RF cavity resonator with low passive inter-modulation tuning element

A tunable RF cavity resonator having reduced generation of passive inter-modulation, the tunable RF cavity resonator including a cavity body, a fixed tuning post at a first end of the cavity body, a dielectric rod in the fixed tuning post interior space with a first spatial gap between the dielectric rod outer surface and the fixed tuning post inner wall, a tuning screw connected to a first end of the dielectric rod, and a tuning element disposed on a second end of the dielectric rod so that the tuning element moves within the interior space of the fixed tuning post while maintaining a second spatial gap between the tuning element outer surface and the fixed tuning post inner wall. Accordingly, the tuning element forms a short section of very low RF impedance transmission line thereby preventing RF signal leakage through the interior space of the fixed tuning post.

NOT APPLICABLE TO THIS INVENTION.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns a re-entrant radio frequency (RF) cavity resonator filter which achieves a low level of generated passive inter-modulation (PIM) by having a tuning element which does not have contact with the fixed tuning post of the re-entrant cavity resonator.

2. Description of the Related Art

Conventional RF cavity resonators are typically tunable so that the resonator may be used across a given range of operating frequencies. Such a conventional RF resonator typically consists of a cavity body, either cylindrical or rectangular, in which a fixed tuning (resonator) post resides inside of the cavity body by being connected at one end to a wall of the cavity body. The fixed tuning post of the typical resonator is generally a tubular, cylindrical shape with a far end opposite the connection to the cavity body being open. The typical fixed tuning post is usually machined as an integral part of the cavity body, and the inside tube of the typical fixed tuning post accommodates a rotatable post that is also cylindrical in nature and that has a metallic tuning element attached at one end near the open end of the fixed tuning post. The metallic tuning element typically moves relative to the fixed tuning post by having screw-type threads on the outer surface of the metallic tuning element which correspond to screw-type threads provided on the inner surface of the inside tube of the fixed tuning post.

The end of the rotatable post which is opposite the metallic tuning element is usually attached to an adjusting knob which protrudes out of the cavity body near the cavity body wall to which the fixed tuning post is connected. Accordingly, the user of a typical RF cavity resonator can adjust the operating frequency of the resonator by turning the adjusting knob which turns the threaded metallic tuning post at the end of the rotatable post within the threads on the inside of the fixed tuning post, thereby linearly moving the metallic tuning element within the fixed tuning post in a controlled manner. As the metallic tuning element is moved relative to the open end of the fixed tuning post, the operating frequency of the cavity resonator is “tuned” to a new frequency.

Unfortunately, although the above-described typical RF cavity resonator achieves the goal of tuning the cavity resonator to a different frequency, there are certain deleterious effects caused by the use of the “screw-type” metallic tuning post mentioned above. Specifically, the metal-to-metal contact between the threaded outer surface of the metallic tuning element and the threaded inner surface of the fixed tuning post causes the generation of passive inter-modulation (PIM) during operation of the cavity resonator. The generated PIM can result in unwanted distortion of the outgoing signal from the cavity resonator.

In addition, repeated tuning of the typical RF cavity resonator can cause the threads on the outer surface of the metallic tuning element and the threads on the inner surface of the fixed tuning post to wear and deteriorate. When the threads become worn and deteriorated, they become mismatched and thereby result in unstable positioning of the metallic tuning element within the cavity body of the cavity resonator. This results in an inability to accurately move the metallic tuning to a specific location, thereby preventing accurate tuning of the cavity resonator in a repeatable and uniform fashion.

Accordingly, it is desirable to have a RF cavity resonator which reduces the amount of PIM generated by contact between the metallic tuning element and the inside of the fixed tuning post. In addition, it is desirable to have a cavity resonator which can be repeatedly tuned in an accurate, repeatable and uniform fashion.

SUMMARY OF THE INVENTION

The present invention solves the above problems by providing a re-entrant RF cavity resonator filter with a metallic tuning element that has no contact with the fixed tuning post of the re-entrant cavity resonator, thereby reducing levels of generated passive inter-modulation, and providing the ability to be tuned in an accurate, repeatable and uniform fashion.

Specifically, according to one aspect of the invention, a tunable RF cavity resonator is provided which has reduced generation of passive inter-modulation. The tunable RF cavity resonator includes a cavity body, a fixed tuning post which is fixed to a first end of the cavity body, the fixed tuning post having an outer wall forming an exterior surface of the fixed tuning post, and having an inner wall forming an interior space within the fixed tuning post, a dielectric rod disposed in the interior space of the fixed tuning post such that a first spatial gap is maintained between an outer surface of the dielectric rod and the inner wall of the fixed tuning post, a tuning screw connected to a first end of the dielectric rod near the first end of the cavity body, and a tuning element disposed on a second end of the dielectric rod opposite the first end of the dielectric rod, and being disposed to move within the interior space of the fixed tuning post such that a second spatial gap is maintained between an outer surface of the tuning element and the inner wall of the fixed tuning post.

Preferably, the fixed tuning post has a cylindrical tube shape with one open end inside of the body cavity, and the dielectric rod has a cylindrical shape. In this regard, the fixed tuning post has a length equal to either 60 degrees or 90 degrees of the center frequency of the cavity resonator. The tuning screw is preferably threaded and connected to a threaded section provided at the first end of the dielectric rod. The second spatial gap between the outer surface of the tuning element and the inner wall of the fixed tuning post is preferably less than 0.005 inches to ensure that the high power multipactor breakdown of the signal is below a cut-off threshold, while still maintaining contact-free tuning movements to prevent passive inter-modulation. The cavity resonator is preferably electrically connected so that the first end of the fixed tuning post is short-circuited (maximum current), and the second end of the fixed tuning post is open-circuited (maximum voltage).

In this manner, the present invention provides a tunable RF cavity resonator that includes a metallic tuning element which has no metal-to-metal contact with the fixed tuning post of the cavity resonator, thereby minimizing levels of generated passive inter-modulation, and that has the ability to be repeatedly tuned in an accurate and uniform fashion.

This brief summary has been provided so that the nature of the invention may be understood quickly. A more complete understanding of the invention can be obtained by reference to the following detailed description thereof in connection with the attached drawings.

DETAILED DESCRIPTION

The present invention is generally directed to a re-entrant cavity resonator filter which achieves a low level of generated passive inter-modulation (PIM) by having a tuning element which has no metal-to-metal contact with the fixed tuning post of the re-entrant cavity resonator. Also, the cavity resonator of the present invention provides the ability to be tuned in an accurate, repeatable and uniform fashion.

Turning to the drawings,FIG. 1Ashows a side view of a tunable RF cavity resonator according to the present invention. As seen inFIG. 1A, tunable RF cavity resonator10is shown which is comprised of several components. Specifically, tunable RF cavity resonator10includes cavity body11, fixed tuning post12, dielectric rod14, tuning element15and tuning screw13. Cavity body11is preferably either a rectangular shape, such as a square, or a cylindrical shape, and has a cavity area disposed inside. Fixed tuning post12is disposed inside the cavity area of cavity body11and is affixed to the first end of cavity body11so that fixed tuning post12is rigidly held in place with respect to cavity body11. Fixed tuning post12is made of a conductive metal and has a cylindrical, tube shape with an open end opposite of the end that is affixed to cavity body11.

Fixed tuning post12acts as a resonator within cavity body11and preferably has a length between sixty (60) to ninety (90) degrees of a frequency which is at the center of the operating frequency bandwidth of cavity resonator10. In other words, fixed tuning post12preferably has a length between ⅙ to ¼ the wavelength of the center operating frequency of cavity resonator10. Preferably, fixed tuning post12is electrically connected through cavity resonator10to be open-circuited (maximum voltage) at the open end, and short-circuited (maximum current) at the affixed end.

Dielectric rod14is a cylindrical shape and is disposed inside of fixed tuning post12such that spatial gap17is maintained between the outer surface of dielectric rod14and the inner wall of fixed tuning post12. In this regard, dielectric rod14is made of a dielectric material and functions to support tuning element15. In addition, dielectric rod14makes the inside of fixed tuning post12behave like a circular waveguide below the cutoff, thereby preventing propagation of radio frequency signals inside fixed tuning post12. As seen inFIG. 1A, dielectric rod14is positioned within the inner tube of fixed tuning post12, is attached at one end to cavity body11by means of tuning screw13, and has tuning element15attached to its other end near the open end of fixed tuning post12. Rotation of tuning screw13moves dielectric rod14in a linear direction parallel to the longitudinal axis of fixed tuning post12, thereby moving tuning element15in a linear direction toward or away from the open end of fixed tuning post12.

The outer surface of tuning element15in the present invention maintains spatial gap16with respect to the inner wall of fixed tuning post12, thereby avoiding any metal-to-metal contact which is commonly associated with conventional cavity resonators that have a threaded tuning element in contact with a threaded inner wall of the fixed tuning post. According to the foregoing arrangement, a user can tune cavity resonator10to a particular center operating frequency by rotating tuning screw13in the appropriate direction thereby moving tuning element15to the appropriate position with respect to fixed tuning post12. Also, tuning element15forms a short section of very low RF impedance transmission line thereby preventing RF signal leakage through the interior space of fixed tuning post12.

FIG. 1Bis a top view of cavity resonator10shown inFIG. 1A. Although tuning screw13is shown at the bottom end of cavity resonator10inFIG. 1A, cavity resonator10is typically positioned in operation in an inverted position with respect toFIG. 1Aso that tuning screw13is shown at the top ofFIG. 1B. In this manner, the user of cavity resonator10can conveniently access tuning screw13to adjust the center operating frequency of cavity resonator10.

FIG. 2is a more detailed diagram of RF cavity resonator10. As seen inFIG. 2, cavity body11is shown in more detail than inFIG. 1A. An explanation of the various elements of cavity resonator10shown inFIG. 2has been previously provided above with respect toFIG. 1Aand will therefore not be repeated here for the sake of brevity. For example, cavity body11, fixed tuning post12, dielectric rod14, tuning element15and tuning screw13are generally arranged and function as described above with respect toFIG. 1A. Accordingly, only those additional elements and more detailed features shown inFIG. 2will be further explained. In this regard, loop contacts25and26can be seen on the end of cavity body11so that RF cavity resonator10can be electrically connected to a signal source for use as a filter, etc. Preferably, RF cavity resonator10is electrically connected so that the first end of fixed tuning post12is electrically short-circuited with maximum current, and the second end of the fixed tuning post12is electrically open-circuited with maximum voltage.

FIG. 2shows that dielectric rod14is positioned within fixed tuning post12, and that dielectric rod14supports tuning element15at one end near the opening of fixed tuning post12. In addition, dielectric rod14is seen to maintain spatial gap17with respect to the inner wall of fixed tuning post12, and tuning element15is seen to maintain spatial gap16with respect to the inner wall of fixed tuning post12. Preferably, spatial gap16is less than 0.005 in order to prevent multipaction breakdown while also avoiding metal-to-metal contact between fixed tuning post12and tuning element15. Also, in the embodiment shown inFIG. 2, spatial gap17between dielectric rod14and fixed tuning post12is greater than spatial gap16between tuning element15and fixed tuning post12.

The tuning mechanism which allows the user to move dielectric rod14, and therefore tuning element15, is now explained in more detail with respect toFIG. 2. In particular, it can be seen inFIG. 2that dielectric rod14has threaded notch section24at then end in which dielectric rod14passes through cavity body11. Tuning screw13is disposed within retainer section27which is attached to the end of cavity body11at which dielectric rod14protrudes. Tuning screw13also has threaded section23which corresponds to threaded notch section24of dielectric rod14. Accordingly, dielectric rod14and tuning screw13are threaded together. Although tuning screw13is rotatable, tuning screw13is maintained in the same linear position with respect to cavity body11because tuning screw13is held in place by the grooves shown in retainer section27.

In this manner, when tuning screw13is rotated, threaded section23of tuning screw13engage threaded notch section24of dielectric rod14, and since tuning screw13is restrained from linear movement by retainer section27, dielectric rod14is moved in a linear direction as tuning screw13is rotated. As dielectric rod14is moved in a linear direction, so is tuning element15moved at the end of dielectric rod14, thereby tuning cavity resonator10to a particular center operating frequency. Preferably, the threads of threaded section23and threaded notch section24are formed so that dielectric rod14and tuning element15are movably translated in a linear direction away from the first end of cavity body11when tuning screw13is rotated in a counterclockwise direction, and so that dielectric rod14and tuning element15are movably translated in a linear direction toward the first end of cavity body11when tuning screw13is rotated in a clockwise direction. Of course, other arrangements of threads will work with the present invention.

According to the foregoing arrangement, RF cavity resonator10can be repeatedly tuned to a center operating frequency by rotating tuning screw13without having any metal-to-metal contact between tuning element15and fixed tuning post12, thereby avoiding generation of passive inter-modulation. In addition, the absence of threaded metal-to-metal contact between tuning element15and fixed tuning post12allows for repeated uniform tuning, unlike conventional threaded-type tuning elements in which the threads eventually wear down, thereby resulting in imprecise positioning of the tuning element.

FIG. 3depicts another embodiment of the invention in which multiple RF cavity resonators10are arranged together in a pattern to form a diplexer for transmission or reception of multiple simultaneous, independent signals on two different frequencies. Preferably, the diplexer ofFIG. 3has a generated passive inter-modulation level of less than −140 dBm. Of course, other arrangements of RF cavity resonators10can be used for a diplexer, and for other functions such as filters, etc.

FIGS. 4A and 4Bdemonstrate the improvement of the present contact-less tuning element RF cavity resonator over the conventional thread-type RF cavity resonator. Specifically, turning toFIG. 4A, graph50is provided which shows the amount of generated passive inter-modulation (PIM) of a conventional RF cavity resonator having a thread-type tuning element over a series of four temperature cycles. Left axis52indicates the measured amount of PIM in dBm, and right axis53indicates the temperature in degrees Centigrade. Bottom axis51indicates the passage of time. As seen by graphed PIM55, the PIM level of the conventional thread-type RF cavity resonator is significantly high with PIM levels up to −98 dbm, with respect to graphed temperature cycles54. As seen below with respect toFIG. 4B, the PIM level associated with the RF cavity resonator of the present invention is much lower than the conventional RF cavity resonator depicted inFIG. 4A.

Turning toFIG. 4B, graph40is provided which shows the amount of generated passive inter-modulation (PIM) of RF cavity resonator10of the present invention having a contact-free tuning element over a series of four temperature cycles. Left axis42indicates the measured amount of PIM in dBm, and right axis43indicates the temperature in degrees Centigrade. Bottom axis41indicates the passage of time. As seen by graphed PIM45, the PIM level of RF cavity resonator10of the present invention is maintained at a low level of less than −140 dbm during the four graphed temperature cycles44.

Accordingly, the arrangement of the present invention results in a tunable RF cavity resonator that includes a metallic tuning element which has no metal-to-metal contact with the fixed tuning post of the cavity resonator, thereby minimizing levels of generated passive inter-modulation, and thereby providing the ability to be repeatedly tuned in an accurate and uniform fashion.

The invention has been described with respect to particular illustrative embodiments. It is to be understood that the invention is not limited to the above-described embodiments and that various changes and modifications may be made by those of ordinary skill in the art without departing from the spirit and scope of the invention.