Microwave Filters for Communication Systems: Fundamentals, Design and Applications

CHAPTER 19

COMPUTER-AIDED DIAGNOSIS AND TUNING OF MICROWAVE FILTERS

As a result of manufacturing and material tolerances, filter tuning is an essential postproduction process. Traditionally, filters have been tuned manually by skilled technologists. The tuning process is not only time-consuming but also expensive, particularly for high-order narrowband filters with stringent requirements. It is a fact that almost all filters used for wireless base stations and satellite applications must be subjected to a postproduction tuning process. For example, in typical satellite applications, channel filters that operate at 4 GHz have very stringent in-band and out-of-band requirements with a design margin of 300 kHz, that is, <0.01%. Such a design margin is needed to accommodate the drift in filter frequency due to temperature, leaving almost a zero margin for design imperfections due to manufacturing tolerances. The specifications for wireless base station filters are as stringent as those for satellite applications.

The complexity of the tuning process depends on the technology that is employed and the filter configuration. For example, tuning is a must for narrowband dielectric resonator filters and narrowband planar filters, since the dielectric constant of the dielectric resonator or the substrate of planar structures can vary from batch to batch. For dielectric resonators with ε_r = 38, a deviation of ±0.5 in the dielectric constant at 4 GHz translates into a frequency shift close ...