Patent Publication Number: US-2022216829-A1

Title: Adaptive microphonics noise cancellation

Description:
RELATED APPLICATIONS 
     This application is a continuation application claiming priority from U.S. patent application Ser. No. 16/952,226 filed Nov. 19, 2020 titled “ADAPTIVE MICROPHONICS NOISE CANCELLATION”. U.S. patent application Ser. No. 16/952,226 is a continuation application claiming priority to U.S. patent application Ser. No. 16/223,777 filed Dec. 18, 2018 titled “ADAPTIVE MICROPHONICS NOISE CANCELLATION”, now U.S. Pat. No. 10,886,877 issued on Jan. 5, 2021, all of which are incorporated herein in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure relates generally to the field of communications, and more particularly to adaptive microphonics noise cancellation. 
     BACKGROUND 
     Microphonics, or microphony, describes the phenomenon wherein certain components in electronic devices transform mechanical vibrations into an undesired electrical signal. Mechanical acceleration, such as vibration or shock, can cause frequency modulation at oscillators, resulting in microphonics phase noise sidebands in signals. Piezoelectrical crystals can be particularly vulnerable to this effect, and mechanical vibration can transiently change the resonant frequency of the crystal and introduce significant phase noise sidebands through inadvertent frequency modulation. This error can propagate and multiply throughout the system, as any oscillator phase locked to the reference oscillator will be affected, such as the sampling clocks for analog-to-digital converters and digital-to-analog converters. 
     SUMMARY 
     In accordance with one example, a system includes a reference oscillator that provides an oscillator output signal and an accelerometer on a same platform as the reference oscillator, such that mechanical acceleration at the reference oscillator is detected at the accelerometer to produce a measured acceleration. A filter assembly, having an associated set of filter weights, receives the measured acceleration from the accelerometer and provides a tuning control signal responsive to the measured acceleration to a frequency reference associated with the system. An adaptive weighting component receives the oscillator output signal of the reference oscillator and an external signal that is provided from a source external to the platform and adjusts the set of filter weights for the filter assembly based on a comparison of the external signal and the oscillator output signal. 
     In accordance with another example, a method is provided for compensating for mechanical acceleration at a reference oscillator. A mechanical acceleration is detected at an accelerometer on a same platform as the reference oscillator to produce a measured acceleration. A tuning control signal responsive to the measured acceleration is provided at a filter assembly having a set of filter weights. The set of filter weights for the filter assembly is adjusted based on a comparison of an external signal that provided from a source external to the platform and an oscillator output signal of the reference oscillator. The tuning control signal is provided to a frequency reference associated with the system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features of the present invention will become apparent to those skilled in the art to which the present invention relates upon reading the following description with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a communications system utilizing a reference oscillator; 
         FIG. 2  illustrates one example of an adaptive weighting component that could be used in the system of  FIG. 1 ; 
         FIG. 3  illustrates one example of a communications system utilizing a reference oscillator that produces an oscillator output signal; 
         FIG. 4  illustrates another example of a communications system utilizing a reference oscillator that produces an oscillator output signal; 
         FIG. 5  illustrates yet another example of a communications system utilizing a reference oscillator that produces an oscillator output signal; 
         FIG. 6  illustrates a further example of a communications system utilizing a reference oscillator that produces an oscillator output signal; and 
         FIG. 7  illustrates a method for compensating for mechanical acceleration at a reference oscillator. 
     
    
    
     DETAILED DESCRIPTION 
     Various examples of the systems and methods described herein provide a noise cancellation system that can be used to generate a tuning control signal that modulates the reference oscillator to cancel or minimize the noise caused by mechanical acceleration at the reference oscillator. To this end, the acceleration at the location is measured and provided to an adaptive filter, comprising an associated set of weights, to generate the tuning control signal. The weights can be adapted, at periodic intervals, according to a measured phase error (or frequency error) of an oscillator output signal of the reference oscillator using an external signal provided to the system to account for changes in the response of the reference oscillator to acceleration. Accordingly, a lower cost, non-ruggedized reference oscillator can be used without a significant increase in microphonic noise or the expense and added weight of a mechanical isolation structure. Further, minor variations among reference oscillators introduced during fabrication can be compensated for without time-consuming testing of individual units. 
       FIG. 1  illustrates a communications system  100  utilizing a reference oscillator  102  that produces an oscillator output signal  103 . The reference oscillator  102  can comprise, for example, an electronic oscillator, such as a Hartley oscillator or a Colpitts oscillator, or a crystal oscillator comprising a piezoelectric crystal. The communications system  100  includes an accelerometer  104  on a same platform  105  as the reference oscillator  102 , configured such that any mechanical acceleration at the reference oscillator is detected at the accelerometer. Accordingly, the accelerometer  104  can continuously or periodically produce a measured acceleration  106  representing that experienced at the reference oscillator  102 . It will be appreciated that for some implementations of the reference oscillator  102 , the oscillator will have varying sensitivity to accelerations from different directions, and the accelerometer  104  can be implemented as a tri-axial accelerometer that measures the acceleration along three mutually perpendicular axes. 
     The communications system  100  further includes an adaptive filter assembly  108  that receives the measured acceleration  106  from the accelerometer and generates a tuning control signal  110  responsive to the measured acceleration  106  according to a set of filter weights. The tuning control signal  110  is provided to a frequency reference associated with the system, in this implementation, the reference oscillator  102 . It will be appreciated, however, that the frequency reference can be another system component that utilizes the output of the reference oscillator  102 . It will be appreciated that the filter weights represent the response of the reference oscillator  102  to acceleration, allowing the filter assembly  106  to correct the oscillator for perturbation caused by the measured acceleration. 
     In some implementations, the response of the reference oscillator  102  to acceleration will vary over time, for example, due to aging of components and changes in the operating environment. Accordingly, the adaptive filter assembly  106  can utilize adaptive weights that are adjusted over time to account for changes in the response of the reference oscillator  102 . Since the response of the reference oscillator  102  to acceleration, in general, varies slowly, the adaptation can be slow relative to the system, ranging, for example, between three hertz and two kilohertz. It will be appreciated, however, that the optimization used to produce the weights will take a certain amount of time to converge, and the adaptation must be performed with sufficient frequency to allow the weights to converge faster that the change in the response at the reference oscillator  102 . Initial values for the filter weights can be set to accelerate convergence of the filter  106  according to known characteristics of the reference oscillator  102 . 
     The weights for the filter assembly  112  can be provided by an adaptive weighting component  114  that receives the oscillator output signal  103  and an external signal  116 . The term “external signal,” as it is used herein, refers to a signal provided from a source external to the platform containing the reference oscillator  102 . Accordingly, the external signal  116  is generated in a manner that is unaffected by the any acceleration experienced at the reference oscillator. The adaptive weighting component  114  adjusts the set of filter weights  112  for the filter assembly based on a comparison of the external signal and the oscillator output signal. The adaptive weighting component  114  can be implemented in digital logic, for example, as a field programmable gate array or an application specific circuit, in software on a non-transitory computer readable medium executed by an associated processor, or in some combination of hardware and software. It will be appreciated that the adaptive filter assembly  106  can be provided with an initial set of weights at the time of manufacture or installation, with the adaptive filter weights  112  provided periodically to adjust for changes in the response of the reference oscillator  102 . 
       FIG. 2  illustrates one example of an adaptive weighting process  200  incorporating an adaptive weighting component  210  that could be used in the system of  FIG. 1 . The adaptive weighting component  210  comprises a demodulator  202  that determines a phase error  203 , Θ c  (n), in the oscillator output signal  204  of the reference oscillator  205  from the oscillator output signal  204  and the external signal  206 . A frequency estimation filter  216  in the adaptive weighting component  210  calculates an instantaneous frequency, f(n), from the determined phase error  203  in the oscillator output signal. In one implementation, the frequency estimation filter  216  can be a differentiator filter with a frequency response, H(f)=j2πf , a phase difference filter, f (n)=Θ c (n)−Θ c (n−1), or any other appropriate implementation. 
     Respective values  222 - 224  for the acceleration along each axis, as measured at the accelerometer  104 , are filtered at respective adaptive filters  226 - 228  and summed, at an adder  230 , to produce a tuning control signal  232 , representing a compensation frequency, f c (n)  232 , which is provided to the reference oscillator  205 . Each of the respective adaptive filters  222 - 224  and the adder  230  can be implemented, for example, as digital logic in at a digital signal processor, an application specific integrated circuit, or a field programmable gate array. It will be appreciated that the adaptive filters  226 - 228  can represent portions of the filter assembly  106  illustrated in  FIG. 1 , and the respective outputs  236 - 238  of the adaptive filters  226 - 228 , in combination, provide the tuning signal  110 . The tuning control signal  232  is provided to the frequency estimation filter  216  and the compensation frequency represented by the tuning control signal can be compared to the instantaneous frequency to produce a frequency error  242 , f c (n). This frequency error  242  can be utilized at a weight computation component  244 , along with the values  222 - 224  for the acceleration along each axis, to produce new weights for the adaptive filters  226 - 228  that minimize the frequency error  242 . Further, the frequency error signal  242  can be provided to the reference oscillator  205  (not shown in  FIG. 2 ) to adjust its frequency as depicted by line  110  in  FIG. 1 . In another embodiment, the frequency error signal  242  can be used for digital correction of the frequency as will be discussed in  FIG. 6 . The weight computation component  244  can employ, for example, algorithms that minimize the mean square of the frequency error, such as the Least Mean Square (LMS) algorithm, the Recursive Least Mean Square algorithm, and gradient descent algorithms. 
     In one example, a Least Mean Square algorithm is used, with a vector, w, of k filter coefficients for each adaptive filter  226 - 228 , where k is a positive integer greater than 1. The measured acceleration values  222 - 224  along each axis at a time, n, can be represented as vectors, α, including the k most recent measurements. For a time n+1, the weights for the filter can be calculated as: 
         w   x ( n+ 1)= w   x ( n )+μα x ( n ) f   c ( n−d )
 
         w   y ( n+ 1)= w   y ( n )+μα y ( n ) f   c ( n−d )
 
         w   z ( n+ 1)= w   z ( n )+μα z ( n ) f   c ( n−d )  Eq. 1
 
     Where μ is a convergence coefficient, selected according to the implementation, and d is a delay that is calculated to temporally align the frequency estimate from the received phase and the measured acceleration to compensate for filter delays along the two signal paths. It will be appreciated that, when the acceleration measured at the accelerometer  104  is low, for example, when a magnitude of the measured acceleration falls below a predefined threshold value, the adaptive weighting component  210  may stop adjusting the weights at the filters  226 - 228  for some time to allow the acceleration vectors to populate with meaningful values for the optimization calculation. 
       FIG. 3  illustrates one example of a communications system  300  utilizing a reference oscillator  302  that produces an oscillator output signal  303 . The oscillator output signal  303  is provided at least to each of receiver front end  304  and a transmitter  305 . An accelerometer  306  on a same platform  307  as the reference oscillator  302 , detects mechanical acceleration at the platform  307 . In the illustrated example, the accelerometer  306  can be implemented as a tri-axial accelerometer. An adaptive filter assembly  308  receives the measured acceleration  309  from the accelerometer  306  and provides a tuning control signal  310  responsive to the measured acceleration to the reference oscillator  302 . 
     A set of weights  312  for the adaptive filter assembly  308  can be determined at an adaptive weighting component  314 . An external, clean signal  316  is received at the receiver front-end  304  and provided to the adaptive weighting component  314 , along with the measured acceleration  309 . It will be appreciated that the oscillator output signal  303  provided to the transmitter  305  is adjusted, at an adaptive filter assembly  308 , to remove the effects of acceleration local to the reference oscillator  302 . Accordingly, a signal  318  transmitted by the transmitter  305  is a “clean” signal like the external signal  316 . The adaptive weighting component  314  can determine a degree of phase error in the oscillator output signal  303  based upon the received external signal  316 . From this phase error, the adaptive weighting component  314  determines appropriate weights for the adaptive filter assembly  308  by minimizing a square of a frequency error derived from the phase error. This can be performed periodically to account for changes to the response of the reference oscillator  302  to acceleration due to aging or changes in the operating environment. 
       FIG. 4  illustrates another example of a communications system  400  utilizing a reference oscillator  402  that produces an oscillator output signal  403 . The oscillator output signal  403  is provided at least to each of a receiver front end  404  and a transmitter  405 . An accelerometer  406  on a same platform  407  as the reference oscillator  402 , detects mechanical acceleration at the platform  407 . In the illustrated example, the accelerometer  406  can be implemented as a tri-axial accelerometer. An adaptive filter assembly  408  receives the measured acceleration  409  from the accelerometer and provides a tuning control signal  410  responsive to the measured acceleration  409 . A set of weights  412  for the adaptive filter assembly  408  can be determined at an adaptive weighting component  414 . 
     An external, clean signal  416  is received at the receiver front-end  404  and provided to the adaptive weighting component  414  along with the oscillator output signal  403  and the measured acceleration  409 . The adaptive weighting component  414  can estimate a phase error  415 , shown as  203  in the example of  FIG. 2 , in the reference oscillator output based upon the received external signal  416 , and determine appropriate weights for the adaptive filter assembly  408  by minimizing a square of a frequency error derived from the phase error  415 . This can be performed periodically to account for changes to the response of the reference oscillator  402  to acceleration due to aging or changes in the operating environment. The adaptive weighting component  414  provides the set of weights  412  to the adaptive filter assembly  408 . 
     It will be appreciated that adaptive filter assembly  408  only compensates for frequency error due to microphonics. Other phase and frequency errors, such as Doppler, crystal drift, and scintillation, are not compensated for at the adaptive filter assembly  408 . To address these sources of error, the phase error  415  can be further provided to a phase locked loop (PLL)  420 . The phase locked loop  420  comprises a phase locked loop filter  422 . In one implementation, the phase locked loop filter  422  is implemented as a low pass filter that removes any unwanted high frequency components present in the estimated phase error. The resulting filtered signal can be combined with the output of the adaptive filter assembly  408  at an adder  424  to provide the tuning control signal for the reference oscillator  402 . 
       FIG. 5  illustrates yet another example of a communications system  500  utilizing a reference oscillator  502  that produces an oscillator output signal  503 . The oscillator output signal  503  is provided at least to each of a receiver  504  and a transmitter  506  operating through a diplexer  507 . An accelerometer  508  on a first platform  510  with the reference oscillator  502 , detects mechanical acceleration at the platform. In the illustrated example, the accelerometer  506  can be implemented as a tri-axial accelerometer. An adaptive filter assembly  512  receives the measured acceleration  513  from the accelerometer and provides a tuning control signal  514  responsive to the measured acceleration. The measured acceleration is also provided to the transmitter  506  for transmission to a second platform  520 . In one example, the first platform  510  is a user terminal in a communications system, the second platform  520  is a satellite access node, and the communication between the first platform and the second platform occurs via a satellite connection. Alternatively or additionally, the first platform  510  can be a mobile platform, for example, implemented on an automobile, a watercraft, and aircraft, a train, or other vehicle. It will be appreciated, however, that other configurations of the system are possible, for example, with one or more user terminals used to correct for vibration at a satellite access node or for correction between two user terminals. 
     The implementation of  FIG. 5  exploits the fact that a signal  522  transmitted from the transmitter  506  on the first platform  510  will contain any of the microphonic error induced by mechanical vibration on the platform that has not been corrected by other means, whereas components located on the second platform  520  will be unaffected by any mechanical acceleration at the first platform  510 . Accordingly, the transmitted signal  522  can be received at the second platform  520  and demodulated at an adaptive weighting component  524  associated with a local receiver (not shown). A phase error in the signal can be determined during demodulation via a frequency reference (not shown) local to the second platform, and the phase error can be used at the adaptive weighting component  516 , to determine appropriate weights for the adaptive filter assembly  512  by minimizing a square of a frequency error derived from the phase error. The determination of the appropriate weights by the second platform also require acceleration information, which is communicated to the second platform by the first platform (not shown). The calculated weights  526  can then be transmitted to the first platform  510  via the receiver  504  for use at the adaptive filter assembly  512 . In an alternative implementation, the adaptive weighting component  516  can be distributed across the first platform  510  and the second platform  520 . In this implementation, a value indicative of the frequency error, such as the frequency error, the phase error, or any other indication that can be used to determine the frequency error, is determined at the second platform  520  can be transmitted to the first platform  510  for use in computing the filter weights  526 . 
     It will be appreciated that the exchange of the accelerometer data and the filter weights represents overhead in the communications system. To reduce this overhead, the rate of updates to the weights can be limited, with the weights updated either periodically or on a predetermined time schedule. When the weights are not being updated, the most recently updated value can be maintained and utilized at the adaptive filter assembly  512  to correct for mechanical acceleration at the first platform  510 . Since the change in the response of the reference oscillator  502  to acceleration changes slowly, gating the update function is this manner allows for a savings of overhead in the system with a minimal loss of accuracy in the oscillator output signal. 
       FIG. 6  illustrates yet another example of a communications system  600  utilizing a reference oscillator  602  that produces an oscillator output signal  603 . The oscillator output signal is used to drive numerically controlled oscillators at least one application specific integrated circuit (ASIC)  605 , as well as an associated receiver front-end  608 . It will be appreciated that this implementation is provided merely for the purpose of example, and that other implementations of the numerically controlled oscillators can be used, such as field programmable gate arrays. An accelerometer  610  on a same platform  611  as the reference oscillator  602 , detects mechanical acceleration at the platform. In the illustrated example, the accelerometer  610  can be implemented as a tri-axial accelerometer. The output  612  of the accelerometer  610  can be provided to each ASIC  605 , via a first analog-to-digital converter (ADC)  613 . Similarly, an external, clean signal  614  is received at the receiver front-end  608  and provided to the ASIC  605  via a second ADC  615 . 
     An exemplary ASIC  605 , containing a numerically controlled oscillator  622  that provides a reference signal for an associated transmitter  623 , is illustrated in detail. In the ASIC  605 , an adaptive filter assembly  624  receives the measured acceleration  612  from the accelerometer  610  and provides a tuning control signal  625  responsive to the measured acceleration  612 . The tuning control signal  625  from the adaptive filter assembly  624  can be supplemented by an additional tuning signal from a loop filter  630  at an associated adder  626 , in order to track other sources of frequency error like Doppler shifts, oscillator drifts, etc. as discussed in connection with  FIG. 4 . It will be appreciated that the numerically controlled oscillator  622  can receive the digital tuning signal, and it is thus unnecessary to convert the digital output of the adaptive filter assembly  624  to an analog signal. 
     Each of the external signal  614 , an output  628  of the numerically controlled oscillator  622 , and the output  612  of the accelerometer  610  is provided to an adaptive weighting component  632 . The adaptive weighting component  632  includes a demodulator (not shown) that estimates a phase error  634 , shown as  203  in the example of  FIG. 2 , in the numerically controlled oscillator output  634  based upon the external signal  614 . This phase error  634  is provided to the loop filter  630  for tracking other sources of frequency error, as described previously. The adaptive weighting component utilizes the estimated phase error, along with the accelerometer output  612  to estimate a frequency error and determine appropriate weights for the adaptive filter assembly  624  that minimize a square of the frequency error. 
     In view of the foregoing structural and functional features described above, an example method will be better appreciated with reference to  FIG. 7 . While, for purposes of simplicity of explanation, the example method of  FIG. 7  is shown and described as executing serially, it is to be understood and appreciated that the present examples are not limited by the illustrated order, as some actions could in other examples occur in different orders, multiple times and/or concurrently from that shown and described herein. Moreover, it is not necessary that all described actions be performed to implement a method 
       FIG. 7  illustrates an example of a method  700  for compensating for mechanical acceleration at a reference oscillator. At  702 , mechanical acceleration is detected at an accelerometer on a same platform as the reference oscillator to produce a measured acceleration. At  704 , a tuning control signal responsive to the measured acceleration is provided at a filter assembly having a set of filter weights. At  706 , the set of filter weights for the filter assembly is adjusted based on a comparison of an external signal that provided from a source external to the platform and an oscillator output signal. For example, a phase error in the oscillator output signal can be determined from the external signal and the oscillator output signal, a frequency error can be estimated and the set of filter weights can be adjusted according to the determined frequency error. 
     It will be appreciated that determining the adjustment to the set of filter weights can be performed locally, remotely, or at a combination of local and remote components. In one example, a signal is generated using the oscillator output signal is transmitted from the platform to a remote platform, and a phase error in the oscillator output signal is calculated from the external signal, which is generated at the remote platform, and the signal generated from the oscillator output signal. The calculated phase error is then transmitted to the platform, and the set of filter weights for the filter assembly is adjusted from the calculated phase error at the remote platform. In one implementation, the set of filter weights for the filter assembly is determined only periodically, such that the accelerometer and the filter are active at times when the set of filter weights is not being determined. 
     At  708 , the tuning control signal is provided to a frequency reference associated with the system to correct for errors caused by the detected acceleration. In one implementation, the frequency reference is the reference oscillator. In another implementation, the frequency reference is at least one numerically controller oscillator driven by the oscillator output signal. It will be appreciated that the tuning control signal can correct for errors other than that caused by the mechanical acceleration. In one implementation, a correction value can be calculated at a phase locked loop to account for additional sources of phase and frequency error, and the correction value can be added to the tuning control signal. 
     What have been described above are examples. It is, of course, not possible to describe every conceivable combination of components or methodologies, but one of ordinary skill in the art will recognize that many further combinations and permutations are possible. Accordingly, the disclosure is intended to embrace all such alterations, modifications, and variations that fall within the scope of this application, including the appended claims. As used herein, the term “includes” means includes but not limited to, the term “including” means including but not limited to. The term “based on” means based at least in part on. Additionally, where the disclosure or claims recite “a,” “an,” “a first,” or “another” element, or the equivalent thereof, it should be interpreted to include one or more than one such element, neither requiring nor excluding two or more such elements.