Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 16

DIELECTRIC RESONATOR FILTERS

The term “dielectric resonator” was first reported in 1939 by R. D. Richtmyer [1] of Stanford University, who showed that dielectric objects can function as microwave resonators. It was not, however, before the 1960s that several pioneering works were published on the behavior of dielectrics at microwave frequencies, including the first reports on microwave dielectric resonator filters by Cohn in 1965 [2,3] and by Harrison in 1968 [4]. However, such filters were not implemented in practical applications because of the poor thermal stability of the dielectric resonator materials available at that time. The dielectric constant of these materials exhibited a significant change in their values with temperature Variation. This in turn caused the temperature drift of the filter center frequency to be as large as 500 ppm/°C.

Advances in the 1970s and 1980s in dielectric materials made it possible to combine high unloaded Q, high dielectric constants, and a small temperature drift in materials suitable for use at microwave frequencies. High-Q dielectric materials with dielectric constants ranging from 20 to 90, and a temperature drift from −6 to +6 ppm/°C, are now commercially available from various manufacturers. Dielectric resonators with ε_r = 29 are commercially available with quality factor-frequency product Q × f values of 90,000; thus, an unloaded Q value of more than 50,000 can be achieved at 1.8 GHz. As the dielectric constant increases, ...