Analysis and design

The structures of the presented dual-mode high-isolation diplexers have been shown in Fig. 1. The overall structures of the third-order and fourth-order circuits are shown in Figs 1(a) and 1(b), respectively. The structures of the circuits are based on the dual-mode SIW resonators, which have the advantages of high quality factor Q and low loss. It can be seen that the number of the required resonant cavities is reduced by half due to the use of dual-passband filters which are based on SIW dual-mode resonators. In addition, the common port of the diplexer is designed with a common resonant cavity structure, so the size of the circuit is more compact without the additional matching circuit.

Fig. 1.

Structures of the high-isolation diplexers. (a) Structure of the third-order diplexer and (b) structure of the fourth-order diplexer.

In order to realize the coupling of the two working modes between the resonant cavities, a coupling structure as shown in Fig. 1 is adopted. The coupling strength is controlled by the size of the coupling window S and the position of the coupling window Sa. The larger the coupling window, the stronger the coupling strength between the two resonators.

The circuit topology of the multi-order diplexer is shown in Fig. 2. R _L and R _H enclosed by dotted frames represent TE₁₀₂ mode and TE₂₀₁ mode of the dual-mode resonator respectively. Both R ₁ and R ₂ represent TE₁₀₁ modes, which are the dominant modes of the single-mode resonators connected at the end of the multi-order diplexer. The dotted lines in Fig. 2 indicate that many dual-mode resonators can be inserted to achieve multi-stage cascade. The use of a dual-mode resonator as the common cavity not only eliminates redundant resonators, but also eliminates additional matching circuits, which is helpful to achieve the miniaturization of the circuit.

Fig. 2.

The circuit topology of the multi-order diplexer.

The resonant frequencies f ₁₀₂ and f ₂₀₁ of the SIW rectangular resonator in TE₁₀₂ mode and TE₂₀₁ mode can be obtained by the following equations:

(1)$$f_{102} = \displaystyle{c \over {2\sqrt {\varepsilon _r\mu _r}}} \sqrt {{\left( {\displaystyle{1 \over {2L}}} \right)}^2 + {\left( {\displaystyle{1 \over W}} \right)}^2} $$

(2)$$f_{201} = \displaystyle{c \over {2\sqrt {\varepsilon _r\mu _r}}} \sqrt {{\left( {\displaystyle{1 \over L}} \right)}^2 + {\left( {\displaystyle{1 \over {2W}}} \right)}^2} $$

Here, L and W are the length and width of the rectangular resonator, respectively.

Figure 3 shows the electric field distributions of the fourth-order diplexer. It can be seen that the electric field distributions of the two paths are orthogonal and there is no direct coupling with each other. Because the two operating modes are not directly coupled in this circuit, the mutual interference between the two passbands is small and there is superior isolation of the diplexer.

Fig. 3.

Electric field distributions of the fourth-order diplexer. (a) Electric field distribution of the low-passband situation and (b) electric field distribution of the high-passband situation.

In order to further separate the two resonant frequencies of the dual-mode resonator at the output ports and improve the isolation between the output ports, two single-mode resonators are connected at the output ports, which are operating in the dominant mode TE₁₀₁.

In order to get superior isolation and better out-of-band rejection of the proposed diplexer, it is necessary to increase the number of the order of the circuit.

Implementation and measurements

Through the above analyses, the two high-isolation dual-mode diplexers are designed and fabricated with the substrate Taconic RF-35. The related parameters of this substrate are as follows: dielectric constant ε _r of 3.5, a thickness of 0.508 mm and a loss tangent of 0.0018. The structures are optimized in Ansys-HFSS. Table 1 shows the final physical sizes of the third-order diplexer circuit. The final physical sizes of the fourth-order diplexer are shown in Table 2. Figure 4 shows the fabricated dual-mode high-isolation diplexers. What's more, all the ports are connected by the type-SubMiniature version A connectors.

Table 1.

The final physical sizes of the third-order diplexer (unit: mm)

Table 2.

The final physical sizes of the fourth-order diplexer (unit: mm)

Fig. 4.

Physical photos of the diplexers. (a) Third-order diplexer and (b) fourth-order diplexer.

The simulated and measured results of the third-order diplexer are given in Fig. 5. The measured center frequencies of the passbands and 3 dB relative bandwidths are 8.9 GHz/2.8% and 9.42 GHz/3.5%, respectively. The measured return losses of the low passband and high passband are less than 18 and 20 dB, respectively, and the insertion losses of the two passbands are 4 and 3.1 dB, respectively. The measured isolation of the entire frequency band is better than 35 dB.

Fig. 5.

Simulated and measured results of the fabricated third-order diplexer. (a) Transmission and reflection characteristics and (b) isolation.

Figures 6(a) and 6(b) show the transmission, reflection, and isolation characteristics of the fourth-order diplexer. The measured center frequencies of the passbands and 3 dB relative bandwidths are 8.75 GHz/2.2% and 9.4 GHz/3.2%, respectively. The measured return losses of the low passband and high passband are less than 16.8 and 14 dB, respectively, and the insertion losses of the two passbands are 4.9 and 3.9 dB, respectively. The measured isolation of the entire frequency band is better than 42 dB. It can be seen that in the operating frequency band, the measured results agree well with the simulated ones, verifying the accuracy of the above design.

Fig. 6.

Simulated and measured results of the fabricated fourth-order diplexer. (a) Transmission and reflection characteristics and (b) isolation.

It can be seen from the simulated results that as the number of the order of the diplexer increases, the measured out-of-band rejection and isolation of the diplexer increase. However, as the number of the order of the diplexer increases, the size and the insertion loss of the circuit also increase. Therefore, it is necessary to make a trade-off based on the required performance when designing the circuit in practice. Table 3 shows the comparison with some prior diplexers. It can be seen that the proposed diplexers have the advantages of high isolation, excellent out-of-band rejection, and good impedance matching.

Table 3.

The comparison with some prior diplexers