диафрагмированные волноводные фильтры / 4433e0f3-24e6-476e-b860-796412035203

Abstract- This paper proposes novel waveguide topologies for obtaining filter responses with arbitrary placed transmis­ sion zeros (TZs). These new structures have a good electrical behavior and allow fast simulation using EM techniques. Several topologies and their corresponding electrical models will be presented, together with amenable synthesis and design procedures. The simplicity of the layout raises these kind of filters as good candidates for useful applications requiring advanced filter characteristics.

Index Terms- Microwave filters, resonator filters, dis­ tributed parameter circuits, Design methodology, equivalent circuits.

I. INTRODUCTION

The current spectral needs lead to more and more stringent requirements for filter designers. For any new set of specifications, the engineer has to choose not only the technology but also a suitable structure that will fulfill the design requirements and, if possible, allow fast EM simulation as well as easy manufacturing.

For producing advanced filtering functions (e.g. asym­ metrical responses, arbitrarily placed TZs) some structures are very well known [1]. Cross-coupled filters with differ­ ent configurations (trisections, cascaded quartets, extracted pole, etc.) are the usual choice. In order to avoid problems and manufacturing issues, non-diagonal couplings struc­ tures (cul-de-sac and others) were recently developed [2]. In general, any chosen topology will exhibit specific advantages and drawbacks. Therefore, we are looking for mechanical simplicity, as well as an easy final optimization stage (if needed), independently of the number of TZs.

If the design specifications force the engineer to use a topology producing TZs, either cross couplings or side resonators (in extracted pole technique) are typically needed [3]. The extracted pole technique has some design issues related to sensitivity. Furthermore, an intensive optimization has to be performed to get the final design. Some other flexible topologies relying on cross-coupling

disadvantage of increasing the number of couplings among the resonators. The addition of non-resonanting nodes can help to improve sensitivity to manufacturing tolerances for narrowband applications, although the optimization of the final structure is still an issue [5].

Some alternative solutions to overcome these problems are introduced in the next section. Filter structures that can be accurately and efficiently analyzed and designed (and optimized if required) are explained in detail. As an additional advantage, the number of transmission zeros can be easily increased, and the flexibility of the final structure allows simple and cheap mechanization processes.

Section III shows how to obtain distributed models for the proposed topologies. These models will allow a step-by-step design procedure in order to obtain the final dimensions of the novel physical structures. Finally, measurements of a manufactured K-band prototype are shown in order to validate the new waveguide structures.

II. PROPOSED TOPOLOGIES

One of the simplest topologies in cross-coupled filters is the trisection. A trisection produces a single TZ at one side of the passband that can be chosen beforehand [6]. Cascaded trisections are commonly used in filter design, since each trisection can manage its own TZ with certain independence from the rest. The main drawback is that the interaction among the resonators within the trisection avoids an easy tuning/optimization process. The TZ is very dependent of the overall trisection structure,thus forcing an intensive optimization.

In addition, waveguide implementations of trisections have an important key parameter that is not very easy to achieve: the non-adjacent 1-3 coupling. The distributed model for the trisection shown in Fig. 1 is analytical, as it will be shown in section III. However, the physical implementation is not straightforward. As a first attempt, the physical layout can be carried out through a long wide open window as shown in Fig. 2, where a side view (E­ plane cut) is shown.