Transmit filters for wireless basestations

Transmit filters for PCS basestation applications will be required to handle peak power levels up to hundreds of watts. To achieve this goal, filters based on high temperature superconductor patch resonators have been investigated. Among these, circular resonators operating in the TM/sub 010/ mode, due to its radial current flow, are particularly promising. However, the large size of these resonators places limits on the frequency and number of poles of planar filters. As an alternative, we have designed and tested 3-pole filters based on the vertical coupling of stripline circular resonators. For operation in the PCS frequency range each resonator was fabricated from two double-sided YBCO films deposited on 2-in. lanthanum aluminate substrates. The measured Q of these resonators at 50 K and 2 GHz was higher than 100,000 and is limited by the dielectric losses of the substrate. The measured circulating power of above 1 MW at 50 K extrapolates to power handling, for a 9-pole, 1% bandwidth filter, of more than 1 kW. A 1.2% bandwidth, 3-pole filter based on this design has been characterized, and does not show any power dependence up to 72 W, the limit of our test setup.