| Summary: | Adopting the deformed stepped impedance micro-strip line structure resonators (SIRs), the external weak coupling hairpin SIR (HSIR) and external strong coupling opening SIR (OSIR) resonators are designed, respectively. These two types of resonators are convenient for creating an ultra-narrow band and broadband filters, respectively, and can broaden the second harmonic passband. A high isolation diplexer without an impedance matching structure is realized by cascading 12-order and 14-order HTS filters composed of OSIR and HSIR resonators with a high isolation T-shaped structure. The diplexer is fabricated on a thin YBCO/MgO/YBCO (a MgO wafer with YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7</sub> thin film deposited on both sides) film with a dimension of 31.85 mm × 17.28 mm, a thickness of 0.5 mm, and a dielectric constant of 9.8. At 77 K, the measured central frequency of the diplexer is 2395 MHz and 3300 MHz, the fractional bandwidths (FBW) are 1.25% and 24.24%, the out-of-band rejections are greater than 60 dB/MHz, the 2<i>f</i><sub>0</sub> are located at 5.1 GHz and 6.6 GHz, the insertion loss is less than 0.20 dB, and the return losses are better than 16 dB and 15 dB. The diplexer has the advantages of a simple design method, compact structure, low insertion loss, high sideband rejection, and high isolation.
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