Such microwave filters are for example employed in wireless communication and may for example realize a bandpass or bandstop filter. In this regard, continuous growth in wireless communication in recent decades has caused more advanced, stricter requirements on filters and on other equipment in a communication system. In particular, filters with a narrow bandwidth, a low insertion loss and a high selectivity are required, wherein such filters must be operable in a wide temperature range. In general, filters must operate at low temperatures in cold environments as well as at elevated temperatures for example after warming of components of a communication system during operation.
To fulfill such requirements, typically microwave filters with a multiplicity of a resonant filter elements, in particular resonant filter cavities, electromagnetically coupled to each other are used. In such filters, in order to fulfill required specifications in an operational temperature range, a mechanism is required to stabilize a resonant frequency against a temperature drift. For this, a housing and a resonator element, for example a resonator rod, of a filter element may be made of materials with different coefficients of thermal expansion (CTE) in order to stabilize the resonant frequency of the entire filter. However, typically such resonant frequency temperature compensation is based on the assumption that all resonant filter elements of the filter resonate at the same frequency. This typically may not be true because as a result of filter synthesis each resonant filter element of a filter may resonate at a slightly different frequency. Consequently, different resonant filter elements may have a different resonant frequency drift caused by temperature variations, possibly resulting in a degradation of filter performance.
Recently proposed topologies called cul-de-sac having a minimum number of couplings for a given response and no diagonal couplings typically are even more temperature sensitive than conventional topologies and require a very precise temperature compensation to profit from their advantages.
There consequently is a need for a method to allow a fine temperature compensation at each single resonant filter element in order to compensate for assembly, mechanical and material tolerances. It in general can be assumed that a filter response can be considered as temperature compensated when all of its resonant filter elements are reasonably well temperature compensated.
Temperature compensated filters may for example employ materials with a low thermal expansion coefficient, for example so called Invar materials. Such materials however are costly. Another option is to combine different materials having suitable thermal expansion coefficients.
Cost-effective coaxial resonator cavities may for example employ a housing of an aluminum alloy comprising a resonator element and a tuning screw made of brass or steel. By computer simulation the dimensions of a resonant cavity may be determined so that the cavity is compensated against frequency drift at its nominal resonator dimensions, at the nominal values of the thermal expansion coefficient and at its nominal frequency. Due to production variances and mechanical and material tolerances, however, different resonant cavities may exhibit different resonant frequency temperature drifts deviating from the nominal resonant frequency temperature drift. This impacts the performance of the overall filter, leading to a degradation in filter performance.
In general, a temperature compensation of a single resonant filter element or of several separate resonant filter elements coupled to a main microwave line is simple and straightforward because the frequency drift of each resonant filter element caused by temperature changes is separated from other resonant filter elements, such that the effects of tuning can be clearly distinguished for the different resonant filter elements. However, more complicated situations occur when multiple resonant filter elements are crossed-coupled, in particular for cul-de-sac topologies in which it by means of currently known technics is practically impossible to distinguish a frequency drift of the particular resonant filter elements from the overall filter response.
The synthesis of microwave filters, in particular microwave cavity filters employing a cul-de-sac topology, is for example described in articles for example by Cameron et al. (“Synthesis of advanced microwave filters without diagonal cross-couplings”, IEEE Trans. MTT, Vol. 50, No. 12, December 2002), by Fathelbab (“Synthesis of cul-de-sac filter networks utilizing hybrid couplers”, IEEE Microwave and Wireless Components Letters, Vol. 17, No. 5, May 2007) and by Corrales et al. (“Microstrip dual-band bandpass filter based on the cul-de-sac topology”, Proceedings of the 40. European Microwave Conference, September 2010). In an article by Wang et al. (“Temperature compensation of combline resonators and filters”, IEEE MTT-S Digest, 1999) a method for temperature compensation of a resonator is modeled, the resonator comprising a tuning screw and a resonator rod being cylindrical in shape and being arranged in a cavity.
From U.S. Pat. No. 6,734,766 a microwave filter having a temperature compensating element is known. The microwave filter includes a housing wall structure, a filter lid, a resonator rod, a tuning screw and a temperature compensating element. The temperature compensating element is joined to the filter lid or the housing and forms a bimetallic composite with the filter lid or housing that deforms with a changed in ambient temperature.
From U.S. Pat. No. 5,233,319 a dielectric resonator is known which comprises two tuning screws, one of which is metallic and the other one of which is dielectric. The two tuning screws are movable with respect to a housing, wherein by moving the metallic tuning screw into the housing a resonant frequency of the resonator can be tuned up, whereas by moving the dielectric tuning screw into the housing a resonant frequency of the resonator may be lowered.