диафрагмированные волноводные фильтры / 9870cb8e-f81b-4f38-93e1-9a4bea0c0ea2

Carlos Carceller, Pablo Soto, Vicente Boria

Grupo de Aplicaciones de Microondas Universitat Polit`ecnica de Val`encia Valencia, Spain

Email: carcarc2@upvnet.upv.es

ABSTRACT—This paper presents a novel filter topology for the implementation of asymmetric responses with transmission zeros (TZs) in rectangular waveguide technology. It is based on a compact folded E-plane arrangement where adjacent resonators are capacitively coupled through rectangular slots, and nonadjacent resonators are coupled through simple inductive windows. The new folded configuration allows the introduction of transmission zeros above the passband, which can be easily controlled. High order filters can be designed by cascading an arbitrary number of resonators in a folded layout. Components based on this novel configuration are amenable to simple manufacturing processes and can be used in high-power environments. A triplexer containing filters implemented with the proposed topology has been designed. Measurements of a manufactured prototype are included to validate the use of this topology in practical applications.

I. INTRODUCTION

With the emergence and rapid development of communication services in the last decades, the need for broader bandwidths and higher data rates has resulted in more stringent specifications for waveguide components. Typical requirements for satellite systems call for filtering functions with asymmetric responses and sharp cut-off slopes, which can only be achieved by the introduction of transmission zeros (TZs) at finite frequencies. The presence of TZs is aimed at concentrating the filter rejection in the frequency band where it is most needed, while relaxing such characteristic where this is not as critical. Compared with all-pole Chebyshev responses, these elliptic or quasi-elliptic transfer functions require lower order filters, thus generally obtaining smaller and lighter hardware with improved insertion losses.

Most of the filters with such transfer functions are based on routing schemes that include couplings between both adjacent and non-adjacent resonators. The relative phase shifts between the multiple signal paths introduced by resonators and coupling elements produce a cancellation (destructive interference) of the signal at certain finite complex frequencies. Depending on the relative sign of the couplings, these complex

This work was supported by the Spanish government under Project TEC2010-21520-C04-01 and BES-2011-045816 grant.

Marco Guglielmi, David Raboso

European Space Research and Technology Centre

European Space Agency

Noordwijk, The Netherlands

frequencies can be located on the imaginary axis (S = JW), and placed above the passband, below or both (if enough signal paths are considered) [1]. The designer must select the topology of the network, and then synthesize a suitable transfer function for this particular topology verifying the electrical specifications. This procedure can be carried out by using analytical techniques [2] or alternatively by direct application of optimization procedures [3]–[5].

Synthesizing a low-degree topology with asymmetric transmission zeros is a relatively simple task, but its practical implementation may not always be so simple. In fact, the choice of the physical structure should so often be regarded as an art, where the designer must look for the best trade-off between physical and electrical behavior. This step is probably the most critical point of the design procedure, since it dramatically affects the final performance of the manufactured component. Simplicity, compactness, modularity and ease to manufacture are always desirable features. In addition, for high-power applications, the use of probes and tuning-elements, although widely extended, is not recommended due to arcing problems [2] as well as multipactor and passive intermodulation (PIM).

In this paper, a novel waveguide filter topology with very good physical characteristics is presented as a flexible solution to all these concerns. The structure is only composed of rectangular waveguides and has E-plane symmetry. Hence, it can be easily manufactured in two identical halves, which reduces undesired effects such as insertion losses and PIM. The use of tuning elements can be completely avoided for relative bandwidths in excess of 1-2%. In addition, its modular nature offers high flexibility to arrange the cavities in a compact form, while allowing the introduction of finite transmission zeros above passband through simple rectangular coupling windows.

II. STRUCTURE DESCRIPTION

The topology proposed in this paper makes use of the recently patented hybrid folded rectangular waveguide (HFRW) filter configuration [6]. The main building block is composed of three rectangular cavities arranged as shown in Fig. 1. Since the different cavities are vertically stacked, the structure

Resonator 3

Resonator 1

Inductive window

(cross-coupling)

Capacitive slots

(main coupling)

Resonator 2

2

Fig. 1: Proposed implementation of the basic triplet block and routing scheme. Solid lines indicate direct couplings whereas dashed lines refer to cross-couplings.

becomes highly compact. Direct coupling between adjacent resonators is provided by openings placed in the bottom/top walls of the cavities.

In order to generate TZs, coupling between non-adjacent resonators must exist. A simple inductive window can be placed between resonators 1 and 3 to provide the crosscoupling. Due to the wide range of coupling values that these windows can provide, the structure offers great control over the location of the TZ within a broad frequency band. However, such TZs can only be located above the passband, since all main couplings are of the same type and crosscouplings are inductive. If the TZs were to be placed below the passband, non-adjacent resonators should be capacitively coupled instead. With the proposed topology, the introduction of negative cross-couplings is possible, although its practical implementation with capacitive irises becomes more difficult as the TZs are placed further away from the passband. In this paper, only transfer functions with above passband transmission zeros will be considered.

One of the major strengths of this filter topology is its flexibility. Starting from the basic triplet, the designer is able to include as many resonators as desired, just by stacking them above or below the preceding resonator. Each additional resonator is simply coupled to the last cavity in the structure through a slot placed on its top or bottom wall. At the same time, this new resonator may be coupled to the second to last one through an inductive window, thus implementing a positive cross-coupling. Responses with multiple TZs can be obtained by simply cascading as many triplets as TZs are required. Figure 2 shows a possible implementation of a filter with two TZs. As it can be seen, the structure is based on the connection of two cascaded triplets that share a common resonator. This implementation allows the designer to control the position of each TZ independently, by simply adjusting the dimensions of the respective triplet.

In addition to selecting the number of TZs, the proposed configuration also offers flexibility in the cavity arrangement to achieve the most efficient use of physical space available,

S 1 3 5 L

24

Fig. 2: Folded configuration with two TZs above the passband.

Fig. 3: Alternative configurations providing one TZ. a) Staircase topology. b) Topology with a 90o cavity rotation that enables coupling through the shortest wall.

or to look for the most suitable layout and port placement. This is indeed a key point in multiplexers. At the expense of reducing the number of potential transmission zeros, the proposed structure can be folded into a staircase configuration (see Fig. 3a). Likewise, it enables a 90o rotation of the resonators to introduce the coupling from the shortest wall, as shown in Fig. 3b.

The E-plane symmetry of the overall structure is another valuable characteristic of this topology. It allows the fabrication of the filter in clam-shell technology without interrupting current lines, and therefore reducing the insertion losses due to manufacturing. In addition, this technology limits the chance of passive intermodulation, an undesired effect in high-power environments. The lack of tuning elements also helps in reducing these high-power effects, even if it implies not being able to compensate for manufacturing defects and restricts the filters to relative bandwiths greater than 1-2% in Ku band. In conclusion, these filters are ideally suited to be located in the output stages of communication payloads, where wide bands are subject to filtering and the signal intensity is considerable.

Compared with classic in-line filters, folded configurations like the one presented in this paper provide a more compact approach, as well as steeper rejection slopes thanks to the introduction of TZs. Folded topologies with an H-plane cavity arrangement [7]–[10] are a classical configuration to provide quasi-elliptic filtering functions. However, these structures

Fig. 4: Response of the designed five-pole filter with one TZ.

Fig. 5: Response of the designed four-pole filter with one TZ.

tend to be bulkier and do not benefit from the advantages that the clam-shell manufacturing process offers to their E-plane counterpart. In [11], an E-plane cavity arrangement is used to design delay elements and self-equalized pseudoelliptic filters. Nevertheless, the coupling scheme presented is not suitable for the implementation of asymmetric responses. A solution to this limitation was presented in [12], where a pair of trisections were cascaded to obtain two TZs above the passband. This approach made use of six cavities to provide two TZ whereas the configuration presented in this paper requires only five, while keeping the input and output ports aligned (see Fig. 2). This reduction in the number of cavities implies a more compact structure as well as reduced losses and weight of the overall component.

III. PRACTICAL APPLICATION EXAMPLE

The proposed topology is applied to the design of a manifold-coupled triplexer. The requirements for this triplexer specify an isolation level greater than 80 dB between the two high-power transmission channels (centered at 11.45 and 12.6 GHz) and the reception channel covering the frequency range between 13.7 and 14.55 GHz, thus requiring the introduction of asymmetric TZ in the responses of the filters serving the lower bands.

The first filter, centered at 11.45 GHz with 600 MHz bandwidth, must feature return losses better than 25 dB in the passband. A 5-pole Chebyshev response with one TZ is chosen to fulfill specifications. The implementation of this filter takes advantage of the novel folded configuration to realize four of the filter cavities with only one cross-coupling between resonators 2 and 4. The fifth resonator, though, is coupled to the rest of the structure through a simple inductive window to allow the placement of the port flange. First, the 5-pole filter without TZ is designed following the cavity-by-cavity methodology described in [13]. Once the complete all-pole filter is built, an inductive window between resonators 2 and 4 is added to generate the TZ. A final optimization is applied

until the TZ is properly positioned. The final response of this first filter is shown in Fig. 4.

Following the same design procedure, a second filter is designed to cover the upper transmission channel (spanning from 12.40 to 12.80 GHz). In this case, a 4-pole filter with one TZ is used to provide the required rejection in the upper frequency band. The full-wave EM response of this filter is depicted in Fig. 5.

Regarding the upper frequency band, a simple high-pass filter is considered. It is composed of a two-step inductive transformer connected to a length of waveguide below cutoff. This waveguide should be long enough to grant isolation greater than 150 dB with the transmission channels.

After designing the three filters separately, they are combined in the triplexer’s manifold. First, the length of waveguide that acts as manifold is directly connected to the port of the high-pass filter. Then, the pass-band filters are vertically stacked above and below the manifold and coupled through slots. In order to achieve a good matching between the manifold an the filters [13], the dimensions of the slots and the first two cavities of each filter must be optimized until the triplexer fulfills requirements.

To validate the design, a prototype has been manufactured in aluminum with silver plating, as can be seen in Fig. 6. The triplexer is split in two halves by the E-plane and each part is realized by a milling technique. Since no currents flow through the splitting plane, the degradation due to manufacturing and assembling processes is minimized.

Figure 7a shows the comparison between the original triplexer design and measured responses. Some differences can be easily identified. However, due to the wide margins that were added to the specifications to cope with manufacturing deviations, and the slight degradations observed within the actual specified passbands, the component was accepted for the intended application. After carrying out the measurements, the convergence parameters of FEST3D simulation tool [14] were increased and the response shown in Fig. 7b was obtained,

Fig. 6: Setup for the measurement of the prototype response.

Fig. 7: a) Original comparison between the designed response and measurements. b) Measured (dashed line) and simulated (solid line) response of the triplexer. The high rejection between the transmission and reception channels was measured with a spectrum analyzer (circles).

which shows a very good agreement between simulated and measured data. However, the simulation time was relatively high (about 5 seconds per frequency point) and full convergence was still not reached. Currently, an improved numerical technique is under development to accurately simulate and design this type of components in very reduced CPU times.

IV. CONCLUSION

In this paper a novel folded topology for rectangular waveguide filters with asymmetric responses (including TZs) is presented. Cavities are arranged in a compact E-plane configuration that enables the introduction of cross-couplings

via simple rectangular windows (magnetic coupling) in order to generate TZs above the passband. The proposed structures are compact and flexible, and also eliminate the need for tuning elements in filters with bandwidths greater than 1-2%. In addition, the practical implementation of filters based on this configuration can be carried out by manufacturing the component in two symmetrical halves split by the E-plane. This technique, when combined with an absence of tuning elements, makes these filters suitable for high-power applications, since they are hardly affected by non-linear effects such as passive intermodulation and multipactor. As a proof- of-concept example, a triplexer with E-plane symmetry has been designed using the proposed topology for the bandpass filters. A prototype for the triplexer has been manufactured and its response successfully compared with simulations.

ACKNOWLEDGMENT

The authors are indebted with Mr. Davide Smacchia, from ESA-VSC high-power laboratory, for his assistance with the manufactured triplexer.

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