диафрагмированные волноводные фильтры / ce367e26-59b7-4292-b8ca-cf6b6628e71b

Abstract- A broadband band-stop filter with T-shape diaphragms loaded in a rectangular waveguide is described to obtain good stopband characteristics. Singleand doublediaphragm loading structures are taken into consideration, followed by analysis of different physical dimensions on the whole characteristics. The stop-bandwidth of double-diaphragm loaded filter can cover the single TE10 mode operation band of standard rectangular waveguide WR-62 with large stopband attenuation. In addition, the filter can be used for waveguide mode suppression in modern high-frequency circuits with metal shielding.

I.INTRODUCTION

Nowadays, increasing demands have been growing in radio frequency and microwave domains, including all sorts of components, circuits or devices, passive or active, etc. Since each of the above ones usually operates in a certain bandwidth, filters with specified characteristics are critically important for better transmission and reception of signals. Up till now, it is still a challenge to design a good band-stop filter (BSF) with high attenuation of unexpected propagation modes in the operation frequency band and good transmission outside the desired band. Various filter configurations have been studied for good performances, such as size compact and simple structure, wide stopband and passband, high power operation.

Traditional BSFs are usually realized by adding single or multiple resonant structures in planar dielectric substrates [1-2] or metallic waveguides [3], which can be easily transformed from well-known low-pass prototypes. For example, cascaded arrangement of resonant “block structures” can be used to block propagation of a number of modes across a specific band [4], and better than 40dB attenuation over a 5% bandwidth can be achieved. Introducing multiple transmission zeros by multiple coupled resonators can improve the out-of-band rejection and the passband selectivity [5], with advantages of compactness and flexibility. Non-resonating nodes may be bilaterally placed on waveguide or micro-strips and coupling electro-magnetic fields by slots, to design band-reject filters [6]. Composed of two layers of metal surfaces, gap waveguide can form stopband to design Chebychev band-pass response [7] by packaging for an irregular perfect electric conductor. Actually, the most popular scheme of waveguide BSF is to periodically or non-periodically add inserts in waveguide housing, such as E-plane metal iris, optimized fin-lines, and so forth [8]. Furthermore, frequency-selective surface could be alternatively introduced to realize size reduction, by controlling adequately

the resonant response of each FSS [9], and multiple attenuation poles in stopbands, providing both a passband and attenuationpole frequencies in stopbands.

In this paper, a novel band-stop filter (BSF) with T-shape diaphragms loaded in a standard rectangular waveguide WR62 is designed to obtain broad stop-band characteristic. BSF configuration is presented, and then singleand doublediaphragm loading structures are considered, followed by parametric analysis of different dimensions on the whole performance. As the double-diaphragm loading structure is implemented, very broad stop-band is obtained over the entire TE10 mode operating band of the waveguide. The filter can be employed not only for band-notching filtering but also for waveguide mode suppression in planar-cavity hybrid circuits.

II.CONFIGURATION DESCRIPTION

Fig. 1(a) shows the configuration of the proposed band-stop filter with T-shaped diaphragms loaded in a rectangular waveguide, where a/b/c, h, p and ih/iw, represent waveguide sizes, diaphragms height, loading space and arm width of the printed T-shaped diaphragm, respectively. The standard WR62 waveguide, can operate over a frequency band 12-18GHz with single dominant TE10 mode.

a

(a) Configuration (b) Equivalent circuit model Fig. 1. Configuration and equivalent circuit model of the proposed band-stop filter with T-shaped diaphragms loaded in a rectangular waveguide.

It is already known that periodically loaded full-height E- plane inserts may bring to shunt inductive effect. While in this design, T-shape diaphragms are positioned in cross-section of WR62, perpendicular to the propagating direction. As shown in Fig 1(b), one inserted T-shape metal diaphragms can be regarded as a series of capacitance-inductance, shunting between two broad walls of waveguide and forming a stopband response with one transmission zero in stopband. Doublediaphragm loading may bring more transmission zeros to obtain broader stop-band and steeper transition bands.

III.RESULTS, ANALYSIS AND DISCUSSION

During this research, equivalent circuit estimation is firstly executed to obtain initial geometrical parameters, including the sizes of T-shape diaphragm and loading space for doublediaphragm loading. Simulation software HFSS is employed to perform frequency sweep and parametric analysis. Simulation results show that the filter characteristics will be more sensitive to the loading space (p), the width of vertical arm (iw), thickness (ix) and height (h) of the diaphragm. The initial values of (iw, ix, h) are derived as (1.0mm, 0.5mm, 3.7mm).

A. Single-diaphragm loading, various iw and various ix:

In parametric analysis of single-diaphragm-loading BSF, changing rules of the filter characteristics can be numerically obtained when one size is gradually changed under the others to be fixed. Fig. 2 shows the variation of |S11| and |S21| curves corresponding to different widths of vertical arm. It is observed that there is only one resonant frequency caused by single T- shape diaphragm loading and it shifts to a higher point when iw becomes larger. Since single diaphragm can be equivalent to a series L-C resonator shunting between the broad walls of waveguide, the resonant frequency inevitably varies with the variance of the equivalent L or C value caused by the adjustment of diaphragm dimensions. A 0.4mm variation of iw will lead to about 0.6GHz of frequency shift and the stop-band width of single-loading BSF with the given physical parameters attains about 2GHz.

Frequency (GHz)

Fig. 2. Variation of S-parameter curves corresponding to different iw, width of vertical arm, in single-diaphragm loading BSF.

Corresponding to different diaphragm thickness (ix) (from 0.1mm to 0.9mm with step 0.4mm), the resonant frequency of the BSF tends to be shifted to a higher point when ix becomes larger. A 0.4mm variation of ix will bring to about 0.3GHz of frequency shift while the |S11| parameters in the higher passbands are certainly influenced, just like those in section A.

B. Single-diaphragm loading, various h:

The variation of |S11| and |S21| curves corresponding to different diaphragm height (h) are plotted in Fig. 3. It can be observed that the resonant frequency of the BSF shifts to a

lower point when diaphragm height h becomes larger, contrarily different from those in the section A and B. A 0.4mm variation of h will bring to about 0.4GHz of frequency shift while |S11| in the lower pass-band are influenced by changing h.

Frequency (GHz)

Fig. 3. Variation of S-parameter curves corresponding to different diaphragm height h in single-diaphragm loading BSF.

C. Double-diaphragm loading, various iw, and various ix:

Simulation results verify that very wide stopband can be obtained if equivalent L and C values are appropriately set by adjusting physical sizes of the single T-shape diaphragm. As a result of single resonator, transition bands are usually very wide and flat, which can be improved by introducing additive resonant frequencies. In the following sections, doublediaphragm loaded BSF will be demonstrated as the leading actor, through the influence analysis of the loading space p and the same three sizes as the former sections discussed.

Frequency (GHz)

Fig. 4. Variation of S-parameter curves corresponding to different iw, width of vertical arm, in double-diaphragm loading BSF.

Fig. 4 shows the variation of |S11| and |S21| curves corresponding to different width of vertical arm. As observed from Fig. 4, double-diaphragm loading brings to two resonant frequencies, forming broad stop-band, and the transition bands appear much steeper, especially in lower frequency band. Then,