Low-loss and tunable millimeter-wave filters using spherical dielectric resonators

Millimeter-wave band-pass filters using spherical dielectric resonators are presented. The dielectric spheres are sandwiched between metal plates and are excited by a simple microstrip line structure on a thin-film circuit board. As such, these filters could also be implemented in the back-end-of-line layers of an integrated circuit. A single resonator, based on a diameter 0.6 mm alumina ceramic sphere, is shown to resonate with high unloaded Q-factor of 750 at 170 GHz. A three-sphere band-pass filter is measured showing <5 dB insertion loss and 0.4% bandwidth at 170 GHz. A concept for mechanically tuning of a two-sphere band-pass filter is demonstrated for a filter operating around 105 GHz. The measured filter shows approximately 5 dB insertion loss and <0.5% bandwidth and its passband can be varied over 3 GHz of frequency, or 3%. Technological challenges are discussed.