Abstract
The cellular communication market is expanding dramatically, with bands near 1-2GHz being auctioned for use. The frequency spectral regions allocated for cellular communication are rather narrow, so they are certain to have a high density of communication channels. Thus, proper filtering of transmitting and receiving signals into the allocated bands, and precise channelization of the multiple simultaneous signals within each band, are essential for providing a high standard of customer service. HTS filters are expected to be used on circuit systems of the base station as the means to maximize frequency use. Bandpass filters are typically realized by appropriately coupling a set of resonant elements which may be in the form of three-dimensional cavities, bulk rods, or resonant structures. Though planar circuits like microstrip are especially attractive because of their compactness and low cost, even cooled copper metals are unable to achieve a Q value of even 1000. This low Q value is insufficient to support the low insertion loss and sharp filter skirts needed for selecting communication frequency bands. To receive circuit systems of the base station, a nine-pole, 3.7% bandwidth HTS filter designed by microstrip resonator configuration would have a dissipation-induced insertion loss of only 0.4dB including connectors at 898MHz and would have much better out-of-band rejection. However this HTS microstrip-type filter typically needs a single 2-inch wafer. The cooling system requires small-sized cooled components for long-life reliability. There have been some ideas devised for small-sized HTS filters. Filter circuits, one-third the previous size, have been successfully designed by using hair-pin circuits. In addition, a two-pole, 0.5% bandwidth HTS hair-pin-type filter has an insertion loss of less than 0.2dB including connectors loss at 5GHz. A more difficult challenge is to realize the transmission of HTS filters, for the filter must have a power-handling capability more than 10W. In cellular base station, combined different channel signals are combined and amplified to the required power level. A stringent linearity of the power amplifier is required for low intermodulation distortion. However such an amplifier would not be able to satisfy the intermodulation regulations outside the provider's licensed frequency band and it may reduce the spurious-free dynamic range of the base station receiver. HTS filters can improve this intermodulation problem by utilizing a sharp filter skirt and by conserving in-band power through low loss. This low-loss property makes the amplitude gain and size of the power amplifier small, and prevents energy loss caused by heating. Two typical filters were designed by using wide microstrip lines having low impedance and by an elliptic disk circuit pattern having two dipole resonating modes. For the HTS disk filter of two-pole Tchebyscheff type, 42dBm power transmission in 2% bandwidth and 0.3dB of insertion loss was confirmed at 5GHz.
