Abstract:
A crystal oscillator having a plurality of quartz crystals that are manufactured so that the directional orientation of the acceleration sensitivity vector is essentially the same for each crystal. This enables convenient mounting of the crystals to a circuit assembly with consistent alignment of the acceleration vectors. The crystals are aligned with the acceleration vectors in an essentially anti-parallel relationship and can be coupled to the oscillator circuit in either a series or parallel arrangement. Mounting the crystals in this manner substantially cancels the acceleration sensitivity of the composite resonator and oscillator, rendering it less sensitive to vibrational forces and shock events.

Description:
RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/969,332 filed Aug. 16, 2013, titled “CRYSTAL OSCILLATOR WITH REDUCED ACCELERATION SENSITIVITY”, which is a continuation of U.S. patent application Ser. No. 13/474,361 filed May 17, 2012, now U.S. Pat. No. 8,525,607 titled “CRYSTAL OSCILLATOR WITH REDUCED ACCELERATION SENSITIVITY”, which is a continuation of U.S. patent application Ser. No. 12/613,336 filed Nov. 5, 2009, now U.S. Pat. No. 8,188,800, titled “CRYSTAL OSCILLATOR WITH REDUCED ACCELERATION SENSITIVITY”, which claims benefit under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/112,634, filed Nov. 7, 2008, entitled “CRYSTAL OSCILLATOR WITH REDUCED ACCELERATION SENSITIVITY”, the entireties of which are hereby incorporated by reference herein and made a part of this specification. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of the invention relate generally to crystal oscillators and more specifically to crystal oscillators with reduced acceleration sensitivity. 
     2. Description of the Related Art 
     Quartz crystals are commonly used to control the frequency of electronic oscillators. Although quartz is one of the most stable materials available for fabricating a high frequency resonator, certain limitations can become apparent in precision applications. For instance, changes in the ambient temperature cause the resonant frequency to change. In addition, the natural frequency of a quartz resonator can also be affected by applied acceleration forces. In some situations, these effects on the frequency are relatively small and can go undetected (˜0.0000002% per g of applied force). However, in many applications, an oscillator must operate in an environment which subjects it to levels of vibration or shock, where the resultant frequency shifts can be significant and can limit system performance. These deleterious effects are a well known problem and a major concern of oscillator designers. 
     Acceleration forces applied to a crystal oscillator assembly will cause a shift in the operating frequency. If these forces are in the form of periodic or sinusoidal vibration, frequency modulation will appear as sidebands to the carrier at the vibration frequency, the amplitude of which is determined by the amount of frequency shift. When the acceleration forces are in the form of random vibration, an increase in the broadband noise floor of the oscillator will occur. Either of these conditions may result in serious degradation of system performance in a noise sensitive application. Shock pulses due to handling or other environmental events can cause a jump in the frequency which may result in circuit malfunction such as loss of lock in phase locked loop or GPS tracking applications. 
     There are generally two classes of methods to minimize the effects of acceleration forces on crystal resonators. The first class is known as active compensation. In active compensation, an acceleration sensor is used to detect the characteristics of applied forces and a signal is then processed and fed back to the oscillator circuit to adjust the frequency by an equal magnitude but in the opposite direction from the acceleration induced shifts. This method can be effective over certain vibration frequency ranges, but it requires a relatively complex circuit and can be very expensive to implement. 
     The second class is referred to as passive compensation. Passive methods do not attempt to sense the applied acceleration. Generally, in passive methods, the crystal resonator or resonators are constructed using special methods that render them somewhat insensitive to acceleration forces. Passive methods can be effective, but they generally require an involved and difficult fabrication process to produce the required crystal or composite crystal assembly. 
     In view of these complications, one attempt has been to cancel acceleration sensitivity including determining a dominant sensitivity axis of the resonators and then mounting the resonators with the dominant sensitivity axes in an anti-parallel arrangement. However, in aligning the resonators according to a supposedly dominant axis, such methods do not take into account the sensitivities along the other axes. As a result, the exact maximum magnitude and direction of a crystal&#39;s acceleration sensitivity characteristic is not accounted for in such methods, and it is less effective in minimizing the effects of acceleration forces. 
     SUMMARY OF THE INVENTION 
     It is more effective to account for the magnitude and direction of the total acceleration sensitivity vector by summing or taking into account sensitivity in all three axes of the resonator. The acceleration sensitivity of a quartz crystal can be characterized as a vector quantity, commonly denoted as Γ. The frequency shifts that are induced by external acceleration are therefore determined by both the magnitude and direction of the applied forces. The fractional frequency shift Δf/f under an accelerations {right arrow over (a)} is given by 
                 Δ   ⁢           ⁢   f     f     =       Γ   →     ·       a   →     .             
By measuring the components of Γ in three mutually perpendicular directions which are perpendicular to the faces of the crystal or resonator package, it is possible to calculate the exact maximum magnitude and direction of the Γ vector. Forces will have the most effect when they are imparted to the crystal in a direction that is parallel to this vector.
 
     When two essentially identical crystals are aligned so that their vectors are in opposing directions or anti-parallel and coupled electrically in combination to the oscillator circuit, the vectors will cancel, rendering the composite resonator less sensitive to acceleration forces. Such an approach, however, has been difficult to achieve. Two crystals must be carefully matched and physically oriented so that the vectors are anti-parallel. Crystal resonators exhibit substantial variation in the direction and magnitude of the vector. The vector direction can vary as much as 60° even with resonators that have been identically manufactured. Measurements of many crystals have shown that the acceleration vector is not oriented relative to the crystallographic axes in any consistent manner even in identically designed and manufactured crystals. Based on these challenges, it has been necessary to physically manipulate the mounting orientation of the crystals to achieve the anti-parallel relationship. Vector inconsistency generally requires the use of complicated equipment such as an adjustable 3-axis gimbal mounting apparatus to individually align each crystal precisely as needed to achieve significant cancellation. Therefore, achieving vector cancellation involves a difficult and time consuming process of measuring, adjusting and mounting the crystals in an effective manner. Manufacturers have generally avoided this approach because it is so difficult and expensive. Embodiments of the present invention eliminate this burdensome process by configuring an oscillator with crystals that have been manufactured so that the direction of the Γ vector points in a consistent and predictable direction in each crystal, the direction being relative to the normal mounting plane. 
     Embodiments of the present crystal oscillator include a plurality of crystals mounted with the acceleration sensitivity vectors in an essentially anti-parallel relationship. This helps to cancel the effects of acceleration or vibration on the output signal of the oscillator. Due to the vector nature of this characteristic, when the crystals are so aligned, cancellation of the acceleration effects will occur. Embodiments of this oscillator use crystals which have been manufactured so that the acceleration sensitivity vector points in a consistent and predictable direction relative to the mounting surface of the resonator. 
     In one embodiment, crystals are selected which have the same acceleration sensitivity vector magnitudes, within a certain tolerance. Complete cancellation of the sensitivity vectors can occur if the sensitivity vectors are of the exact same magnitude. However, significant cancellation can be achieved if the magnitudes of the sensitivity vectors differ. 
     The crystals are preferably contained in individual crystal packages or resonators. The crystals or resonators are then mounted to an oscillator circuit which is configured to sustain periodic oscillations. In one embodiment, the crystals are coupled in pairs so that the first crystal is inverted with respect to the second crystal. This inversion can be achieved by rotating the first crystal 180° around either the x axis in the y-z plane or around the y axis in the x-z plane. Because the crystals have been manufactured so that the direction of the vector is substantially the same for all crystals, the vectors can be aligned in an essentially anti-parallel manner without the need to measure and characterize the vector direction of each crystal and then manipulate the orientation of the mounting plane of the crystals. The crystals are preferably coupled in a way that allows them to function as a single composite resonator. This allows for the construction of an acceleration and vibration resistant crystal oscillator. 
     One embodiment of an oscillator comprises an electronic circuit configured to initiate and sustain periodic oscillations, a plurality of crystal resonators having acceleration sensitivity vectors aligned in a consistent and predictable relationship to the normal mounting plane of the resonator, wherein said crystal resonators are coupled to the electronic circuit such that the acceleration sensitivity vectors of at least two of the crystals are in an essentially anti-parallel relationship, and wherein said crystal resonators function as a single composite resonator controlling the frequency of oscillation. The oscillator can have crystal resonators electrically coupled in parallel or the oscillator can have crystal resonators electrically coupled in series. The crystal resonators can also be mechanically mounted by rotating at least one crystal resonator 180° around either the x or y axis. 
     Also, the resonators can be mounted such that said resonators are disposed on opposite sides of an oscillator substrate such that they may be coupled to the oscillator circuit by their normal mounting means. The resonators can be disposed side by side on the same surface of an oscillator substrate with the first resonator inverted 180° and mounted on its top with connections to the substrate. The oscillator can be configured such that the resonators are disposed back to back on the same surface of an oscillator substrate said resonators being mounted on their sides so that electrical connection to all of the electrodes may be made directly to the substrate. 
     An embodiment of the present invention also includes a method for improving the acceleration resistance of a quartz crystal controlled oscillator comprising (a) manufacturing a plurality of crystal resonators having acceleration sensitivity vectors that point in substantially the same direction relative to the mounting surface of the resonator, (b) determining the magnitude of the acceleration sensitivity component in the axis normal to the mounting plane of the crystal for all crystals in the group, (c) selecting a first crystal and a second crystal that exhibit acceleration sensitivity magnitudes that are substantially the same within a predetermined tolerance, (d) mounting the second said crystal such that its mounting plane is rotated 180° either around the x axis in the y-z plane or around the y axis in the x-z plane relative to the first crystal, aligning the two acceleration sensitivity vectors in a substantially anti-parallel arrangement, (e) coupling said crystal pair to the oscillator circuit so that the combination performs as a composite resonator to control the frequency of the oscillator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a crystal oscillator assembly having a composite dual crystal resonator. 
         FIG. 2  illustrates a cross-sectional view of the composite resonator indicating the relative relationship of the crystal blanks and the mounting structure. 
         FIG. 3  illustrates a cross-sectional side view of an embodiment of the invention wherein the two matched crystals are mounted on opposite sides of an oscillator substrate. 
         FIG. 4  illustrates an embodiment wherein the two matched crystals are mounted side by side on the same surface of the oscillator substrate. 
         FIG. 5  illustrates a further embodiment of the oscillator wherein the two crystals are mounted on the oscillator substrate with the packages turned on their sides. 
         FIG. 6  illustrates an embodiment of an oscillator including a composite dual crystal resonator on a substrate containing the circuitry to implement a Temperature Compensated Crystal Oscillator (TCXO). 
         FIG. 7  illustrates an embodiment of an oscillator wherein the composite crystal resonator is included in an Oven Controlled Crystal Oscillator (OCXO). 
         FIG. 8  illustrates the axis definitions of a single rectangular quartz crystal resonator. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 1 , an embodiment of a crystal oscillator  30  can be configured with a composite dual crystal resonator in which the crystals are mounted with their normal mounting planes parallel to the surface plane of the circuit substrate  18 . The oscillator  30  includes a first crystal resonator  10  and second crystal resonator  12  which are both electrically connected to the circuit with conducting jumpers  22  to form composite resonator  15 . Preferably, the crystal resonators  10  and  12  have been manufactured so that their F or acceleration sensitivity vectors  14  and  16  are pointing in the same direction relative to the normal mounting planes of the resonator packages  36  and  38 . Therefore, when the first crystal  10  is inverted with relation to the second crystal  12  and mounted on top of it with the mounting planes parallel to each other, the acceleration sensitivity vectors  14  and  16  are essentially anti-parallel or pointing in opposing directions. While an arrangement with the acceleration sensitivity vectors exactly anti-parallel is preferred, vectors that are nearly anti-parallel may still provide the desired acceleration insensitivity. For instance, in some embodiments, the desired acceleration insensitivity benefits can be achieved where the acceleration vectors are within 5° off of anti-parallel. However, where the acceleration vectors are arranged more than 10° off of anti-parallel, the benefits can substantially diminish. Therefore, it is preferable that the acceleration vectors be within 10° of anti-parallel, and it more preferable that the vectors are within 5° of anti-parallel. 
     In one embodiment, the preferred crystal for the oscillator is a rectangular resonator strip crystal known as an “AT” cut crystal. This particular cut of crystal has a very low temperature coefficient with the inflection temperature near +25° C. so that frequency variations are minimized in most applications. The temperature characteristic of a quartz crystal is primarily determined by the angle that the resonator wafer is cut from a quartz bar relative to the crystal lattice. While the implementation of an anti-parallel cancellation technique is well suited to the AT cut, it may also be accomplished with any other family of cuts having an acceleration sensitivity in three axes. 
     The crystal resonators can be configured in an inverted position by rotating the first crystal  10  resonator 180° around either the x or y axis and directly mounting the first crystal on top of the second crystal  12 .  FIG. 8  illustrates an embodiment of a rectangular resonator  10  and a three axis coordinate system  50  which defines the axes relative to the faces of the resonator. The z axis ({right arrow over (z)}) points outward from the top of the package (a major face). The x axis ({right arrow over (x)}) points outward from the side (the long minor face). The y axis ({right arrow over (y)}) points outward from the end of the package (the short minor face). 
     The illustrated configuration effectively causes cancellation of the acceleration sensitivity of the composite resonator due to the vector nature of the crystal acceleration parameter. Crystal electrode pads  27  and  28  can be connected to the circuit with conducting jumpers  22  so that they can be operated either in parallel or series configuration in the oscillator circuit. The crystal oscillator  30  can also be configured to include a circuit substrate  18  supporting passive and active oscillator components  20 . 
       FIG. 2  shows a cross-sectional side view of an embodiment of a composite resonator  15  exposing the first internal quartz crystal blank  24  of first crystal resonator  10  and the second internal quartz crystal blank  26  of the second crystal resonator  12 . Electrode pads  27  can be connected to the electrode deposited on of crystal blanks  24  and  26 . Circuit traces within the crystal packages connect pads  28  to the electrode deposited on the other side of the crystal blanks  24  and  26 . Preferably, the crystals are configured as part of a crystal resonator package which can include circuit traces, electrodes, the crystals, and other resonator materials. In constructing the composite resonator, first crystal resonator  10  can be rotated or inverted 180° and placed on top of the second crystal resonator  12 . The first resonator  10  is preferably rotated 180° around the x or y axis, as shown in  FIG. 8 , so that planes  36  and  38  are mounted parallel to one another, and the acceleration sensitivity vectors  14  and  16  are aligned essentially anti-parallel. Although the vectors of the preferred embodiment are aligned exactly anti-parallel, other embodiments may have desired acceleration benefits where the vectors are aligned within 5° and 10° of anti-parallel. Conductive straps  22  can connect the electrode pads  27  and  28  of the first crystal resonator  10  to the electrode pads  27  and  28  of the second crystal resonator  12 . The crystal resonators can also be coupled together using other means such as adhesive, use of the substrate, etc. 
       FIG. 3  illustrates an embodiment of an oscillator wherein the matched crystal resonators  10  and  12  are disposed on opposite sides of an oscillator circuit substrate  18 . In this manner, the crystal resonators  10  and  12  can each be attached to a substrate and circuit while maintaining the acceleration resistant positioning relative to one another. Preferably, the substrate has a uniform thickness so that the mounting plane of the first crystal  36  is parallel to the mounting plane of the second crystal  38 . As the orientation of the first crystal has been inverted 180° around either the x or y axis, the acceleration sensitivity vectors  14  and  16  are essentially anti-parallel, pointing in opposite directions. Although the vectors of the preferred embodiment are aligned exactly anti-parallel, other embodiments may have the desired acceleration benefits where the vectors are aligned within roughly 5° and 10° of anti-parallel. The electrodes on crystal blanks  24  and  26  can be connected through the crystal package to the electrodes  27  and  28 . The electrodes can then be connected together by conductive circuit board via the circuit elements  40  and  42  which complete the connection to the oscillator circuit either in a parallel or series configuration. 
       FIG. 4  shows a further embodiment of an oscillator wherein two matched crystal resonators  10  and  12  are mounted side by side on the same surface of an oscillator substrate  18 . Crystal resonator  10  is inverted or rotated 180° about the x or y axis and mounted on its top surface so that the mounting planes  36  and  38  are parallel and the acceleration sensitivity vectors  14  and  16  are essentially anti-parallel. Although the acceleration vectors of the preferred embodiment are aligned exactly anti-parallel, other embodiments may have desired acceleration benefits where the vectors are aligned within roughly 5° and 10° of anti-parallel. The crystal resonators  10  and  12  can be coupled directly to the substrate and can also be coupled to one another. Preferably, the crystal resonators have been manufactured so that the Γ vectors  14  and  16  are pointing in a consistent direction relative to the mounting surface of each crystal. The electrode pads  27  and  28  of the inverted crystal  10  can be connected to the oscillator substrate  18  with connecting jumpers  22 . 
       FIG. 5  shows a further embodiment of an oscillator wherein the two matched resonators  10  and  12  are mounted on their sides with their normal mounting surfaces  36  and  38  facing outward in opposite directions. This allows crystal pads  27  and  28  to be electrically connected to the oscillator substrate  18  directly without requiring additional connecting jumpers. Acceleration sensitivity vectors  14  and  16  are thereby aligned anti-parallel in the horizontal plane. While it is most beneficial for the Γ vectors to be aligned exactly anti-parallel, substantial acceleration benefits can be achieved if the vectors are aligned within 10° of anti-parallel. The crystal resonators  10  and  12  can be mounted to the substrate and can also be coupled to one another. The oscillator can also include active or passive elements  20  configured on the substrate  18 . 
       FIG. 6  illustrates an embodiment of an oscillator  60  wherein the two matched resonators  10  and  12  are mounted in an inverted position or back to back with the mounting plane of resonator  10  rotated 180° around the x axis in the y-z plane. The acceleration sensitivity vectors of the crystal resonators  10  and  12  are arranged essentially anti-parallel. While it is most beneficial for the Γ vectors to be aligned exactly anti-parallel, substantial acceleration benefits can be achieved if the vectors are aligned within 10° of anti-parallel. This composite resonator is disposed on an interconnecting substrate  64 . Conductive straps  22  can connect the two resonators together and to the interconnecting substrate  64 . A TCXO circuit  62  can also be mounted on the substrate  64  in order to produce a temperature compensated crystal oscillator  60 . The TCXO circuitry  62  generates a correction signal to compensate and minimize the frequency drift of the resonator as the ambient temperature varies. 
     A TCXO with acceleration sensitivity vector cancellation based on embodiments of the invention has a g-sensitivity less than 0.05 parts-per-billion (ppb) or 5×10 −11  per g of applied acceleration force. This is at least an order of magnitude improvement compared to other TCXOs currently available. Also, when operating under random vibration, such a TCXO can improve the phase noise by more than 40 dB compared to conventional TCXOs. In an embodiment of an acceleration sensitivity cancelling TCXO, the frequency stability can be ±1 ppm over −40° to +70°. The input supply voltage can be +3.3 Vdc to +5 Vdc at 10 mA. Also, the low phase noise output can be CMOS compatible with 50/50±5% duty cycle. This can provide electronic frequency control for precise tuning or phase locking applications. 
       FIG. 7  illustrates a further embodiment of an oscillator  80  wherein the two resonators  10  and  12  are mounted in an inverted or back to back position with the mounting plane of resonator  10  rotated 180° around the x axis in the y-z plane. The acceleration sensitivity vectors of the crystal resonators  10  and  12  are arranged essentially anti-parallel. While it is most beneficial for the Γ vectors to be aligned exactly anti-parallel, substantial acceleration benefits can be achieved if the vectors are aligned within 10° of anti-parallel. This composite resonator is then mounted to a planar oven substrate  72 . On the substrate is also a heat source  68 , temperature sensor  66 , oscillator circuit  70  and oven control circuit  69  which proportionally controls and stabilizes the heat source  68  to maintain the resonators at a precise temperature even when the outside or ambient temperature varies. The oven controlled crystal oscillator is housed within a package consisting of supporting header  74  and cover  76 . Therefore, the oscillator  80  is less sensitive to acceleration forces and is also oven controlled. 
     It should be pointed out that while what has been described here are several embodiments of the invention, it may be possible to implement various modifications and variations without departing from the intent and scope of the invention. Although the invention presented herein has been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that the invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. Thus, it is intended that the scope of the invention herein disclosed should not be limited by the particular embodiments described above, but should be determined only by a fair reading of the claims.