Abstract:
A shear-mode quartz resonator designed to mechanically oscillate at a predetermined frequency and electronic circuits for inhibiting oscillation of the shear-mode quartz resonator in response to externally applied mechanical forces which otherwise induce mechanical vibration of the shear-mode quartz resonator at frequencies significantly less than the predetermined frequency. The shear-mode quartz resonator includes a cantilevered quartz beam having relatively large metallic electrodes attached on opposite sides thereof, the relatively large metallic electrodes, in use, being coupled to an external oscillator circuit. The beam also has relatively smaller sense and rebalance electrodes attached on the same opposite sides of said quartz beam as said relatively large metallic electrodes. The relatively smaller sense and rebalance electrodes being coupled, in use, with said electronic circuits for inhibiting oscillation of the shear-mode quartz resonator in response to externally applied mechanical forces.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     This invention was made under US Government Contact No. HR001-10-C-0109 and therefor the US Government may have certain rights in this invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is related to U.S. patent application Ser. No. 12/835,610 filed Jul. 13, 2012 and entitled “Piezoelectric Resonator Configured For Parametric Amplification” the disclosure of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present technology relates to the field of resonator devices. 
     BACKGROUND 
     Electronic systems have become ubiquitous in many modern societies, wherein these systems may be used to perform various tasks electronically, such as to increase the ease and efficiency with which certain tasks may be carried out. Oftentimes, it is useful in such electronic systems that an electrical signal be created with a particular frequency, such as to provide a stable clock signal for digital integrated circuits. Resonator devices are frequently used in oscillators to generate the aforementioned particular frequency. 
     Prior art resonator devices tend to be sensitive to external vibrations which affect the stability of the particular frequency which they are intended to generate when connected within an oscillator. Therefore there is a need to reduce the sensitivity of resonator devices to external vibration. In the prior art this problem has been addressed by utilizing a separate inertial sensor to detect such external vibrations and to attempt to correct changes in the oscillators frequency by pulling the frequency within the sustaining circuit of the oscillator. However, this increases the overall size of the system (since there is a separate inertial sensor added to the mix) and it does not properly correct for non-linear mechanical behavior within the quartz of the oscillator resonator for large vibrations. Prior art devices have used separate inertial sensors for detecting vibrations of the quartz resonator and correcting the changes in the oscillator frequency by pulling the frequency of the oscillator electrically within the sustaining circuit of the oscillator. However, this increases the overall size of the system and does no correct for non-linear mechanical behavior within the quartz for large vibrations. 
     What is needed is a more accurate way to inhibit mechanical vibrations otherwise induced in quartz resonators due to environmental shock while continuing to allow the quartz resonators to vibrate at their normal frequencies. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In this patent we describe a method and apparatus to use the quartz resonator as both an oscillator resonator and as it own accelerometer for detecting mechanically induced vibrations and then to force rebalance the quartz resonator using additional force rebalance electrodes disposed on the quartz resonator and using electrostatic forces therewith. This inhibits or reduces externally induce vibration strains in the quartz resonator which would otherwise cause undesirable variations in the frequency of oscillation of the quartz resonator. This technique also results in a better method for reducing vibrationably induced increases in phase noise. 
     A mechanical resonator designed to mechanically oscillate at a predetermined frequency and electronic circuits for inhibiting oscillation of the resonator in response to externally applied mechanical forces which otherwise would induce mechanical vibration of the resonator at frequencies significantly less than the predetermined frequency. The resonator includes a cantilevered beam, preferably made of quartz, having relatively large metallic electrodes attached on opposite sides thereof, the relatively large metallic electrodes, in use, being coupled to an external oscillator circuit. The beam also has relatively smaller sense and rebalance electrodes attached on the same opposite sides of said beam as said relatively large metallic electrodes. The relatively smaller sense and rebalance electrodes being coupled, in use, with said electronic circuits for inhibiting oscillation of the shear-mode quartz resonator in response to externally applied mechanical forces. 
     The described technology also relates to a method of dynamically damping a quartz resonator comprising the steps of: disposing sense and rebalance electrodes on opposite side of a quartz beam of said quartz resonator; disposing opposing sense and rebalance electrodes on structural members supporting and at least partially surrounding said quartz beam, opposing sense and rebalance electrodes being disposed in a confronting relationships with corresponding sense and rebalance electrodes on said quartz beam; using a first electronic circuit coupled to said sense electrodes on said quartz beam and on said structural members to sense movement of said beam in response to an externally applied shock; and using a second electronic circuit couple to said first electronic circuit and to said rebalances electrodes on said quartz beam and on said structural members to apply a counter-acting electrostatic force to said quartz beam to counter-act significant movement of said quartz beam in response to said externally applied shock. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a three dimension view of a resonator with force and rebalance electrodes on both the quartz beam and on neighboring structure; 
         FIG. 2  is a bottom view of the quartz bar with sense and rebalance electrodes taking a picture frame configuration; 
         FIG. 3  is a schematic diagram of a preferred embodiment of the sense electronics coupled to the sense electrodes of the resonator; 
         FIG. 4  is a schematic diagram of a preferred embodiment of the rebalance electronics coupled to the sense electronics and to the rebalance electrodes of the resonator. 
     
    
    
     DETAILED DESCRIPTION 
     It is known through experimentation and modeling that the vibrational sensitivity of quartz shear-mode resonators is largest in the out-of-plane direction (or Y-axis direction of the typical cuts of shear-mode quartz resonators). Typical out-of-plane of vertical sensitivities are ˜10 −9  fractional deviation per g while the in-plane sensitivities are several orders of magnitude smaller. Thus, it is highly desirable to reduce the out-of-plane sensitivity using the capacitive rebalance techniques described herein where inertial forces can be compensated for at the resonator itself if small gaps are available for capacitive sense and electrostatic force rebalance, similar to the operation of MEMS-based accelerometers. However, the resonators need to be thin (10 μm) so that this rebalance technique is ideally suited for UHF shear mode devices. 
     Capacitive sense and force rebalance electrodes are added to a shear-mode quartz resonator and an electronic loop is used to (i) sense movement of the shear-mode quartz resonator and (ii) supply voltages to the force rebalance electrodes to counteract against such movement. By adding additional electrodes on the quartz surface of the hear-mode quartz resonator, the substrate, and a capping wafer, differential capacitive sense and force rebalance can be achieved. 
     The shear-mode quartz resonator is preferably made using an integrated quartz MEMS process taught by U.S. Pat. No. 7,237,315 which allows the gap spacings between the sense and force rebalance electrodes electrodes to be reduced to the range of 2-10 microns. This allows large electrostatic forces to be produced with small voltages and large capacitive sensitivity to the movement of the quartz resonator. 
     Since the frequency stability of an oscillator is dependent on the frequency stability of the resonator, any strain in the resonator due to vibration can introduce unwanted frequency instabilities. For quartz shear-mode resonators, the largest vibration sensitivity is known to occur for vibrations perpendicular to the shearing plane. The concept disclosed herein is to use electrostatic forces rebalance to prevent the quartz resonator from moving in its vertical direction normal to the shearing plane and thus reduce a high level of vibration-induced frequency instability. Since the shear-mode resonances are generally much higher in frequency (10 MHz-1 GHz) than induced mechanical vibrations (10 Hz-2 KHz), the frequencies of the sustaining circuit for the oscillator (e.g., Pierce, Colpitts, Clapp, etc.) are greatly different from the force rebalance loop and the electrostatic rebalance forces do not interfere with the much higher frequencies at which the shear-mode quartz resonator is intended to oscillate. 
     The disclosed technique ideally works best for shear-mode resonators with fundamental mode resonances in the UHF range. In this case, the quartz thickness of the shear-mode quartz resonator is below about 5 microns and can produce displacements in the nm range for an induced one g vibration. This amount of displacement can easily be detected with capacitive sense electronics capable of attofarad (aF) detection. Calculations show that for typical dimensions of the capacitive sense electrodes (100s of square microns), gap spacings of roughly 5 microns, and nm range motion, capacitive sense electronics can be used to detect mg level vibration for force rebalance. Since typical vibration sensitivities for inherent quartz resonators is 10-9 fractional frequency deviation per g, this allows the frequency stability to be improved to &lt;10-11 even in the presence of large vibrations greater than one g in the vertical direction. Since the vertical vibration sensitivity is about two orders of magnitude larger than in the in-plane directions, the overall vibration sensitivity can be greatly reduced. 
     The capacitive sense and force rebalance electrodes can be arranged in various configurations on the quartz plate and on the substrate and capping wafers. In one configuration, shown in  FIG. 1 , the groups of capacitive sense and force rebalance electrodes  100 ,  200  are placed at the end of the cantilevered quartz beam  10  of the depicted shear-mode resonator and are also placed in opposition to the electrode on the beam  10  on a cap  14  and a substrate  12 . The cantilevered quartz beam  10  is supported at one end on a pedestal  11  relative to the substrate  12 . The cap  14  surrounds the cantilevered quartz beam  10  and is also supported by the substrate  12 . 
     Conventional top and bottom electrodes  16 ,  18  are used with an external oscillator circuit (e.g., the Pierce, Colpitts, Clapp, etc. oscillators noted above) to generate high frequency oscillations (for example in the UHF range). Since such external oscillator circuits are well known they are not shown herein. Moreover this invention is concerned with the groups of sense and force rebalance electrodes  100 ,  200  rather than the conventional top and bottom electrodes  16 ,  18 . 
     The group of sense electrodes  100  comprises four electrodes, two of which (electrodes  102  and  106 ) are disposed on beam  10 . Opposing the top most sense electrode  102  on the beam is an electrode  104  disposed on cap  14 . Opposing the bottom most sense electrode  106  on the beam is an electrode  108  disposed on substrate  12 . 
     Electrodes  102  and  104  are spaced apart by a distance of preferably of about 5 microns and form the plates of a first variable capacitor C s1 . Electrodes  106  and  108  are also spaced apart by the same distance (preferably by about 5 microns) and form the plates of a second variable capacitor C s2 . 
     The group of rebalance electrodes  200  comprises four electrodes in this embodiment, two of which (electrodes  202  and  206 ) are disposed on beam  10 . Opposing the top most rebalance electrode  202  on the beam is a rebalance electrode  204  disposed on cap  14 . Opposing the bottom most rebalance electrode  206  on the beam is a rebalance electrode  208  disposed on substrate  12 . 
     Rebalance electrodes  202  and  204  are spaced apart by a distance of preferably of about 5 microns or less and rebalance electrodes  206  and  208  are similarly spaced apart by a distance of preferably of about 5 microns or less. Additional sets of force rebalance electrodes are be utilized if desired. 
     In other embodiments, the force rebalance electrodes can surround (or nearly surround) the shear-mode electrodes in a picture frame to reduce mechanical motion very near the shear-mode active region at electrodes  16 ,  18 . See  FIG. 2  which is a bottom view of the quartz bar with electrodes  18 ,  106  and  206  shown. Note how sense electrode  106  surrounds electrode  18  and the rebalance electrode  206  also surrounds electrode  18 . Similar picture frame electrodes would be disposed on the top side of beam  10  as well as on the substrate and on cap  14  in opposition to the corresponding electrodes on beam  10 . 
     In  FIG. 2  the sense electrode is larger (see  106   LARGE ) at the distal end (remote from pedestal  11 ) of the cantilevered quartz resonator  10  to maximize its detection sensitivity while the force rebalance electrode preferably surrounds electrode  18  to prevent or deduce bending under high Q loads. 
     The picture frame shaped sense  106  and rebalance  206  electrodes of  FIG. 2  may include openings in their picture frame shapes to allow wiring from the inner electrodes to more conveniently reach substrate  12  and thence the electronic circuits to which the resonator is connected in use. Alternatively, multi-level wiring techniques could be used to allow wiring from the inner electrodes to more conveniently reach substrate  12  and thence the electronic circuits to which the resonator is connected in use. 
     A preferred embodiment of the sense electronics is shown in  FIG. 3 . In this embodiment, single-sided square wave clock signals (f clk ) are applied to sense electrodes  104  and  108 . Note that the two square wave signals are 180° out of phase with respect to each other and that the square wave oscillator frequency is about 1 GHz in this embodiment. The single-sided square wave clock signals (f clk ) should ideally be higher in frequency than the oscillator clock frequency controlled by resonator  10  to help prevent noise from coupling into the electronics. In one embodiment might well include a multiplier so that f clk  is then some multiple of (i.e., a higher frequency than) the oscillator clock frequency controlled by resonator  10 . 
     The variable capacitors formed by the upper opposing electrode pair  102 ,  104  and the lower opposing electrode pair  106 ,  108  are shown as C s1  and C s2 , respectively, each having a nominal value of 15fF in this embodiment. Capacitors C s1  and C s2  are shown in phantom lines since they are inherent in the design of the resonator as opposed to being added discreet electronic components. The arrows through C s1  and C s2  are shown in opposite directions to reflect that fact that movement of beam  10  causes one capacitor to increase in value while the other capacitor decreases in value. 
     The sense electronics of  FIG. 3  has a limited bandwidth so that it does not respond to the normal high frequency vibrations of beam  10  yet it does response to the relatively low frequency vibrations of beam  10  induced externally. Preferably the sense electronics of  FIG. 3  has bandwidth of about 1 KHz. The Buffer should preferably be a low-noise op amp such as a Texas Instruments operational amplifier model LMH6624. 
     In  FIG. 3 , i signal  is the AC differential current developed across the sense capacitors (sense electrodes  102  &amp;  104 ;  106  &amp;  108 ). At no applied acceleration, this signal is zero. For motion of the beam upward, this signal is positive in this embodiment and for motion of the beam downward, this signal is negative in this embodiment. It is the filtered current at a low pass bandwidth (of about 1 KHz) which floods through R dc  to produce a filtered voltage V x  that is buffered and becomes a control signal V out . V x  is either positive or negative depending on whether the quartz beam was moved upwardly or downwardly, respectively, in response to a applied acceleration due to an external shock, for example. 
     The sense electronics of  FIG. 3  outputs the control signal or voltage V out  which if other than 0 volts is trying to counteract externally induced vibrations. The control voltage V out  is applied to the rebalance circuit of  FIG. 4 . The control voltage V out  is positive for a positive vertical displacement of the cantilevered beam  10 . This is based on the clock input polarities on the differential capacitive sense electrodes  104 ,  108  and the summing junction  302  as shown in  FIG. 3 . Control signal V o  is then applied to a set of inverting integrators  402 ,  404  for applying the force rebalance voltages to rebalance electrodes  204 ,  208  in cap  14  and on substrate  12 . The force rebalance counter electrodes  202 ,  206  on the quartz beam  10  can be held at zero potential. Using differential capacitive sensing and force rebalance, the quartz cantilevered beam can be maintained in its undeflected position (due to external acceleration inputs) at all times which prevents changes in its nominal frequency of vibration otherwise due to externally induced vibrations (from such external acceleration inputs). 
     The signal from the capacitance sensing buffer output V out  is applied to both a standard inverting integrator  404  and a unity gain inverter  400  followed by an inverter integrator  402  in the rebalance circuit of  FIG. 4 . The time constants of inverting integrators  402 ,  404  is preferably set to be about 1 msec. The outputs of the inverting integrators  402 ,  404  are passed through diodes  406 ,  408  and then to the appropriate force rebalance electrode  204 ,  208  each with a parallel resistance load  410 ,  412 . The inverting integrators  402 ,  404  allow the voltage V out  from the sensing buffer output to be integrated and adjusted to null and hold the position of the cantilever beam  10  near its zero deflection point under applied external vibration (accelerations). The diodes  406 ,  408  are used to block the signal to the opposing force rebalance electrode so that the electrostatic force is always applied in the needed direction to counter the external applied acceleration. The time constants of the integrators (R1×C1) are made equal and should be ideally about 50 μsec to allow compensation of vibration signals up to frequencies of about 1 KHz. This is only one embodiment of the force rebalance loop, and a person skilled in the art should be able to envision other embodiments. Functionally the circuit of  FIG. 4  provides negative feedback to damp out the mechanical oscillations which might otherwise be induce in the device due to an externally applied shock or vibration. Numerous feedback circuits are possible in addition to the integral feedback of  FIG. 4 . Some feedback circuits may incorporate proportional as well as integral feedback and various filters to ensure stability and performance. These circuits should be well known to those of skill in the art. These circuits could involve using microprocessor based systems to integrate and switch the appropriate signals to the force rebalance electrodes or by applying DC biases to the sense electrodes instead of AC varying biases. 
     The force rebalance electrode size should be scaled in size based on the upper range of vibration to be sensed by the sense electrodes  100  and the resulting voltages supplied to the electrodes  204  and  208  of the rebalance electrodes  200  by the electronics described herein. Using charge pump electronics in CMOS the rebalance voltages may exceed 30 v. 
     This concludes the description including preferred embodiments of the present invention. The foregoing description including preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations are possible within the scope of the foregoing teachings. Additional variations of the present invention may be devised without departing from the inventive concept as set forth in the following claims.