Hermetically Sealed Oscillator With Dielectric Resonator Tuned Through Dielectric Window by Adjusting Screw

This invention relates to microwave dielectric resonator apparatus and, more particularly, to such apparatus as oscillators that are stabilized in output frequency by a dielectric resonator.

Heretofore, dielectrically stabilized microwave oscillators have been provided in which a dielectric resonator puck stores microwave energy coupled thereto by a driving circuit. This causes the puck to resonate at a microwave frequency to which the output frequency of the oscillator becomes stabilized.

It is conventional to make such known oscillators mechanically tunable to compensate for frequency drift due to aging. In that case, the dielectric resonator is generally associated with a tuning slug movable into and out of positions at which it variously interferes with external field lines of the puck which correspond to a portion of the stored microwave energy in the puck, thereby to alter the microwave resonant frequency of the puck to which the oscillator output frequency becomes stabilized. To this end, the tuning slug, the dielectric resonator puck and the driving circuit have all been enclosed in a non-hermetic housing. Typically, the tuning slug has been disposed directly over the puck at one end of a shaft threaded into an overlying wall of the housing, the other end of the shaft lying outside the housing and being slotted to receive a screwdriver by which an operator mechanically positions the slug toward and away from the top of the puck.

While the known mechanical tuning arrangement is satisfactory in dielectrically stabilized microwave oscillators where, for example, active or negative resistance elements of the puck driving circuits are individually encapsulated for environmental protection, such as Gunn diodes, it is inherently unsatisfactory in dielectrically stabilized microwave oscillators having puck driving circuits which employ active elements which are not individually encapsulated, notably GaAsFETs, and which must be hermetically housed for environmental protection. Such driving circuits may even include buffer amplifier stages and other circuitry having exposed semiconductor devices which, independently of whether the active elements individually are encapsulated or unencapsulated, require hermetic protection from the environment.

The reason why the known mechanical tuning arrangement is inherently unsatisfactory where hermetic protection from the environment is required is simply that the known arrangement renders it virtually impossible to preserve the required hermetic protection for any considerable length of time due to contaminating leakage at the threaded shaft of the tuning slug. While a mechanical positioning of the tuning slug with a bellows arrangement would overcome the leakage problem, it would introduce intolerable mechanical hysteresis and be intolerably microphonic. Thus, up to now, mechanical tuning of hermetically sealed dielectrically stabilized microwave oscillators has not been feasible, and electronic tuning, such as by Varactors, has been the only practical way to achieve tuning access into a hermetically sealed enclosure.

An object of the invention is to provide improved means for rendering the dielectric resonator of a microwave dielectric resonator apparatus mechanically tunable about its resonant frequency from without a hermetic chamber in which the resonator is disposed.

Another object of the invention is to provide a microwave dielectric resonator apparatus with an improved arrangement for mechanically and selectively altering the resonant frequency of the resonator without penetrating a hermetic chamber enclosing the resonator.

Another object of the invention is to provide an improved microwave dielectric apparatus of the kind in which a dielectric resonator puck stores microwave energy coupled thereto by a driving circuit and in which both the puck and the driving circuit are disposed in a hermetically sealed enclosure.

Another object of the invention is to provide an improved dielectrically stabilized microwave oscillator of the kind in which the oscillator output frequency becomes stabilized to the microwave resonant frequency of a dielectric resonator enclosed in a hermetic chamber together with a driving circuit for the resonator.

Another object of the invention is to provide an improved mechanically tuned microwave oscillator stabilized in output frequency by a dielectric resonator.

According to one aspect of the invention, a dielectric resonator puck and a driving circuit for causing the puck to resonate at a microwave frequency are housed in a hermetically sealed chamber, externally of which a movable tuning slug is mounted together with cooperating means for mechanically positioning the slug, the chamber being impervious to microwave fields except at a local wall area thereof which transmits out of the chamber at least a portion of the field lines developed around the puck when the puck is driven into resonance, the transmitted portion being in a path permitting this portion to be interfered with by the tuning slug for altering the microwave resonance frequency of the puck.

According to another aspect of the invention, there is provided a method of obtaining a fine tuning adjustment of the resonant frequency of a dielectric microwave resonator from a location outside of a hermetically sealed chamber which encloses the resonator in common with microwave circuitry energizable to develop fringing fields at microwave frequencies that link with the resonator to excite it into a state of resonance. The method is carried out by forming the hermetically sealed chamber with a bounding structure which has the property of confining microwave fields within the chamber except at a window portion thereof transparent to such fields; positioning the dielectric microwave resonator relative to the window portion so that part of the field developed about the resonator by the fields circulating therein during its state of resonance passes through the window portion; movably disposing outside the hermetically sealed chamber proximate the window portion a slug of solid material capable of influencing the field part as a function of the positional relationship of the slug with the field part and of thereby altering the distribution and amount of the energy associated with the resonator current and stored in the field about the resonator, any such alternation being effective to alter the resonant frequency of the resonator; and adjusting the position of the slug relative to the field part so as to obtain the fine tuning adjustment of the resonant frequency of the resonator.

The aforesaid and further objects, advantages and aspects of the invention will be more clearly understood from the ensuing detailed description when read in conjunction with the appended drawings, in which:

FIG. 1 is an isometric exterior view of a microwave dielectric resonator oscillator;

FIGS. 2 and 5 are elevational views, partly in section, showing alternative interior structures for the oscillator exteriorly depicted in FIG. 1; and

FIGS. 3 and 4 are elevational views, partly in section, illustrating modifications of the interior structures shown in FIGS. 2 and 5.

In FIGS. 1, 2 and 5, the invention is illustrated as embodied in a microwave dielectric resonator oscillator 10 having a rectangular box-like housing 12 formed of a non-loss metal such as aluminum which confines the microwave fields.

Referring particularly to FIG. 1, an adjusting screw 14 for use with a screwdriver in mechanically tuning oscillator 10 is threaded within a small cylindrical sleeve 16 perpendicularly projecting from the bottom of housing 12. The back of housing 12 carries an output jack connector 18 adapted for connection to a female fitting of a coaxial cable in order to couple the oscillator output from a hermetic feed-through terminal (not shown) to the coaxial cable in a conventional manner. On the left side of housing 12, a d.c. input terminal 20 and a ground terminal 22 are mounted as hermetic feed-through terminals for respectively providing oscillator 10 with d.c. bias and reference potentials necessary for its operation.

With reference to the showing in FIG. 2 of one interior structure of oscillator 10, housing 12 includes a hermetically sealed chamber 24 bounded at the top by a cover 26 which also serves as the top of housing 12. The bottom of hermetic chamber 24 is provided by a substrate 28 of a suitable dielectric material transmissive to microwave fields and capable of forming a hermetic seal, such as alumina or quartz, parallel to cover 26. The bounds of hermetic chamber 24 are completed by side portions of housing 12 to which the peripheries of cover 26 and substrate 28 are eventually soldered or brazed to provide chamber 24 with its hermetic integrity.

Substrate 28 supports on its upper surface a dielectric resonator puck 30 and microwave driving circuitry 32 along the puck and has on its opposite surface a metalizing film constituting a ground plane 34. A portion of the lower surface of substrate 28 underlies puck 30 and, being free of the metallizing film, provides substrates 28 with an r-f window 36 therethrough for microwave field lines developed around the resonating puck 30. A cup-shaped non-hermetic chamber 38 is formed in housing 12 directly beneath window 36 for receiving the microwave field lines; and a circular metallic tuning slug 40 mounted at the inner end of adjusting screw 14 is disposed in non-hermetic chamber 38 to be selectively movable into different interfering relationships with the window transmitted field lines by mechanically turning the screw 14 in the internal threads of sleeve 16 and a continuation thereof in a housing hole extending the sleeve interior into non-hermetic chamber 38.

Resonator puck 30 is a precision ground cylinder of a suitable dielectric material, such as barium titanate, purified to an extremely high degree and having a dielectric constant in the range of said from 30 to 42. Microwave driving circuitry 32 includes etched microstrip elements integrated with one or more negative resistance active elements, such as gallium arsenide field effect transistors (GaAsFETs), and is energized by applying a d.c. bias voltage thereto by way of hermetic feed-through terminal 20 (FIG. 1). The intercoupling association of such microwave driving circuitry, by way of the fringing fields it develops when energized, with such a dielectric resonator puck to form a dielectrically stabilized microwave oscillator is well-known. Equally known is the resultant functioning of the puck as a resonant circuit which determines the oscillator output frequency and in and around which a microwave field is created by the fields circulating in the resonating puck, the output of the driving circuitry 32 feeding microwave energy to the puck to induce natural resonance which in turn feeds the input of the driving circuitry to stabilize the oscillator frequency. In fact, such fields represent the energy stored in the puck and the microwave field is a manifestation of that stored energy. According to the invention, part of that field passes through window 36 to be influenced by the tuning slug 40 as a function of the positional relationship of the slug with the field part. Thus, the distribution and amount of the energy associated with the puck current and stored in the field about puck 30 is altered, thereby effectively altering the resonant frequency of the puck without placing the mechanically movable tuning slug 40 in the hermetic chamber 24 enclosing the puck and its microwave driving circuitry 32.

Referring now to FIG. 5, wherein like parts are identified by like reference numerals, the alternative interior structure for oscillator 10 (FIG. 1) is distinguished from the structure shown in FIG. 2 by the addition of a Kovar carrier 42 upon which electrical ground plane 34 uniformly rests and which has a cut-out coextensive with the window 36 of substrate 28. Kovar carrier 42 facilitates the assembling of the apparatus, and lowers the thermally induced mechanical stress between the housing and the substrate.

The variant shown in FIG. 3, wherein like parts are again identified by like reference numerals, permits the screwdriver positional adjustments of tuning slug 40 to be made proximate the unsupported upper side of dielectric resonator puck 30, i.e. at a non-hermetic location directly overlying the puck. To this end, cup-shaped non-hermetic chamber 38 is now formed in a cover assembly 44 of a modified housing 46; and a quartz or alumina window 48, transparent to microwave fields, hermetically seals the mouth of cup-shaped chamber 38 at its interface with hermetic chamber 24. Magnetic field lines about puck 30 accordingly pass upwardly through window 48 to be interfered with by tuning slug 40. Thus, the under surface of puck-supporting substrate 28 is now entirely metallized with a chrome-gold film to provide an uninterrupted electrical ground plane 50 for circuitry 32; and a Kovar carrier 52 sandwiched between the electrical ground plane and the bottom of housing 46 omits the circular cut-out required in carrier 42 of FIG. 5.

Reference is now made to the variant illustrated in FIG. 4, wherein like parts are once again identified by like reference numerals. Here, a movable tuning slug 52 is slid in a direction substantially parallel to the base of dielectric resonator puck 30 in order to achieve its different positional states of interference with the microwave field developed in and around puck 30. To this end, a modified housing 54 has a base provided with a tapped horizontal hole 56 in which an adjusting screw 58 is threadedly engaged and which opens into a non-hermetic chamber 60 directly underlying window 36. A rod-shaped metallic tuning slug 62 of diameter less than that of hole 56 is loosely connected to the inner end of adjusting screw 58 and rests on the floor of non-hermetic chamber 60 so that it will be slidably positioned into and out of non-hermetic chamber 60 in response to mechanical tuning of adjusting screw 58 by a screwdriver. Accordingly, the influence of tuning slug 62 on the part of the puck's surrounding microwave field transmitted out of hermetic chamber 24 through window 36 is a function of the positional relationship of the slug with that field part, as determined by the slug's sliding movements.

While the invention has been particularly shown and described with reference to microwave dielectric resonator oscillator embodiment thereof, it will be understood by those skilled in the art that the invention may be embodied in other microwave dielectric resonator apparatus. For example, the invention may be embodied in a mechanically tunable microwave filter which employs a hermetically protected dielectric resonator puck determinative of the critical frequency of the filter. It will moreover be understood by those skilled in the art that various alterations in form and detail may be made in each of the embodiments of the invention shown and described herein. For example, the construction of cover assembly 44 (FIG. 3) may be simplified by removing ceramic window (alumina, quartz, etc.) 48 and the fixed metal supporting portion thereof up to the supporting shoulder at the top of housing 46 and, by way of substitution, hermetically attaching the mouth of a ceramic cup to the underside of the remaining plate-like fixed metal portion so that the cup encloses tuning slug 40 with the bottom of the cup being disposed over the top of dielectric resonator puck 30.