диафрагмированные волноводные фильтры / d7153ebe-9e9b-4dbf-8700-dc50f5a802a9

Laetitia Estagerie, Ludovic Carpentier,

Service Hyperfréquences et Temps/Fréquence CNES - DCT/RF/HT

BPi 2013, 18 avenue Edouard Belin, 31401 Toulouse Cedex 9

Abstract—This paper introduces a new concept capable of obtaining a wide frequency excursion, keeping high electrical performances. To reach such performances, the use of waveguide resonators is chosen, actuated by a mechanical system is made. Starting from a classical 2 pole E-plane filter, which consists of thin walls implemented along the propagation axis inside a waveguide, we make both the filter external coupling and the frequency tunable by moveable walls. Thus, on the two pole presented, it allows to control the resonance frequency of the system. A simulated 2-pole filter is continuously tuned from 8,37 GHz to 10,04GHz (thus 20%) with a quality factor Q of 6000. It was fabricated by a low cost 3D plastic printing, in order to validate the principle with a fast prototyping technique.

Keywords—E-plane filters, Continuously tunable filters, High- Q filters, Bandpass filters, 3D plastic printing, additive manufacturing.

I.INTRODUCTION

The principle of tunable filters is to modify their operating frequency and/or bandwidth using a controlled command. The main interest is to reduce the number of filters for a given application. They also have potential in dynamic spectrum radio operation by adjusting themselves according to the conditions and application needs.

The drawback of a tunable device implemented on a filter involves losses, with some limitations on the frequency range. Here is the concern of this paper : conciliate a large tuning range while maintaining a high Q factor.

Planar circuits with striplines or coplanar lines etched on a substrate give the advantage to implant easily the tunable device without adding significant losses. Moreover, they provide a good integrability. These technologies take the advantage of the capacitance shift using MEMS [1], varactors [2], BST capacitors [3] implemented directly on the line with λ/2 resonators, SRR [4] resonators and so on. Although,

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performances of such a system do not allow to be used in high performance (unloaded factor under 100) system or high power handling circuits.

To improve the performances, other systems using the same tunable SIW resonators [5] have been developed. The integrability is still good, the unloaded factor can reach several hundred.

Finally, to reach very high electrical performances and power handling, cavity filters are preferable. Many tuning devices have been proposed like cut lines linked by a switch to modify a coupling [6], piezoelectric actuators associated to a moveable membrane on top of an evanescent cavity [7], with capacitive tunable elements techniques used to control each coupling [8]. In [9] this membrane can be actuated directly by an electrostatic force. Other examples using ferromagnetic materials can be used [10] providing a high sensitivity. However all these technologies do not provide Q factor higher than 1000. Better Q can be obtained using a moveable top an a cavity resonator [11] or using moveable dielectric part in a cavity[12]. MEMS implemented on a wall placed inside a

Fig. 1: View of the two pole continuously tunable filter

cavity can also be used to efficiently change the resonator electric size [13] or directly the electrical length of E-plane walls [14]. Frequency can also be shifted by an optical control using N-type silicon dice doped with phosphorus [15]. We aim have to propose an alternative solution for a widely tunable 2 pole E-plane waveguide filter.

II.E-PLANE FILTER

A. Presentation

The first step is to consider a simple waveguide having a section a.b with its own cut off frequency given by (1).

Considering the TE10 propagation mode, the E-field vector belongs to the transverse plane along the propagation. Besides, it is well known that the insertion of a metallic part collinear to the E-field vector acts as an electric wall as the tangent part of E is equal to zero in such condition. So, adding thin metallic walls along the propagation vector (fig. 2) has an equivalent effect closed to a regular rectangular iris. Indeed, as the wall does not fill all the cavity section, a coupling is possible from each parts of the metallic plane. Thus, we can imagine that a pseudo closed TE101 mode cavity is created as illustrated in blue on figure 1.

Fig. 2:Left, TE01 mode E-field ; Right, pseudo cavity inside a waveguide

B. Link between couplings and E-plane filter

Fig. 3: Diagram of the two pole E-plane filter

Considering a two pole filter with two of such pseudo cavity in a waveguide (fig.3), 3 main dimensions LQ, Lk and Lf have been optimizes as follow :Lf changes the resonance frequency fr, Lk modifies the inter resonator coupling k and LQ strongly impacts the Input/output coupling Qe.

These 3 dimensions have been optimized according to Thebitchev Synthesis [16], giving Qe and k, at 10 GHz.

III.SIMULATION OF A 2 POLE-FILTER

A.Description

Considering that almost no energy goes out from a tight slit

along the propagation vector on the top of a waveguide,we decided to place in this position the elements used for mechanical movement (and therefore frequency tuning). The part inside the slit is not metalized in order to avoid the generation of a coaxial effect. As seen in fig.4, the chosen mechanical movement is a translation along the z-axis.

The principle is to make the two Input/output walls moveable in order to adjust the length of the resonators in order to control the resonance frequency.

A first measurement points out several drawbacks. Indeed, the first version has a guide in the bottom of the cavity so as to guide correctly the walls. However, that creates contact failures which leads the system to low performances. Thus, on the new design, guide elements are deleted and a gap is taken into account between the septa and the bottom of the cavity.

In such configuration, walls have to be very short to provide the desired coupling, involving difficulties for the fabrication and a breakable system. To solve this problem, the walls are offset to one side of the cavity.

Fig. 4: View of the two pole E-plane filter

Different dimensions have been optimized to provide well matched response along the tunable range.

A lid (fig.4) is created over the slit in order to limit as much as possible the EM field leakage thorough this aperture. The wall are coated with a silver lacquer.

B. Coupling study

As the dimensions are all fixed, there is no action to control the input/output couplings and inter resonators one. Thus a trade-off value is chosen. The curve fig.5 shows the evolution of Qe with the frequency.

Fig. 5: Evolution of Qe with the frequency

The coupling increases with the frequency. The behaviour is the same with the inter resonator coupling. Thus, an increase of the passband with the frequency is expected.

IV. RESULTS

Based on the simulations made on Ansoft HFSS, we expected to achieve a tuning range from 8,37 GHz to 10,04 GHz (20%) with a passband from 48 MHz to 255 MHz corresponding to a 10 mm translation of the moving parts. The return loss is better than 15 dB over 8,50 GHz (fig.6). The losses, considering classical silver coating (σ = 47,6 S.µm-1) are lower than 0,66dB. The unloaded factor goes from 6000 (low frequency) to 6500 (high frequency). Considering the

conductivity of the lacquer (σ = 23 S.µm-1), the losses reach 1,4 dB and the unloaded factor

V.FABRICATION

A.3D plastic printing characteristics

This filter has been manufactured with a very quick and low cost fabrication technique : 3D plastic printing by fused deposition modeling. For our applications, this technique allows a first step to quickly validate some principles, without reaching the accuracy of a regular machined metallic fabrication. A CAD file of the structure is sent to the printer, which, layer by layer, print it with a 0,25mm in diameter melted ABS plastic wire. Another polymer made of Polylactic Acid (PLA) is used to create any needed supporting parts during fabrication. At the end, by plunging the system into a Na2CO3 bath, the sacrificial plastic (PLA) is dissolved, without damages to the wanted ABS part.

The advantages of such manufacturing technique are low cost (around a hundred €) and the fabrication time (few hours).The next step is about metal coating of the plastic device. The inner part of the waveguide has been coated with lacquer containing silver monoparticles by manual application [17]. For big parts, electroless bath is possible. The system is dried into an oven at 70°C to burn the organic materials in the lacquer.

The accuracy of this printing is around 100µm and a measurement of roughness was made. This one is different according to the direction. Along the y-axis it is less than 60 µm and it can reach 100µm in the two other directions. This value is typically due to the melted plastic wire through the printer nozzle.

B. Two pole fix plane filter

To first validate the feasibility of such a system, we designed a two pole E-plane filter at 10,5GHz with a passband of 150 MHz. Fig.7 shows the comparison between measurements and simulation of the two pole filter. A good agreement is observed here, demonstrating the relevance to use 3D printing for prototype fabrication these frequencies.

C. Two pole tunable E-Plane filter

The tunable filter has been fabricated as well. The frequency is measured (fig.7) from 8,5 GHz to 10,0 GHz -18%- and we succeed to obtain the two pole filter without any compensation device. No spurious mode are found on the measured range from 8 GHz to 11 GHz. Measured losses are between 6,4 and 1,6 dB. By keeping only one resonator, we were able to measure the Q-factor of a single resonator. Its value remains between 1000 and 1250 over the tuning range. The decrease of the excursion frequency observed fig.8 between simulation and measurement is due to the coating which reduces the translation range. In the simulation fig.8, a conductivity of 23 S/µm is used. This is the conductivity of the lacquer which has been mesured.

Fig. 8: Comparison between simulation and measurements

VI. UP TO A 4 POLE FILTER WITH CONTROL OF EACH

COUPLING

From our experiment, we foresee to design a four pole filter, with only one command and the possibility of controlling each coupling.

This concept is currently under development in the Xlim research institute.

VII. CONCLUSION

A low cost and fast 3D printing technique with plastic has been used for the creation of filters around 10 GHz. For prototyping purposes, it has been demonstrated that standard and even tunable filters can be fabricated. We then succeed in achieving a 1,75% BW filter at 10,53 GHz having 0,5 dB of insertion losses. A tunable filter showing a tuning range of 22% around 10 GHz has also been achieved. Its Q factors remains between 1000 and 1250 over the whole tuning range. Of course, better Q can be obtained by regular machining technique, but plastic printing offers a cheap and fast possibility to easily validate RF design. The next step is to develop a 4-pole tunable filter which will be mechanically tuned by only one command. Moreover we plan to control each coupling between the different resonators, still with a Q factor over 1000.

ACKNOWLEDGMENT

This work has been sponsored by CNES and Thales Alenia Space. The 3D printer was provided by Lycée Turgot in

Limoges, France. We want to thank M. Gramond for its availability and mastering of the 3D printer.

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