Design and Fabrication of Compact HTS Duplexers Using a CQ Structure With a High $Q_{u}$ Resonator

We designed a compact high-temperature superconductor (HTS) duplexer on a substrate. The duplexer consists of a T-junction with discontinuous impedance and two cascaded quadruplet (CQ) filters with a bandwidth of 20 MHz centered at 5.0 and 5.1 GHz, respectively. The unloaded quality factor Q
_u
of the proposed resonator used for the CQ filter was about 98000, and the smallest line and gap width were 10 μm. We applied a CQ structure to the bandpass filter with one pair of transmission zeros. The isolation of the HTS duplexer could be designed at less than 35 dB. We fabricated the HTS duplexer by using a 300-nm single-sided YBCO film deposited on a sapphire substrate 10 mm × 10 mm in size. We measured the frequency response of the duplexer between 4.8 and 5.15 GHz. We found a separation of the two passbands and sharp out-of-band rejection with the pair of transmission zeros. The isolation between the two filters was better than 32 dB. However, it was necessary to adjust the measured center frequency shifts about 150 MHz to the low-frequency side from the design results. Through this method, we were able to fabricate a miniaturized HTS duplexer with high selectivity.