Broadband Temperature Compensated Microwave Cavity Oscillator

This invention relates generally to the field of microwave RF communications, and particularly to microwave cavity oscillators.

Microwave based RF communication devices are known. Typically, in communication devices that operate above 2 GHz, transmitter and receiver oscillators are comprised of metal encapsulated cavities that serve to stabilize a free running, active oscillating device such as a Gunn diode. In general, such cavities will support oscillation of the active device at a single frequency, which frequency will be determined by the inside dimensions of the cavity, the dimensions and material of purposely inserted tuning element, and to some extent, the characteristics of the load.

Through the use of tuning elements that can be inserted into the cavity, the frequency can be preselected very accurately as appropriate to accomodate a given system's requirements. Unfortunately, temperature variations will cause this frequency to drift. In particular, the material that defines the cavity will expand or contract slightly with temperature changes. Also, the active device itself will vary its activity somewhat with temperature variations, as will the load characteristics. All of these changes cause a resultant change in the oscillation frequency, and thereby degrade the performance of the device.

To deal with this problem in microwave cavity oscillators, the prior art discloses two alternative solutions. The first, and more popular approach, is to specifically design the oscillator for a given tightly limited frequency range, through choice of materials and mechanical connections. In this way, an oscillator can be designed to remain acceptably stable over a workable temperature range. The great drawback to this approach is the limited tuning range of the resultant oscillator. In addition to increased costs of design and construction to match each desired tuning range to acceptable temperature performance, the manufacturer, distributors, and users of such oscillators must maintain a large inventory of oscillators, since they must be able to accommodate needs over a wide frequency range.

The second prior art approach requires external frequency detection circuity to monitor for drift and to use this information in a feedback loop to control the frequency of oscillation accordingly. This approach represents an expensive alternative, and has found little use to date in many frequency bands above 20 GHz.

A need therefore exists for a broadband temperature compensated microwave cavity oscillator that can be tuned over a wide tuning range, and that can be easily temperature compensated to remain accurate within that tuning range. As an example of the accuracy required, devices intended for operation in the 21.2 GHz to 23.6 GHz radio band can drift no more than plus or minus 0.03% over an ambient operating temperature range of minus 30 degrees C. to positive 50 degrees C.

These and other needs are substantially met through provision of the broadband temperature compensated microwave cavity oscillator disclosed herein. This oscillator includes generally a housing having a resonant cavity formed therein, an active device located within the resonant cavity for creating the microwave signals, a tuning element for selecting a frequency of oscillation, and a temperature compensation unit internally associated with the resonant cavity for causing the selected frequency of oscillation to automatically remain substantially stable over a standard operating temperature range.

In one embodiment, the temperature compensation unit can include a second tuning element that is insertable within the cavity to thereby vary the resonant characteristics of the cavity and thereby allow the resonant frequency of oscillation to be temperature compensated. This second tuning element can be supported by a support shaft that has a different coefficient of expansion with temperature from that of the housing. Appropriately configured, the second tuning element will tend to be removed from the cavity as temperature increases, and thereby compensate the resonant characteristics of the cavity.

So configured, the temperature compensation unit will not only compensate for variations in the cavity itself, but can also compensate for frequency variations that occur due to temperature as regards the active device and any load associated with the oscillator.

These and other attributes of the invention will become more clear upon making a thorough review and study of the following description of the Best Mode For Carrying Out the Invention, wherein:

FIG. 1 comprises a perspective exploded view of a first embodiment;

FIG. 2 comprises a detail side elevational section view of this first embodiment;

FIG. 3 comprises a detail side elevational section view of a second embodiment;

FIG. 4a comprises a detail closeup view of a third embodiment;

FIG. 4b comprises a graph relating to the third embodiment; and

FIGS. 5a-5c are graphs depicting the operation of an uncompensated and compensated oscillator as regards the first and second embodiments.

Referring now to the drawings, and in particular to FIGS. 1 and 2, the invention can be seen as depicted generally by the numeral 10. The invention (10) includes generally a housing (11) having a resonant cavity (12) formed therein, an output port (13) formed to allow access to the resonant cavity (12), an active device (14) for producing the desired oscillations, and a first tuning rod (16) to allow the frequency of oscillation to be selectively tuned. The invention (10) also includes a temperature compensation unit (17) that includes a second tuning rod (18) and a support shaft (19). Each of the above generally referred to components will now be described in more detail in seriatim fashion.

The housing (11) can be comprised of a substantially rectangular shaped block formed of an appropriate metal, such as copper. The resonant cavity (12) can be formed therein in accordance with well understood prior art technique, and the output port (13) formed through one side of the housing (11) to allow controlled escape of the desired microwave oscillations. As well understood in the art, an output iris (21) can be soldered to the housing (11) to control escape of the microwave oscillations through the output port (13).

A partially threaded opening (22) can be provided through the housing (11) to allow implacement of the active device (14). The active device itself can be a Gunn diode as well understood in the art. Gunn bias chokes (23) are positioned with respect to the active device (14), with at least a portion of the choke having insulating medium, such as Mylar tape (not shown), mounted thereabout, all in accordance with well understood prior art technique. An associated shaft (26) passes through an appropriate insulating washer (28) and threaded bushing (29), the latter being threadably insertable into the opening (22) provided therefore in the housing (11). So configured, the active device (14) can be properly located within the cavity (12) as depicted.

Another threaded hole (31) can be provided through the housing (11) to accommodate a first tuning rod comprised of an appropriate dielectric material. This first tuning rod (16) passes through a bushing (32), which bushing (32) can be threadably inserted into the hole (31) provided therefore. So configured, the first tuning rod (16) can be selectively inserted into or withdrawn from the cavity (12), such that the first tuning rod (16) can affect the frequency of oscillation.

The temperature compensation unit (17) includes a second tuning rod (18), a support shaft (19), and a support chamber (33). The support chamber (33) comprises an extension of the housing (11) and includes a hole (34) formed therethrough, which hole (34) connects to the resonant cavity (12) of the housing (11). The second tuning rod (18) and an associated bushing (36) connect atop a support shaft (19) having appropriate chokes (37) configured therewith. The support shaft (19) affixes at one end to a plate (38) that can be attached to one end of the support chamber (33) through use of screws (39) or other appropriate affixing members. Importantly, the support shaft (19) should be comprised of a material having a different coefficient of expansion with temperature than that of the housing (11). For instance, Invar may be used. The significance of this will be made more clear below.

Other additional elements may be appropriate to add, depending upon the application. For instance, a dielectric power adjuster (41) and an associated bushing (42) may be provided through a hole (43) provided therefore to allow the power output of the oscillator to be controlled. As depicted in FIG. 2, this power adjustment rod (41) should be positioned exterior to the iris (21). Also, in a frequency modulation application, a modulation varactor (44) may be mounted within the cavity (12) through use of an appropriate hole (46) provided through the housing (11) and an appropriate shaft (47), chokes (48), insulating washer (49) and bushing (51) as configured. In accordance with well understood prior art technique, the varactor (44) can be appropriately controlled to frequency modulate the output of the oscillator.

Operation of the invention (10) can now be described.

With reference to FIG. 5a, the frequency drift of a high Q (low loss) cavity oscillator closely approximates a linear function with respect to temperature. As temperature increases, the metal cavity expands, and the resonant frequency of the cavity decreases as depicted.

The dielectric material (such as ceramic or glass) comprising the tuning rods of the invention (10) also can affect the frequency of oscillation. This relationship, however, does not constitute a linear one. As depicted in FIG. 5b, the frequency of oscillation will decrease in a nonlinear way as the insertion depth of the tuning rod into the cavity increases. Although nonlinear, this relationship is predictable, and hence allows the temperature compensation of the invention (10) to be realized.

The support shaft (19) of the temperature compensation unit (17) has a lower temperature coefficient of expansion (K) than the surrounding material. The shaft connects at one end distal from the cavity (12), such that as temperature increases, the shaft (19) will expand less than the support chamber (33) and hence function to extract the second tuning rod (18) from the cavity (12).

By constructing the second tuning rod (18) of sapphire (K=5 ppm/deg C.), the support shaft (19) of Invar (a nickel-iron alloy, K=2 ppm/deg C.), and the housing (11) of copper (K=17 ppm/deg C.), a controlled temperature compensated configuration will result. The longer the length of the support shaft (19), the greater the resulting movement. With a shaft of 3.302 cm (1.3 inches) in length, approximately 0.00508 cm (0.002 inches) of tuner movement from negative 30 degrees C. to positive 70 degrees C. can be obtained.

Since the slope of the tuning curve (FIG. 5b) increases at an almost constant rate as the tuner is inserted, a greater degree of frequency change will result when the tuner is moved slightly when deep within the cavity (12) than when it has barely been inserted. This sensitivity, or slope of the curve as depicted in FIG. 5b, can be seen in FIG. 5c, and constitutes a linear function. Therefore, the amount of compensation can clearly be varied by changing the depth of the tuner into the cavity. Since this can also change the operating frequency, the first tuning rod (16) must be kept relatively short so as not to affect the temperature compensation (i.e., its relative depth within the cavity (12) remains substantially the same over the expected temperature range).

To appropriately tune and compensate the oscillator, the second tuning rod can first be adjusted for compensation, and the oscillator then tuned back to the desired frequency with the first tuning rod (16).

Through provision of the above described invention (10) this microwave cavity oscillator can be tuned and compensated over the full bandwidth of the oscillator through relatively simple appropriate placement of the first and second tuning rods vis-a-vis the cavity (12). No parts need be replaced to make the oscillator compatible at a new frequency, and no expensive external feedback mechanisms need be provided. As a result, a single part can be utilized to replace what previously required an inventory of up to 48 different parts, thereby contributing significant cost savings to all relevant parties.

Referring now to FIG. 3, an alternative embodiment can be seen wherein the first tuning rod (16) also connects to a support shaft (61) that is affixed at one end to the housing (11). By this configuration, both the first tuning rod (16) and the second tuning rod (18) will move relative to the cavity (12) with temperature variations. In this way a greater compensation range can be made available.

With reference to FIG. 4a, yet another embodiment can be seen to comprise a device having only a single tuning rod (16), wherein the tuning rod (16) connects to a support shaft (61) as depicted in FIG. 3 and has an enlarged end (64) to provide a linear as versus nonlinear relationship as depicted in FIG. 4b. Although this particular embodiment will not provide the same high precision of temperature stability as the embodiments depicted above, substantial drift reduction does result and may be appropriate for use in certain applications.

Those skilled in the art will recognize that various modifications and changes could be made with respect to the invention without departing from the spirit and scope of the claims. It should therefore be understood that the claims are not to be considered as limited to the precise embodiments described in the absence of explicit limitations directed thereto.