**7.3 Filtering crossover**

In the traditional filter crossover circuit with five cavities, to further reduce the circuit size, the four peripheral dual-mode cavities are changed to single-mode resonators whose fundamental mode is TE101 [63, 64], and the cavity arrangement is different, but the signal filtering selectivity of the crossover is enhanced in [64]. By increasing the number of cavities, and flexibly arranging single-mode and dual-mode resonator cavity arrangements, a variety of filter crossovers with different center frequencies and bandwidths are realized [65]. The circuit only retains one dual-mode resonator, and four CPW half-wave resonators are cascaded at the four ports, which further reduces the circuit size [66]. In addition, square SIW cavities with TE102 and TE201 modes are used as building blocks, and multi-channel filtering crossover is designed and fabricated by rationally arranging coupling ports and feed ports [67].

**Figure 46** shows the electric field distribution of a filter crossover circuit with a single resonator. The circuit is based on a four-mode resonant SIW rectangular reso-

#### **Figure 46.**

*Electric field distribution in the crossover: (a) TE103 mode, (b) TE104 mode, (c) TE201 mode, (d) TE202 mode [68].*

nant cavity, and the middle through holes and slots are used to adjust the electric field distribution. Compared with the above-mentioned circuits with multiple resonant cavities, on the one hand, the structure only uses a single resonant cavity, which further reduces the size, and on the other hand, the operating frequencies of the two channels in the above structure are the same, but the center frequencies of the two channels in this structure are different, which realizes the dual-frequency crossover of the two-channel signals and expands the application range.

In [69], the circuit structure is still in the traditional cross form, but the SIW evanescent-mode (EVA) resonator is applied. The coupling coefficient between the resonators is controlled by diodes, which makes the coupling coefficient of the adjacent cavities 0, and the diagonal cavities are not 0, thus completing the isolation of two signals. Piezoelectric actuators are utilized to form a tunable filtering channel, and the application range of the crossover is expanded.

### **7.4 Crossover with balanced structure**

The above-mentioned crossover circuits are all used for single-ended circuits, but considering the reduction of signal noise interference, the crossover circuits applied to balanced circuits are also studied. The multi-layer circuit topology is the main structure.

Based on the traditional filter crossover circuit, five SIW cavities are cascaded as shown in **Figure 47**. The CM is suppressed in this structure. Under differential mode (DM), the crossover part is realized by orthogonal modes TE102 and TE201.

Another balanced structure is shown in **Figure 48**. The 1–3 channels and the 2–4 channels are arranged in different layers, and the signals are transmitted through the middle slots. TE202 and TE204 in a single rectangular cavity are orthogonal to each other to complete the cross isolation of the two signals.

**Table 6** shows the performance parameter of the crossovers following different technological approaches.

**Figure 47.** *The crossover circuit layout [70]. (a) 3D figure; (b) Planar graph.*
