**6.2 Multiple dual-mode resonators**

**Figure 38** shows a six-port balanced SIW diplexer by using dual-mode resonators [55]. Because the electric field responds differently to each mode and the characteristics of differential signals, there is better isolation performance between two channels. The electric field distribution of the lower channel and the higher channel is shown in **Figure 39**. Additionally, differential input can enhance common-mode rejection and increase the stability of circuits.

**Figure 37.** *Topology of the proposed SIW CSRR diplexer.*

**Figure 38.** *Structure of the balanced diplexer.*

**Figure 39.** *Electric field distributions in (a) lower channel. (b) Higher channel [55].*

### **6.3 A common dual-mode and multiple single-mode resonators**

A wide passband SIW diplexer with a common half-mode dual-mode resonator and multiple single-mode resonators is shown in **Figure 40**. The first half-mode dual-mode resonator is used to flexibly allocate fractional bandwidths. The rest of the resonators are single-mode couplings, which almost be mapped independently. **Figure 41** shows the electric field distributions in this diplexer at 3.5 and 5.0 GHz, respectively. Different channels are dominated by different modes.

The schematic of a tunable SIW diplexer with various single-ended and balanced ports is shown in **Figure 42** [59]. Resonator A is a common dual-mode resonator, Resonator B and A form the lower channel, while Resonator C and resonator A form the upper channel. The bottom of each resonator is covered with a silver disk, which attaches to a piezo disk. Piezoelectric actuator is used to tune the operating passband. The single-end structure and balanced structure are both fabricated and have state-of-the-art performance.

**Table 5** shows the performance parameter of the diplexers following different technological approaches.

**Figure 40.** *Structure of the fabricated prototype [58].*

**Figure 41.** *Electric field distributions in the proposed SIW diplexer [58].*

**Figure 42.** *The schematic of the tunable diplexer [59].*


#### **Table 5.**

*Comparisons with different diplexers.*
