диафрагмированные волноводные фильтры / 8ead47af-c6c1-4850-b20d-91fcebf1a3be

Since the late 1960s, when first Intelsat satellite series were launched, the number and applications of satellite communication systems have grown exponentially. In the last four decades, the use of

satellite systems has been extended from traditional fixed telecommunication systems to cope with mobile, navigation and remote-sensing applications [1]. Microwave components, and in particular filters, are key elements present in the payloads of the aforementioned satellites [2], [3]. A very deep review of many different filter topologies and corresponding electrical characteristics can be found in [4]. However, as described in [2], the specific implementation of filters for satellite communication systems are mainly dependent on the space mission as well as on the required filtering function within the corresponding payload.

Satellite filters can also cover a wide range of frequencies (from a few hundreds of megahertz up to 40 GHz), depending upon the specific service

provided by each payload. For instance, navigation and mobile satellite systems are typically operated in the L (1–2 GHz) and S (2–4 GHz) bands, whereas remote sensing applications are mostly offered in C (4–8 GHz) band. In the commercial communication area, due to the increasing demand of high-quality services, the operating frequency band has evolved towards the K u (12–18 GHz) band [2], whereas higher spectral bands (20–30 GHz) are also being considered [3].

To fix the scope of this article, we will consider the

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communications payload of a satellite system, where microwave filters are typically inserted [2]. A generic block diagram of such a payload can be seen in Figure 1, where the division of the frequency band into a number of channels is detailed. This channelization is required because of practical constraints due to nonlinearities and noise effects in high power amplifiers, which allow the required level for the transmitted signals to be obtained. The division into channels and the consequent recombination is

Output Waveguide

Fourth Section

Third Section

Second Section

Resonant Cavity

Coupling Window

Input Waveguide

w1

Another strategic area in the practical development of microwave filters for satellite communication systems is related to the design process, where a wide variety of techniques and methodologies can be followed (see a detailed review in [32]). Since extremely fast and precise electromagnetic analysis methods are already available, recent efforts are being devoted to their integration within modern CAD tools, which are typically driven by optimization algorithms. A wide survey of technical contributions on this research area

can be found in [7] and [8]. Even though optimization methods are extremely useful, their direct application to the filter design problem may not result in good solutions. They should be wisely combined with synthesis techniques, such as those based on network theory and coupling matrices [33]–[35], which provide good initial points that help to find the optimal solution with reduced computational efforts. This combined strategy has been recently applied to the fast and automated design of waveguide filters and diplexers for direct use in space systems [36]–[38].

Finally, we will briefly describe the basic features of the software package FEST 3D (Full-Wave Electromagnetic Simulation Tool), which has been used for the analysis and design of the waveguide filters described in the next sections. This CAD tool, supported through several research funding programs of ESTEC, ESA (European Space Research and Technology Centre, European Space Agency), is based on the advanced modal methods described in [21]. Among such analysis methods, the previously cited two-dimensional (2-D) and 3-D BI-RME techniques are included, thus allowing the software to cope with a wide range of complex waveguide devices. FEST 3D is also provided with synthesis and design capabilities of waveguide filters and diplexers, which are based on the theoretical approach reported in [37]. Furthermore, since the software tool is focused on space applications, some additional modules for dealing with high power effects are available. For instance, FEST 3D can predict the multipactor breakdown level of waveguide based components using the method described in [39] and can also perform the analysis of passive intermodulation (PIM) in waveguide connections [40].

Inductively Coupled Rectangular Waveguide Filters

The first example under consideration is a simple implementation of a direct-coupled cavity filter in rectangular waveguide technology, which is composed of waveguide cavities coupled through thick inductive irises (see Figure 2). This waveguide topology implements the classic half-wave prototype, where the

techniques are used. For such purposes, the rounded corners effects introduced in the H- or E-plane of the manufactured waveguide filters have been rigorously considered during the analysis and design stages (see details in [9]). An alternative application of the low-cost milling technique produces rounded corners in the cross section of the waveguides, as shown in Figure 5. Making use of this novel topology, a four-pole inductive filter with a

(a)

bandpass response of 300 MHz centered at 11 GHz has been completely designed as proposed in [23].

In this case, the CAD tool employed has optimized the lengths of the coupling windows and cavities of the filter (see such parameters in Figure 5). This choice has widely increased the numerical efficiency of the complete design procedure, since the optimization of the length parameters does not imply any change in the cross section of the rounded corner waveguides. Therefore, the modal chart of such arbitrarily shaped waveguides does not need to be recomputed at each optimization step. Once the lengths of all involved waveguides have been determined, a prototype has been manufactured using this alternative fabrication technique. As shown in Figure 6(a), the filter is composed of several pieces, which must be cascaded to build up the complete structure. To minimize the additional losses introduced by this fabrication technique as much as possible, the junction of consecutive pieces takes place at the half plane of each waveguide resonator.

For validation purposes, the simulated results provided by our CAD tool at the design point are successfully compared with measurements of the manufactured prototype in Figure 6(b). In this case, and using the same personal computer employed in the previous example, a global effort of 204 s has been required for obtaining the simulated results in the whole frequency band (1,000 points). Therefore, it is also concluded that the complete design procedure of this filter has been performed in reasonable development times.

E-Plane Metal Insert Filters

improved selectivity and insertion loss performance. Such electrical responses have been traditionally required by narrow-band channel filters [44], [45].

For implementing dual-mode filters, square and circular waveguides have been typically used [2], since they support the dominant mode in vertical and horizontal polarizations. To understand the operation of these compact filtering devices, a classical implementation using square waveguide cavities and tuning screws has been first considered, whose topology is shown in Figure 9(a). The screws rotated 45◦ are used for controlling the coupling levels between the two orthogonal modes of each cavity, whereas the other screws allow the independent frequency tuning of each resonant mode. The required intercavity coupling levels between each pair of resonant modes are performed through an evanescent rectangular waveguide. The same kind of rectangular iris is also employed for coupling the input and output waveguides to the corresponding square cavities. As shown in Figure 9(a), the orientations of the two modal coupling screws are perpendicular, which realizes a modified elliptic-function response with two transmission zeros [44].

The dual-mode filter topology described above has been used to obtain a four-pole bandpass response of 400 MHz bandwidth centered at 15 GHz with two transmission zeros. For such purposes, our CAD tool has optimized the lengths of all waveguides, the width and height of the central rectangular iris, and the penetration depths of the tuning screws. In Figure 9(b), we successfully compare our simulated results for the designed filter with the numerical data generated by the FEM-based Ansoft HFSS code. The same personal computer used in the previous examples was employed to generate these results (150 frequency points), with a global CPU effort of 229 s.

It is widely recognized that the use of tuning screws in dual-mode filters can severely affect their power handling capability as well as increase the passive intermodulation risks. These restrictions are relevant for dual-mode components, which are typically used as channel filters in output multiplexers of satellite payloads. For such reasons, some novel topologies of dualmode filters without tuning screws, based on rectangular (almost square) [46], [47], circular [48] and elliptical [49] resonators, were proposed during the 1990s. A

GHz

(b)

Figure 7. Standard E-plane filter in rectangular waveguide technology: (a) 3-D geometry and 2-D view of the metal insert and (b) scattering parameters.

t

b

Figure 8. Periodically loaded E-plane waveguide filter:

(a)3-D geometry and 2-D view of the metal insert and

(b)scattering parameters.

practical realization of a dual-mode filter in circular waveguide technology without tuning screws is presented in Figure 10, where it can be noticed that only elliptical irises are used.

Each cavity of the dual-mode filter shown in Figure 10 contains two elliptical irises, the first one rotated 45◦ for realizing the modal coupling of the two degenerate modes and the second one (not rotated) with major and minor axis dimensions chosen for tuning each independent resonant frequency. The cross section of a central elliptical iris is also designed for recovering the needed coupling levels between the pairs of resonant modes placed at adjacent cavities. Finally, the input and output waveguides are coupled to the first and last waveguide resonators, respectively, through two elliptical irises. For designing this filter, the dimensions of all elliptical irises, as well as the lengths of the circular cavities, have been determined.

In this case, our flexible CAD tool has been used to provide a four-pole elliptical bandpass response of 2% relative bandwidth centered at 11 GHz. For generating the two transmission zeros of such electrical response, the modal coupling elliptic irises of the filter must be orthogonally oriented, as it is shown in Figure 10. Once the geometrical dimensions of the designed dual-mode filter were determined, a real prototype was manufactured. In Figure 11(a) we can see the pieces of the filter, which include the required elliptical irises and circular waveguide cavities. After assembling all these pieces, a very compact filtering structure is finally obtained for space applications. An excellent comparison between the measurements of the fabricated filter response and the predicted simulated results is offered in Figure 11(b). It is relevant to remark that such experimental results were obtained without using any tuning screw in the prototype, which was possible due to the accuracy of the manufacturing technique employed (i.e., spark eroding).

Evanescent Mode Waveguide Filters

of-band performance and sharp selectivity. In addition, these filters are very competitive in terms of mass and volume due to their below-cutoff waveguide housing. As a practical example, we have used this compact geometry to realize a seven-pole bandpass response of 250 MHz bandwidth centered around 7 GHz. The design parameters determined by our CAD tool were the lengths of the evanescent waveguide sections and of the ridge waveguides (see Figure 12), whereas the housing dimensions and the ridge width and height were previously fixed for optimizing the filter length

Input

Waveguide

Modal Coupling Screw

and its stopband behavior. A prototype of the designed filter was manufactured using a conventional milling technique. As shown in Figure 13(a), the structure is composed of two symmetrical pieces including the cor-

Figure 11. Manufactured prototype of the dual-mode filter in circular waveguide technology: (a) view of the internal pieces and (b) scattering parameters. Courtesy of ESTEC, ESA.

responding metal ridges, which must be assembled together to build up the complete filter.

The experimental measurements of the manufactured prototype are included in Figure 13(b), where they compare well with the simulated results of the designed structure. Nevertheless, some slight variations between both responses are observed in the passband, which are attributed to manufacture tolerances. For this structure, small perturbations in the penetration depths of the upper and lower metal inserts seriously affect the ridge gap dimension, which is identified to be a very sensitive parameter. Additionally, slight assembly misalignments can easily spoil the required symmetry of this filter design. To overcome such drawbacks, a potential alternative consists of using an asymmetrical configuration, with metal inserts placed only in one wall of the

waveguide housing.

A practical implementation of an asymmetrical evanescent mode waveguide filter, which uses tuning screws instead of rectangular cross-section metal ridges, is shown in Figure 14(a). In these structures, the use of post-assembly tuning screws can also be useful for overcoming response degradations due to manufacture tolerances. Using this asymmetrical configuration, a Chebyshev bandpass electrical response with five poles, 300 MHz bandwidth, and center frequency of 10 GHz was realized. For such purposes, the CAD tool optimized the penetration depths of the screws, as well as the lengths of below-cutoff waveguide sections placed between such screws.

In Figure 14(b), we successfully compare the simulated results provided by our CAD tool for the designed asymmetrical filter with numerical data, obtained with Ansoft HFSS based on the FEM method. To compute our simulated results (200 frequency points) with the same personal computer of previous cases, a CPU time of 35 s was only needed, which was very suitable for design purposes.

Output Waveguide

Figure 12. Symmetrical evanescent mode waveguide filter with centered ridges.

Conclusions

All-metal waveguide filters have been widely used in satellite payloads since the advent of the first space communication systems four decades ago. In this period, such technology has been continuously evolving due to the increasing requirements demanded to these components. This article has tried to describe this historical evolution, and its theoretical and technological features. Special attention has also been paid to present the most recent advances in the development of CAD tools for satellite filters, which are based on full-wave electromagnetic analysis methods.

Several practical examples of waveguide filters for space communication systems were considered. For each waveguide filter type, we

Figure 13. Manufactured prototype of the evanescent mode filter: (a) view of the two symmetrical pieces with ridges and (b) scattering parameters. Courtesy of Alcatel Alenia Space (Spain).

have described a standard configuration, its typical electrical response, and the potential use within a satellite communications payload. Then, we presented some of the technological novelties related to each filter category under study. The two first examples included in this article are classical implementations of direct-coupled cavity filters, i.e., the inductively coupled rectangular waveguide filter and the E-plane metal insert filter. For both of them, standard topologies and common manufacturing processes were outlined. In the case of inductive filters, we have also shown how modern CAD tools can cope with the undesired mechanization effects due to low-cost mass-production techniques. For the E-plane technology, a more compact alternative solution with an improved stopband electrical performance was proposed. Later on, the concept of dual-mode filters was introduced. Two solutions, the first one based on square waveguide cavities and tuning screws and the second one using circular cavities and elliptical irises without tuning elements, were detailed. Finally, we focused on compact solutions for the input and out-

Figure 14. Asymmetrical evanescent mode waveguide filter with tuning screws: (a) geometry and (b) scattering parameters.

put stages of satellite payloads, thus considering the evanescent mode waveguide filter technology.

Acknowledgments

The authors would like to thank ESTEC, ESA (in particular Dr. Marco Guglielmi) and Alcatel Alenia Space (Spain) for providing the manufactured prototypes shown in the manuscript. They also want to acknowledge their research colleagues and Ph.D. students at Valencia (Spain), who have contributed with some of the numerical and experimental results included in this article.

References

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