Miniaturized High Isolation Microstrip Diplexer

Microwave diplexer is an essential component to make the modern multi-band multi-service transceiver system have the ability to use a common antenna, and its property will affect the performance of overall RF system directly. The modern wireless system requires the diplexer have the characteristic of light weight, compact size, deep stopband rejection and high isolation. Due to its simple topology and tunable electrical property, the stepped impedance resonator (SIR) has been widely used in the design of microwave components. Through studying the characteristic of the conventional SIR, two improved SIRs are proposed in this paper, one using spiral compact microstrip resonant cell (SCMRC) instead of the low impedance section of SIR and the other with slotted line in the low impedance section of SIR. These two improved SIRs have much better performance than the conventional SIR. The main work of this paper includes two sections:By using the slow-wave structure SCMRC instead of the low impedance section of the conventional SIR, a novel structure called SCMRC-SIR is proposed. Compared to the conventional one, the SCMRC-SIR has a lower fundamental resonant frequency and has the ability to generate a finite transmission zero above its fundamental resonant frequency. Based on the proposed SCMRC-SIR, two fourth-order chebychev response bandpass filter are designed. Two filters operate at the GSM uplink band and GPS bands, and have a same fractional bandwidth of 8%. The simulated results show that two filters have compact size, sharp transition band and deep stopband rejection. By using a T-junction to connect these two compact and high performance bandpass filters, a microstrip diplexer is built. The fabricated diplexer has a compact size of 0.12λg×0.21λg. The measured results show that the fabricated filter has a high isolation of -50dB at 900MHz and -55dB at 1570MHz.By etching slotted lines in the low impedance section of the conventional SIR, a novel structure called SSIR is proposed. Compared to the abovementioned SCMRC-SIR, the SSIR has a compacter size and better spurious frequency rejection. Based on the proposed SSIR, two chebychev response bandpass filter with the same fractional bandwidth of 10% and operating at GSM uplink band and GPS band are designed. These two filters have the advantage of compact size, sharp transition band and deep stopband rejection. By using a T-junction to connect these two compact and high performance bandpass filters, a high performance diplexer is designed, simulated and fabricated. Compared to the diplexer utilizing SCMRC-SIR, the diplexer based on SSIR has a compacter size (0.11λg×0.14λg) and higher isolation (-61dB at 900MHz and-57dB at 1570MHz). Furthermore, as the author known, this diplexer has the smallest circuit area in the case of realizing about the same isolation compared to the reported ones.