Microwave filter having a temperature compensating element

#1. Field of the Invention

#This invention relates generally to the field of electronic filters. More particularly, the present invention provides a microwave filter having a temperature compensating element.

#2. Description of the Related Art

#FIG. 1 is a cross-sectional view of a known coaxial microwave filter;

#FIG. 2 is a cross-sectional view of a microwave filter having a temperature compensating element joined to an inside surface of the filter lid;

#FIG. 2A is an alternative embodiment of the microwave filter shown in FIG. 2 in which the temperature compensating element has a cylindrical neck portion with a screw-threaded bore for receiving the tuning screw;

#Referring now to the remaining drawing figures, FIG. 2 is a cross-sectional view of a microwave filter 30 having a temperature compensating element 32 joined to an inside surface of a filter lid 14. The filter 30 includes a housing wall structure 12, a cavity 13, the filter lid 14, a resonator rod 16, and a tuning screw 18. In addition, the filter includes the temperature compensating element 32 fixedly joined to the inner surface of the filter lid 14. Operationally, the temperature compensating element 32 causes the filter lid 14 to bow outward as the filter temperature increases, creating an equal (or substantially equal) but opposite frequency drift as that caused by the thermal expansion of the housing 12, 14. In this manner, the frequency drift caused by the temperature compensating element 32 counteracts the frequency drift caused by the thermal expansion of the housing 12, 14, thus stabilizing the filter 30.

#The housing wall structure 12 preferably includes four external walls 34, and a plurality of internal walls 36 that define a plurality of cavities 13 within the housing wall structure 12. The cavities 13 are preferably covered by the filter lid 14 which is fixedly mounted to the top of the housing wall structure 12. The cavities 13 are preferably interconnected in an array by openings or irises (not shown) within the internal walls 36 of the housing wall structure 12 in order to form a continuous path between an input port (not shown) and an output port 38

#The resonator rod 16 projects upward from a bottom wall 15 of the housing wall structure 12, preferably with one resonator rod 16 at the center of each cavity 13. The tuning screw 18 is adjustably mounted through the filter lid 14 opposite the resonator rod 16, and is received in a bore 19 in the top of the resonator rod 16. Preferably, the tuning screw 18 mates with a screw-thread in a bore extending through the filter lid 14 along an axis 21, and may be adjusted to a desired depth within the bore 19. In a preferred embodiment, the filter 30 includes a tuning screw 18 corresponding to each resonator rod 16, but in other embodiments some resonator rods 16 could have a fixed resonant frequency.