US20020084858A1

US20020084858A1 - Temperature compensated crystal oscillator assembled on crystal base

Info

Publication number: US20020084858A1
Application number: US09/751,206
Authority: US
Inventors: Marlin Luff
Original assignee: CTS Corp
Current assignee: CTS Corp
Priority date: 2000-12-29
Filing date: 2000-12-29
Publication date: 2002-07-04
Anticipated expiration: 2020-12-29
Also published as: US6456168B1

Abstract

A dual-cavity temperature compensated crystal oscillator (100) provides a three-layer ceramic package (110), with a crystal (170) sealed in a well or cavity (148). Oscillator components (180-184) such as a compensation circuit and an oscillator are attached through screened solder onto the back side of the ceramic package (110) and are encapsulated within potting compound or encapsulant. Electrical connection is provided between the oscillator and compensation circuitry and the piezoelectric element (170) to produce a frequency-controlled oscillator. After frequency tuning, a hermetic seal is provided between a cover (160) and ledge (140) to hermetically seal the cavity (148).

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to oscillators generally, and more specifically to housings or packages for temperature-controlled crystal oscillators that are simultaneously compact, rugged, and amenable to mass-production techniques.

2. Description of the Related Art

Seemingly from the first day a wire was attached to a battery to form an electrical circuit, the design of electrical and electronic devices have been driven by demands in the marketplace to provide better reliability and more function in a smaller package using less power, all at a lower cost. Many times these demands are in conflict, and a project manager must evaluate which goals take precedence for the given project. The project may at least then be completed, even if some of the goals were not attained. The attainment of a plurality of market or consumer goals is very significant, and meeting all of the goals is, of course, most desirable.

Nowhere are these goals of better reliability and more function in a smaller package using less power more evident than in the hand-held or personal electronic device marketplace. Many of these devices, such as pagers, cellular telephones, wireless modems and the like are carried with or upon a person at nearly all times of the day, and unnecessary size and weight are very unattractive in the marketplace. Within this market segment, these communications devices additionally demand high quality frequency control for transmissions from and reception by these devices. With limited bandwidth available for personal communications, the channels within which these devices are permitted to operate are relatively narrow, and deviation from those channels can lead to unwanted interference, loss or drop of signals, and other undesired effects. Undesired effects such as these directly translate into lower reliability for the finished product, and consumer dissatisfaction. Consequently, high quality frequency control is required.

To produce a base frequency of oscillation, an electronic oscillator circuit is employed that will typically include a piezoelectric device. Piezoelectric devices can be designed to mechanically resonate at a very precise and repeatable frequencies, and this mechanical resonance is translated by the device from or into an electrical signal. Electrodes are formed upon the surface of the piezoelectric device which enable the application or pick-up of an electric field across some part of the piezoelectric device. In response to electrical stimulation, the piezoelectric device will change physical shape, and, when the electrical signal is at the mechanical resonant frequency of the piezoelectric device or some harmonic thereof, energy lost through the piezoelectric device is at a minimum. Other electrical waves may be applied at frequencies different from the natural mechanical resonance of the crystal, but these electrical waves will be very strongly attenuated by the piezoelectric device, since the mechanical oscillations that might be induced are not within the mechanical resonance of the piezoelectric device.

Electronic oscillator circuits are typically associated with these piezoelectric devices that set up a basic oscillation, and the piezoelectric device is then used to very tightly control the exact frequency at which the oscillator circuit will be resonant. The mechanical properties of the piezoelectric device are obviously very critical to the proper functioning of the oscillator, and to accurate frequency control. Among the many mechanical properties that have been identified as being significant to accurate frequency control are the actual piezoelectric characteristics of the piezoelectric device, one representation of which is the Q, or quality, of the device. Q is measured by the resonant frequency divided by the bandwidth which will pass through the device at less than a three decibel drop, or about seven-tenths of the original amplitude. The higher the Q, the more accurately a frequency may be controlled. Many factors affect the Q of a device, most which are too complex to be considered herein, but of particular significance herein are the loading of a piezoelectric device with mass which is not piezoelectrically active, the piezoelectric material used to fabricate the device, and the particular shape of the device and angle of cut or fabrication with respect to the various crystal axes. Inactive mass tends to lower Q. Quartz tends to have a high Q, while piezoceramic materials are generally lower.

Another important characteristic is the resonant frequency of the device. A third characteristic, or, more accurately, family of characteristics, has to do wit the impact of environmental factors upon the Q, resonant frequency, aging and other similar properties. Primary among these are temperature, humidity, and mechanical shock.

Many of these piezoelectric devices, particularly in the handheld communications marketplace, operate at resonant frequencies in the megahertz range. One Hertz is equal to one full cycle occurring within one second of time. A device mechanically resonant in the megahertz range must vibrate back and forth millions of times each second. As may be appreciated, relatively small changes in a large number of different parameters can have very consequential effects upon the resonant frequency and Q of a piezoelectric device. Mass and temperature are two such parameters.

With regard to mass, even seemingly minute particles, measured in micrograms, that may land on or adhere to the surface of a piezoelectric device will adversely affect the frequency of operation of the device. Furthermore, even the chemical or atomic addition of mass, such as through corrosion or surface adsorption, will alter the resonant frequency of these high frequency piezoelectric devices sufficiently to disrupt their reliable performance. In order to prevent this type of disruption, high quality resonators are generally manufactured in very clean environments such as clean rooms, and are most preferably packaged into a sealed container free of any possible contaminants or corrosive compounds. Vacuums, dry nitrogen, noble gases and the like have all been used as atmospheres for packaged crystal resonators. Unfortunately, the inclusion of other electronic devices may introduce harmful out gassing or other contaminants that may adversely affect the crystal, or the aging characteristics of the crystal.

A second parameter that has very significant impact upon the resonant frequency and Q is temperature. In particular, portable electronic devices are used in a wide variety of environments. The electronic devices are required to operate in a cold, potentially sub-zero environment one day, such as may be encountered out-of-doors in the winter, and then the next day the same electronic device will be used in a warm environment such as a gymnasium or a more tropical climate. It is well known that temperature has an effect on oscillator circuitry. More particularly, as temperature changes, the resonant frequency of an oscillator circuit also changes. While it is possible to fabricate quartz crystals having very low changes in resonant frequency with respect to temperature, this requires cutting the quartz along difficult axes and requires elaborate production equipment, thereby elevating production costs. The resulting parts often are still not completely immune to effects due to temperature and require sorting of individual components, meaning lower manufacturing yields and yet higher production costs. Consequently, the selection of completely temperature insensitive components has not proven to be economically practical for most applications. In order to better address the frequency variations due to temperature, several other approaches have also been proposed in the prior art.

One approach is to place the resonator into a temperature-controlled oven where the resonator will experience no temperature variation. These devices, referred to as ovenized crystal oscillators, require elaborate packaging and substantial electronic circuitry to precisely control the temperature within the oven, thereby increasing both size and cost of the device. For most personal communications devices such as handheld cellular telephones, the size and expense are not acceptable.

Another approach is to design a circuit with other components that can be used to trim, or adjust, the resonant frequency of oscillation of the oscillator circuit. These trimming components are then used to make up for known variations in the frequency of oscillation of the piezoelectric device through a predetermined temperature range. While the trimming components do not normally exactly compensate for the temperature variations, the overall performance of the oscillator can be markedly improved. It is this type of oscillator, referred to as a temperature-controlled crystal oscillator, or TCXO, for which the preferred embodiment was constructed.

In a TCXO, electronic circuitry necessary to provide basic electrical oscillation and temperature compensation will most desirably be placed as close to the crystal as possible, since the temperature being measured will desirably be that of the crystal. The electronic circuit should also be close, owing to the high frequencies of oscillation which are adversely affected by long lead lengths. This has been accomplished in the prior art by including the electronic circuitry adjacent to the crystal, such as illustrated by Shigemori et al in U.S. Pat. No. 6,081,164. Unfortunately, the discrete package which is provided as important physical isolation for the crystal adds volume to the second package, making the finished oscillator quite large. Consequently, this design has not been widely accepted in the handheld marketplace.

Several additional designs, which are commonly owned by the present assignee, have been proposed that have proven to be successful in the marketplace. U.S. Pat. No. 5,438,219 to Kotzan and Knecht, and U.S. Pat. No. 5,500,628 to Knecht, the contents which are incorporated herein by reference for teachings found therein, each illustrate packages which accommodate oscillator and temperature control circuitry within a package which also separately houses a crystal resonator. The novel oscillator packages illustrated therein enabled the manufacture of a reliable package of smaller dimension than was previously possible, reducing the package size from 8.89 mm×8.89 mm×2.79 mm down to 7.11 mm×6.22 mm×2.24 mm. While those dimensions are already quite small, the demand continues for even smaller and lower cost components. Unfortunately, these patents also require that electrical connections be made within a package cavity. This eliminates the use of low-cost screened and re-flowed solder attachment, which is very amenable to high volume production, and instead requires alternatives such as wire-bonding to be used in a greater degree.

SUMMARY AND OBJECTS OF THE INVENTION

In a first manifestation, the invention is a temperature compensated oscillator assembled on a crystal package base. A planar substrate has first and second major surfaces forming opposite sides of the crystal package base. Sidewalls adjacent the first major surface extend upwardly therefrom and form a cavity therewith. The cavity is adapted to receive at least one piezoelectric component, and the second major surface is adapted to receive at least one electronic component. A cover is coupled to the cavity and defines a hermetic enclosure therewith. An encapsulant and the second major surface encapsulate the electronic component.

In a second manifestation, the invention is a method of coupling components to a crystal package. A ceramic package having an open receptacle with a planar exterior is provided. A piezoelectric element is mounted in the open receptacle. The piezoelectric element is frequency tuned, and then hermetically sealed in the open receptacle. Electronic components are affixed to the planar exterior and electrically coupled to the piezoelectric element.

In a third manifestation, the invention is a method for assembling a temperature compensated crystal oscillator. In this method, the steps are: providing a double-sided TCXO package body with a first planar surface and a cavity opposite the first surface; attaching compliant crystal supports into the cavity; placing a piezoelectric device into the crystal supports; hermetically sealing the cavity with a cover; applying a conductive composition onto the first planar surface; positioning electrical components into the conductive composition; affixing and electrically connecting the electrical components to the conductive composition; dispensing encapsulant substantially over the electrical components; and curing the encapsulant to yield an assembled TCXO.

Exemplary embodiments of the present invention solve the inadequacies of prior temperature compensated crystal oscillator circuits by providing a three-layer ceramic package with a crystal sealed in a well or cavity. Oscillator components are attached through screened solder onto the back side of the ceramic package and are encapsulated within potting compound or encapsulant.

A first object of the invention is to enable low-cost mass-production manufacturing techniques, while still ensuring the integrity of sensitive and critical parts within the oscillator. A second object of the invention is to provide a temperature compensated crystal oscillator (or other type of oscillator circuit) with output frequency that deviates less than ±2 parts per million (ppm) over a temperature range of −30° to +85° C. Another object of the present invention is to provide a temperature compensated crystal oscillator with an output frequency that deviates less then approximately ±1 ppm over a one-year aging period. A further object of the invention is to minimize the possibility of cross-contamination of the components by isolating the crystal from other components. Yet another object of the present invention is to provide a temperature compensated oscillator that may be packaged into a smaller package than was heretofore possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, advantages, and novel features of the present invention can be understood and appreciated by reference to the following detailed description of the invention, taken in conjunction with the accompanying drawings, in which: [0021]
FIG. 1 illustrates the preferred embodiment temperature compensated crystal oscillator designed in accord with the teachings of the present invention from a bottom plan view. [0022]
FIG. 2 illustrates the preferred embodiment temperature compensated crystal oscillator of FIG. 1 from a side plan view. [0023]
FIG. 3 illustrates the preferred embodiment temperature compensated crystal oscillator from a cross-section view taken along [0024] line 3′ of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides an improved dual-cavity temperature compensated crystal oscillator (TCXO) [0025] package 100. Within package 100 are included generally a compensation circuit and an oscillator coupled to a substrate. Package 100 receives a piezoelectric element 170 within cavity 148, and an electrical connection is provided between the oscillator and compensation circuitry and piezoelectric element 170 to produce a frequency-controlled oscillator. After frequency tuning, a hermetic seal is provided between cover 160 and ledge 140 to hermetically seal cavity 148. With preferred embodiment package 100, dimensional limitations of the prior art frequency control devices are substantially overcome.
The exact dimensions and geometry of [0026] package 100 can vary widely. Package 100 is particularly adapted for miniaturization, and may be shaped geometrically differently to accommodate differing design needs. The height of TCXO 100 will preferably be sufficient to enclose crystal 170 in cavity 148 and minimize stray or unwanted capacitances between piezoelectric element 170 and associated electronic circuitry. For example, package 100 may have dimensions of only 3.2 mm×5 mm×1.5 mm in one particular embodiment. As a result, package 100 is adapted for placement in an electronic device where it will typically take up a small portion of the total volume of the electronic device.
In the preferred embodiment, [0027] package 100 is most preferably manufactured from materials having substantially similar thermal expansion coefficients, to minimize stresses. Fired or co-fired ceramic materials such as alumina, produced for example through various casting or pressing techniques and having refractory, thick film or thin film metallizations, are suitable materials for body 110. These materials are preferred, owing to an intrinsic relatively low dielectric constant, electrical insulation, moderate thermal conductivity, exceptional chemical, mechanical and environmental resistance including hermeticity, and a low thermal coefficient of expansion compatible with silicon and quartz. Nevertheless, myriads of materials exist that may also perform satisfactorily, as do myriads of processing techniques. Alloys of nickel, iron and cobalt sold under the trademark Kovar, or Alloy 42 and the like, but preferably Kovar because of its coefficient of thermal expansion being substantially similar to the preferred ceramic material of body 110, may preferably be used to fabricate cover 160.
Referring to FIG. 3, [0028] cavity 148 holds at least piezoelectric element 170 and can include other components if desired. However, having piezoelectric element 170 isolated from other components minimizes the possibility of contaminating element 170, which might otherwise result in undesirable frequency and performance changes. More particularly, isolating and physically separating piezoelectric element 170 in cavity 148 from components 180-184 on the exterior surface 114 substantially minimizes the possibility of the solder, organic underfill, and other unwanted contaminants from adversely affecting the output frequency of the piezoelectric element 170, which can occur over time in prior art TCXO's.
The geometric shape, size and composition of [0029] piezoelectric element 170 may vary depending upon the application, so long as it has the characteristics desired at design, such as a stable frequency output over a wide temperature range. Examples can include quartz AT-cut strip resonators, other cuts, shapes and compositions of bulk resonators, and even surface acoustic wave (SAW) devices. Nevertheless, quartz AT strip resonators are most preferred in the present invention owing to their inherent small size, photolithographic means of fabrication, light weight, good temperature stability and aging characteristics, and improved mechanical resilience due to a low mass. The AT-cut quartz strip further exhibits a well-behaved frequency versus temperature relationship from about −40° C. to about 90° C. that may be compensated for electronically.
[0030] Piezoelectric element 170 is mechanically supported on and coupled to couplings 172 and 174. Cantilever mounts, metal foil clips, or springs may be provided, or, alternatively, compliant materials may instead be used to form couplings 172 and 174. Regardless of composition or construction, couplings 172, 174 should provide a compliant electrical and mechanical connection. This allows piezoelectric element 170 and surface 150 to expand and contract at different rates without putting excess stress upon piezoelectric element 170, which in turn minimizes the possibility of undesirably changing the output frequency. As should be understood by those skilled in the art, compliant materials as referred to herein can comprise various materials, so long as they are compliant and have a suitable viscosity so as to minimize the possibility of unwanted spreading or excessive flowing during application. Examples of compliant materials can include but are not limited to any one or more of the following: silver-filled silicone, silicone, epoxy, and silver-filled epoxy. Preferably, silver-filled epoxy is used for very good outgassing characteristics that only minimally degrade performance during aging while enclosed in the inert environment
In an alternatively conceived embodiment, [0031] piezoelectric element 170 may be supported across ledges within cavity 148, and an electrical connection may be made therebetween. Care in placement is necessary in this embodiment so that piezoelectric element 170 sits across the ledges properly to suitably isolate the active region from the ledges. Otherwise there may be undesirable dampening and degrading of the frequency performance of TCXO package 100.
Electrically connected to [0032] couplings 172 and 174 are through- hole vias 176, 178 which are preferably co-fired, refractory metal or the like, for connecting piezoelectric element 170 to the circuitry on the opposing surface 114 of body 110. Through- hole vias 176, 178 interconnect the first major surface 150 of the base substrate of body 110 through to the second major surface 114. Surface 114 of body 110 supports various electronic components 180, 182, 184, and will preferably additionally include circuit traces formed thereon. These traces or wiring patterns may be formed by refractory metallization, thick or thin film, or other satisfactory processes. Wire bonds 186, 188 maybe provided where needed, but more preferably the components placed upon surface 114 will be soldered into place. Most preferably, a solder paste will be screened onto surface 114, and components 180-184 will be placed therein. Then the solder will be reflowed in an oven, furnace or with other satisfactory technique to affix and electrically connect the components to surface 114. As an alternative to solder, conductive adhesives may be screened or otherwise provided and components 180-184 may be placed into the adhesive. The adhesive will then be cured, affixing and electrically connecting the components.
A glob top, conformal coating, encapsulant, molding or overmolding, or [0033] similar covering 190 will be applied after components 180-184 are affixed and electrically connected. One technique for potting or encapsulating the oscillator electronics is described in U.S. Pat. No. 5,640,746 to Knecht and Wille, commonly owned by the present assignee, which is incorporated herein by reference for teachings in that regard. Other techniques which are known generally in the electronics industry for encapsulation or potting are also considered to be incorporated herein as well.
After components are mounted and the encapsulant cured, [0034] TCXO package 100 is frequency tuned. This tuning may be accomplished, for example, by contacting tuning pads 175, 177 with appropriate equipment to determine the resonant frequency of element 170 and simultaneously adding or removing small amounts of mass. Mass loading of piezoelectric element 170 by adding mass decreases the frequency of resonance until a desired frequency is achieved, while removal of mass increases the frequency of resonance. Mass is typically added by either adding metal or other atoms through various metallization techniques, chemical reactions or oxidation of the electrode metal. Ion milling and other suitable techniques may be used for removal of mass. The possibility of unwanted metal or gasses contacting and adversely affecting other electronic components in cavity 148 during tuning is eliminated in the preferred embodiment because these components are isolated on the other side of body 110. Moreover, these additional components may not even be coupled to package 100 until the tuning step has been completed, depending upon the manufacturing sequence selected.
Next, [0035] cover 160, preferably comprising a metal such as Kovar, is welded, brazed or soldered onto metallization formed on ledge 140. A good weld can provide a seal to enclose an inert gas within cavity 148 or alternatively maintain a vacuum therein.
Once [0036] package 100 has been assembled, it may be electrically and mechanically coupled to a circuit board located in another device. Surface 116 of body 110 is configured to facilitate mounting onto circuit boards or similar substrates. A plurality of contacts 124 formed upon surface 116 are suitably connected to respective castellations 120, which are plated half holes or metallizations formed upon the exterior sidewalls 112 of body 110. As can best be seen in FIG. 3, a small cavity 138 is also provided which spaces cover 160 away from the circuit board. This helps to ensure that cover 160 will not form unwanted electrical contacts or short-circuits within the circuit board traces when contacts 124 are connected thereto, or mechanically interfere with the proper connection between contacts 124 and the circuit board.
[0037] Contacts 124 and castellations 120 may be formed from a variety of suitable techniques, including refractory metallization, thin or thick film, or other suitable techniques as are known in the art. The exact count or placement of contacts 124 and castellations 120 are not critical to the workings of the invention. However, contacts for DC power, ground and oscillator output are most preferred for the proper working and connection of the invention. A connection through which a voltage may be applied to adjust frequency will also preferably be provided. Other connections which are beneficial to the manufacture and testing of an oscillator may also be provided, in accordance with the needs of a particular embodiment at the time of design.
The method for making [0038] package 100 generally includes: (1) providing a double-sided TCXO package body 110; (2) providing compliant crystal supports 172, 174 appropriately onto surface 150; (3) placing quartz crystal 170 into cavity 140; (4) frequency tuning quartz crystal 170 by mass loading and removal, while actuating quartz crystal 170 electrically; (5) hermetically sealing cavity 148 with a metal cover, by placing and then sealing cover 160 with a seam weld, solder seal, compression weld or the like around a periphery thereof; (6) stenciling solder or conductive adhesive onto surface 114; (7) placing components 180-184 into the stenciled conductive; (8) reflowing the solder or curing the adhesive; (9) wirebonding, where necessary, any components to mating conductors on surface 114; (10) dispensing a glob top epoxy or encapsulant substantially over the components 180-184; (11) curing the encapsulant; and (12) testing the finished TCXO package 100. This process is particularly adapted for use in the mass production of temperature compensated crystal oscillators.
While the foregoing details what is felt to be the preferred embodiment of the invention, no material limitations to the scope of the claimed invention are intended. Further, features and design alternatives that would be obvious to one of ordinary skill in the art are considered to be incorporated herein. The scope of the invention is set forth and particularly described in the claims hereinbelow. [0039]

Claims

I claim:

1. A temperature compensated oscillator assembled on a crystal package base, comprising:

a planar substrate having first and second major surfaces forming said crystal package base;

sidewalls adjacent said first major surface and extending therefrom and forming a cavity therewith;

said cavity adapted to receive at least one piezoelectric component and said second major surface adapted to receive at least one electronic component;

a cover coupled to said cavity and defining a hermetic enclosure therewith; and

an encapsulant and said second major surface encapsulating said at least one electronic component.

2. The oscillator of claim 1, wherein said second major surface is adapted to receive screen printed solder paste.

3. The oscillator of claim 1, wherein said second major surface is primarily covered by a curable material and further contains a wire bonded integrated circuit therein.

4. The oscillator of claim 1, wherein said extending sidewalls terminate at a surface mountable end portion including a plurality of contacts adapted to facilitate connection to an electrical device.

5. The oscillator of claim 1, wherein said extending sidewalls include electrically conductive castellations which carry electrical power, ground and oscillator output signals.

6. The oscillator of claim 1, wherein said cover comprises a metal.

7. The oscillator of claim 1, wherein said substrate and said extending sidewalls are composed of a ceramic.

8. The oscillator of claim 7, wherein said cover is composed of an alloy of nickel, iron and cobalt.

9. The oscillator of claim 1, wherein said substrate and said extending sidewalls comprise materials having substantially similar thermal expansion coefficients.

10. The oscillator of claim 1, further comprising a ledge formed on an interior surface of said extending sidewalls and spaced from said first major surface, upon which said cover may be mounted.

11. The oscillator of claim 10, further comprising a plurality of castellations extending from said second major surface along said extending sidewalls to a surface mountable end portion including a plurality of contacts adapted to facilitate connection to an electrical device, said plurality of electrical contacts electrically coupled to said castellations.

12. The oscillator of claim 1, wherein said cover is coupled to said extending sidewalls by a hermetic seal.

13. A method of coupling components to a crystal package, comprising:

a) providing a ceramic package having an open receptacle with a planar exterior bottom;

b) mounting a piezoelectric element in said open receptacle;

c) frequency tuning said piezoelectric element;

d) hermetically sealing said piezoelectric element in said open receptacle;

e) affixing electronic components to said planar exterior bottom; and

f) electrically coupling said electronic components to said piezoelectric element.

14. A method for assembling a temperature compensated crystal oscillator, comprising the steps of:

providing a double-sided TCXO package body having a first planar surface and a cavity opposite said first planar surface;

attaching compliant crystal supports into said cavity;

placing a piezoelectric device into said crystal supports;

hermetically sealing said cavity with a cover;

applying a conductive composition onto said planar surface;

positioning electrical components into contact with said conductive composition;

fixing said conductive composition to said electrical components; and

encapsulating said electrical components to yield an assembled TCXO.

15. The method of assembling a temperature compensated crystal oscillator of claim 14, further comprising frequency tuning said piezoelectric device while actuating said piezoelectric device electrically.

16. The method of assembling a temperature compensated crystal oscillator of claim 14, further comprising testing the assembled TCXO.

17. The method of assembling a temperature compensated crystal oscillator of claim 14, further comprising wirebonding components to mating conductors on said first planar surface.

18. The method of assembling a temperature compensated crystal oscillator of claim 14, wherein said step of applying is selected from the group of stenciling and screenprinting.

19. The method of assembling a temperature compensated crystal oscillator of claim 14, wherein said step of hermetically sealing further comprises placing and then sealing said cover around a periphery thereof.

20. The method of assembling a temperature compensated crystal oscillator of claim 15, wherein said step of frequency tuning further comprises mass loading said piezoelectric device.