the time delay from filter bank. The oversampling ratio can be determined from
The MEMS sensor 704 is mechanically connected to the oscillator 716 directly or indirectly for sensing the frequency and magnitude of the vibration present. The sensor 704 measures the acceleration along three orthogonal axes (e.g., xaxis, yaxis, and zaxis). The signal from the MEMS sensor 704 is altered by the spectrally compensated response associated with each of the orthogonal axis. Each signal is multiplied by the corresponding corrective factors. The corresponding corrective factors may be determined via a lookup table based on the measured acceleration vector. In one implementation, the lookup table is precomputed using the setup shown in
The signals from each accelerometers 718,720, 722 are altered by the spectral response associated with each of the orthogonal axis. Each signal is multiplied by the corresponding corrective factors. The corresponding corrective factors may be determined via a lookup table based on the measured acceleration vector. Then, the corrected outputs are summed 724. The signal is then multiplied by a predetermined gain factor 726. Then, the signal is passed through a LPF 728. The corrective signal is then fed to the oscillator 730.
The pure analog implementation (e.g., implementation 702) provides real time correction. The mixed signal has the advantage over analog implementation because the mixed signal can provide calibrated weighting of the corrective signal from a lookup table and a customizable frequency response. The disadvantage is that there is more digitally induced time delay in the mixed signal implementation 700 compared to the analog implementation 702 and also lack in the degrees of freedom to counter act frequency response without paying a significant time delay penalty.
The modules (e.g., filters, ADC, DAC, summer) described in 700, 702 may be implemented as either software and/or hardware modules and may be stored in any type of computerreadable medium or other computer storage device. For example, each of the elements (e.g., filters) described herein may be implemented in circuitry that is programmable (e.g. microprocessorbased circuits) or dedicated circuits such as application specific integrated circuits (ASICS) or field programmable gate arrays (FPGAS). In one embodiment, a central processing unit (CPU) could execute software to perform the functions attributable to each of the modules described herein. The CPU may execute software instructions written in a programming language such as Java, C, or assembly. One or more software instructions in the modules may be embedded in firmware, such as an erasable programmable readonly memory (EPROM).
The placement of the MEM sensor is configured to accurately record the vibration which the oscillator sees. As depicted in
During fabrication of the vibration compensated crystal oscillator, the oscillator is placed on a vibration table (e.g., vibration table 104 of
In some implementations, the method 900 begins by sensing vibrations of an oscillator (902) along one or more axes. For example, the vibration may be sensed by one or more accelerometers.
In some implementations, a corrective factor associated with each axis is determined (904). The determination is based on the vibration measurements. For example, the determination may be effected by the processing circuitry by matching the detected vibrations to predetermined corrective factors. For example, a lookup table may be referenced to determine the corrective factors. The corrective factors may be predetermined using equation (20).
In some implementations, the oscillator is controlled based on at least one corrective factor (906).
To illustrate the capabilities of the apparatus and methodologies described herein, exemplary results are presented.
The experimental data shown in
Experimental improvement is shown up to 400 Hz with significant performance increase below 100 Hz. The corrective limitations past 400 Hz is found to be related to MEMS sampling rate and time delay induced by DSP required for appropriate correction.
The limitations at higher frequency shown in
Two different topologies are described herein, pure analog and mixed signal approach, which increase a designer ability to provide excellent phase noise and shortterm stability under both vibrating and static environments.
Obviously, numerous modifications and variations are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.
Thus, the foregoing discussion discloses and describes merely exemplary embodiments of the present invention. As will be understood by those skilled in the art, the present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Accordingly, the disclosure of the present invention is intended to be illustrative, but not limiting of the scope of the invention, as well as other claims. The disclosure, including any readily discernible variants of the teachings herein, defines, in part, the scope of the foregoing claim terminology such that no inventive subject matter is dedicated to the public.