диафрагмированные волноводные фильтры / 232efdd7-2253-447a-b740-8335b8eb7a3a

Abstract—This paper presents a continuously tunable bandpass filter fully made by plastic additive manufacturing. This filter is designed to provide a constant absolute bandwidth around 5.7 GHz using a mechanical movement. A 3D spiral ribbon made of metallized plastic is designed to create a 4th order Chebyshev bandpass filter by making it rotate in a standard WR159 waveguide. The obtained filter experimentally provides a 325 MHz +/- 35 MHz bandwidth while having its operating frequency be continuously tuned from 5.45 to 5.64 GHz (7.47% tuning range). The return loss remains better than 10 dB over the whole tuning range. Even if the insertion loss are rather high (1.6 to 3.6 dB) mainly due to the low conductivity used for the plating of the plastic parts, this proof concept demonstrates the validity of such design and of the use of additive manufacturing for prototyping new concepts of tunable waveguide filters.

Keywords—additive manufacturing; tunable filter; microwave; plastic; 3D printing

I. INTRODUCTION

Tunable filters are now a common topic in order to provide concrete solutions to replace redundant filters in telecommunication front-ends. Tunable cavities or waveguide resonators are the first choice when low insertion losses are researched. Different tuning mechanisms are thoroughly tested in order to provide a high tuning range while degrading as less as possible the filter losses. Discrete tuning can very efficiently be obtained using MEMS switches [1] that can create an equivalent reconfigurable inner waveguide wall and thus modifying the filter frequency. Dielectric resonators and MEMS switches can be used to maintain a high Q like in [2]. Continuously tunable resonators while keeping a high Q are mostly obtained using mechanical elements such as movable metallic perturbers [3],[4],[5] or moving one cavity wall position [6],[7],[8]. One extremely efficient way is to actually use one flexible membrane as a cavity wall and to deform it using piezoelectric actuators [9] or electrostatic force [10]. Additive manufacturing has been recently used to create ceramic perturbers dedicated to the creation of continuously tunable cavity filters [11] and for the prototyping of tunable E- plane filters using mechanical movement [12]. The latter has successfully proved the relevance of using additive manufacturing of metallized plastic parts as proof of concepts for tunable filter for input stages of telecom satellite for example. However this initial concept cannot provide an absolute constant bandwidth.

The proposed paper is in direct relation with this last work but instead of using a translation of thin metal walls, a spiral ribbon is rotated inside a waveguide to produce a tunable filter capable to maintain an absolute constant bandwidth.

The next section will firstly present the principle of this tunable filter with the expected performances. Section III will explain the fabrication and assembly of the filter as well as the measured performances. Section IV will finally conclude this paper.

II. TUNABLE FILTER CONCEPT

The proposed filter is using the typical E-plane filter configuration proposed in [12]. Five thin metallic walls are still used in a regular rectangular waveguide in order to create four resonant TE101 cavities (Fig. 1). In order to tune the cavities resonances, the distance has to be changed but if an absolute constant bandwidth is researched, the width of the metallic walls have to be changed accordingly to provide the needed coupling as the operating frequency is changing.

Fig. 1 Cross-cut view of the proposed 4th order Chebyshev bandpass filter: the electrical field is plotted considering a frequency of 5.7 GHz

In order to make such movement and tuning continuous, we propose here to actually connect all the walls in a unique kind of spiral ribbon (Fig. 2) whose width changes continuously as the ribbon rotates. This part is thought to be made in a single part using additive manufacturing.

Fig. 2 Proposed spiral ribbon made in a single part.

The filter concept proposed in this paper can be seen in Fig. 3. The ribbon is rotating in such a way that it goes through a standard WR159 waveguide.

Fig. 3 CAD views of the 4th order bandpass filter

The spiral ribbon dimensions have therefore being optimised to provide an absolute constant bandwidth over the whole tuning range. The design procedure is based on the optimisation of the different sub-parts of the ribbon for different discrete intermediate positions i.e. for different given rotation angles. If enough discrete positions are considered, a true continuous tuning while keeping an absolute bandwidth can be obtained to some extent. We consider that the maximum acceptable tuning range is reached when the obtained bandwidth and return loss start being too far from the wanted performances. The obtained theoretical S parameters are displayed in Fig. 4 and the expected performances are shown in Table I.

A theoretical tuning of 8.45% from 6.001 to 5.494 GHz is expected for such filter while maintaining a 224.5 MHz +/- 11.5 MHz bandwidth. Considering that the different part will be made out of plastic and then fully covered with a silver paint, the simulations have been made considering an ideal conductivity of 5 S/µm. With such conductivity, the cavities unloaded Q factors are expected to be between 770 and 1300. The filter inner size is 40.4 mm x 48 mm x 172 mm.

TABLE I

THEORETICAL PERFORMANCES OF THE 4TH ORDER BANDPASS

FILTER

III. FABRICATION AND MEASUREMENT

The Fused Deposition Modelling (FDM) technique used in [12] is applied again here for the prototyping of the proposed tunable filter. This fabrication technique using the ABS plastic has shown to be accurate enough for filters working in low frequency (less than 8 to 10 GHz typically) with a typical accuracy of +/- 200 µm [13]. The FDM has been chosen here for its low manufacturing cost especially for big parts like the one proposed in this paper. The FDM printer automatically generates the supports needed during the fabrication. They are easily removed afterwards using washing soda.

The filter is thus made in three parts, each of them being individually covered with 3 layers of silver paint, the layers being applied manually using a small brush. Each layer is dried at ambient temperature for 24 hours before applying another one. The obtained parts can be seen in Fig. 5. Taking into account the flanges that are added for its measurement, the overall size of the experimental device is 60 mm x 93 mm x 200 mm.

Fig. 4 Theoretical S parameters for 3 different angles (initial, intermediate and final rotation angles).

Fig. 5 Left: Metallized plastic parts before assembly; Right: assembled filter once connected to a VNA.

The measured S parameters for five different rotation angles of the spiral ribbon can be seen in Fig. 6. Table II summarizes the measured results. An absolute constant bandwidth of 325 MHz +/- 35 MHz is obtained for operating frequencies between 5.45 and 5.89 GHz while having a return loss better than around 10 dB. This bandwidth is about 45% higher than the simulated value and the experimental tuning range is 7.47% which is a bit lower than the simulated value of 8.45%.

Considering the assembly tolerances of the different parts and the unwanted flexibility of the spiral ribbon, we mainly explain the experimental discrepancies by these uncontrolled parameters. Some uncontrolled EM field leakages certainly occur here and specially during the ribbon rotation. Moreover a recent similar activity showed that the conductivity of the silver paint used here is rather close to 2 S/µm [13] instead of 5 S/µm as initially considered in our simulations. The estimated experimental unloaded Q factors are thus between 90 and 260 and would certainly be greatly increased with a more accurate fabrication, assembly and a much better metallisation technique since unpredictable EM filed leakage have occurred.

However, the low cost proof of concept of a continuously tunable bandpass filter that we were expecting is obtained.

TABLE II

MEASURED PERFORMANCES OF THE 4TH ORDER BP FILTER

(a)

(b)

(c)

Fig. 6 Measured S parameters: (a) S21 ; (b) S11 ; (c) S22.

IV. CONCLUSION

A continuously tunable waveguide filter has been proposed and experimentally demonstrated. Additive Manufacturing (AM) appears to be extremely helpful for RF designers as a quick, efficient and low cost way to create proofs of concept of microwave devices that can be as complex as such tunable bandpass filter that is able to provide an absolute constant bandwidth. Since the key part of this device, i.e. a 3D spiral ribbon, is rather complicated to fabricate by standard means, AM brings another benefit here by making such complex part easily feasible. Despite some low manufacturing accuracy and quality of assembling brought by the FDM fabrication of the plastic ribbon (and therefore low insertion loss), the objective of getting a functional proof of concept of a new tunable waveguide filter is fully achieved here.

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