DESIGNING THE SIW DUAL-MODE DIPLEXER
The preceding analysis has shown that two kinds of SIW dual-mode resonators can be utilized to design the channel filters of the diplexer. Figure 5 shows the proposed structure of this circuit.
Figure 6 shows the simulation results of the diplexer realized with two kinds of coupling circuits at a center frequency of 5 GHz and 5.25 GHz. Capacitive perturbation is adopted for the lower-channel filter at 5 GHz, while the magnetic coupling with the NRN is adopted for the upper-channel filter at 5.25 GHz. The dimension of the upper-channel filter is slightly smaller than that of the lower-channel filter. The relative bandwidths of the channel filters are 1.96 percent and 2.06 percent, respectively. The SIW dual-mode channel filters have high-frequency selectivity due to the transmission zeros on each side of the channel
Zero-degree feeding-line technology is adopted in the diplexer to reduce the circuit size further and improve the isolation. The design of the T-junction connected to the two channel filters requires specifying only the ta and tb lengths, as shown in Figure 5. The length of the proposed microstrip line T-junction is much less than a conventional SIW T-junction. This allows the proposed microstrip line T-junction to be viewed as a zero-degree feeding line. This allows the proposed diplexer to be more compact with lower design complexity. The input and output feeding-line impedance is set to 50 Ohms. If the port1 GD(1,1) group delay meets the conditions of Equation 4, the channel filters can be designed independently, where CH1_GD(1, 1) and CH2_GD(1, 1) are the group delays of the two passbands. The circuit sizes are optimized to meet Equation 4, simplifying the diplexer and channel filter designs.
The design process of the diplexer can be summarized as follows:
- Determine the necessary quality factor and coupling coefficient of the two desired passbands based on the parameters of the circuit design
- Using these values, design the two channel filters of the diplexer and the T-junction
- Combine the channel filters with the microstrip T-junction into a diplexer
- Optimize the T-junction and the diplexer sizes for the desired results
RESULTS AND DISCUSSION
The proposed SIW dual-mode diplexer was optimized in ANSYS HFSS and verified by experiments. The optimized dimensions of the diplexer shown in Figure 5 are shown in Table 1.
Figure 7 shows the SIW dual-mode diplexer fabricated using standard single-layer PCB fabrication processes. The substrate uses Taconic RF-35 with a permittivity of 3.5, a thickness of 0.508 mm and a loss tangent of 0.0018. The diplexer is 36 mm × 72 mm or λg × 2 λg, where λg is the guided wavelength at the center frequency of the first passband. Figure 8a shows the simulated and measured insertion loss and return loss results of the SIW dual-mode diplexer. Figure 8b shows the measured versus modeled isolation results for the diplexer. The measured center frequencies are 5 GHz and 5.25 GHz and the relative bandwidth of the two channels are 1.96 percent and 2.06 percent. Both channels are well matched, with the measured return loss greater than 20 dB. The measured insertion loss of the two channels are 2.01 dB and 2.13 dB, respectively and the measured isolation is greater than 45 dB in both channels. While there is good agreement between the simulated and measured results, errors can be attributed to the fabrication and measurement process and the losses of the SMA connectors.
Table 2 compares the proposed diplexers and reported results. Compared with the references, the diplexer described in this article is smaller, has higher isolation between the two channels and has lower insertion loss. In addition, two transmission zeros near the two sides of each passband have greatly improved the frequency selectivity of the diplexer presented in this article.
Table 2.CONCLUSION
This article has described a compact, high-isolation SIW diplexer with high-frequency selectivity. The electromagnetic perturbation and magnetic coupling between the NRN and the even mode have introduced two transmission zeros on both sides of each passband. This technique improves the frequency selectivity of each channel of the duplexer and the isolation between the two channels. The diplexer has been designed, fabricated and measured with good agreement between the simulated and measured results.
ACKNOWLEDGEMENT
This work was partly supported by the National Natural Science Foundation of China (Grant No: 62171097).
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