Filters have a wide range of applications in wireless communications, military, and technology. The development of microwave and millimeter wave circuit technology requires that these filters should have low insertion loss, compact structure, small size, light weight and low cost. Rectangular waveguides and microstrip lines traditionally used as filters have been difficult to meet this requirement. The substrate integrated waveguide (SIW) technology provides a good choice for designing such filters.
SIW's dual-membrane resonators have a pair of degenerate modes that can be separated by adding a perturbation element to the resonator. Therefore, the perturbed resonator can be viewed as a double-tuned circuit. After the decoupled degenerate mode is coupled, two poles and one zero are generated. Therefore, the dual membrane filter also increases the stopband attenuation while reducing the size. It also enables a narrower percentage bandwidth. However, the dual membrane filter has the disadvantages of high power loss and large insertion loss. To this end, this paper proposes a new design method for SIW cavity double membrane filter.
The high power capacity and low insertion loss characteristics of the SIW can compensate for the inherent shortcomings of the dual membrane filter. Moreover, the input/output adopts a direct transition conversion structure, which also reduces the loss of the coupling gap.
l Double film resonance principle and frequency adjustmentSIW is a new type of artificial integrated waveguide. It is formed by embedding two rows of metallized holes in the dielectric layer of a planar circuit. These two rows of metallized holes form the narrow wall of the waveguide. Figure 1 shows the substrate integration. Schematic diagram of the structure of the waveguide. Such planar waveguides are not only easy to integrate with microwave integrated circuits (MICs) and monolithic microwave integrated circuits (MMICs), but SIW also inherits the advantages of high quality factor, low radiation loss, and ease of design of conventional rectangular waveguides.
1.1 substrate integrated waveguide resonator
In general, the closer the oscillation frequencies of the two circuits are, the smaller the coupling required for energy conversion between the two circuits. Since there are orthogonal relationships among the numerous modes in the resonant cavity, in order for these modes to be coupled to energy exchange, the ideal structure must be perturbed. However, in order to maintain the original form of the field structure, this disturbance is small. Therefore, this paper selects the degenerate main modes TE102 and TE201 of SIW, and their electric field distribution diagram is shown in Fig. 2. Because TM and TEmn (n10) cannot be transmitted in SIW. Therefore, on the one hand, it can be ensured that the coupling can be realized in the case of small disturbances, and the original structure of the field can also be ensured.
It is assumed that the length, width and height of the rectangular cavity shown in Fig. 3 are a, b, and d, respectively. Since TEmn (n10) cannot be transmitted in the SIW, the calculation of the resonant frequency for the SIW resonator
The formula is as follows: 
For the two modes with the same resonant frequency, there are the following relationships:
The selected work degenerate mode, using equations (1), (2), (3) to determine the initial size of the rectangular waveguide cavity, and then combined with the relevant literature, can determine the size of the SIW cavity. Figure 3 shows the basic structure of its metal rectangular resonator.
1.2 Double film SIW resonator and its frequency adjustment
Cylindrical waveguides, rectangular waveguides, and microstrip lines can be used as dual membrane filters. However, some typical two-film design methods (such as adjusting screws, internal corner machining, adding a cross recess to a microstrip patch, etc.) are not suitable for SIW cavities. Some literature mentions the use of chamfering, punching, feed disturbance and other disturbances to apply to the SIW cavity. Therefore, this paper chooses to cut two identical square chamfers on the symmetrical angle of the SIW cavity as the perturbation mode. The resonant frequency of the disturbing cavity is divided into two different frequencies, f1 and f2. The average of the two frequencies (f1+f2)/2 and the resonant frequency f0 of the original cavity are often not equal. Similarly, the coupling of the input/output sections also causes a translation of the resonant frequency. This will result in two situations: one is (f1+f2)/2"f0; the second is (f1+f2)/2"P"
Whether it is larger or smaller depends on the coupling structure. In the first case, the frequency shift can be adjusted by increasing the size of the cavity; and in the second case, the equivalent size of the cavity can be reduced by reducing the size of the resonator or opening a slit in the cavity. To adjust. Of course, it is also possible to adjust the two cases without adjusting. In practical engineering applications. Requires s "λ / 20, when the SIW works in the high frequency band, in order to meet the above conditions, the metal column radius and the spacing between them are often required to be so small that processing is very difficult. At this time, the first case can be utilized to achieve good filtering performance at a higher frequency with a larger size and to reduce the processing difficulty; and for the second case, a lower resonance frequency can be achieved with a smaller size. A good filtering performance is achieved to achieve miniaturization of the filter. This article is to effectively use the second case to design a filter with good performance and small size.
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