диафрагмированные волноводные фильтры / 9b2ee8b6-a89e-4e13-b9f8-2441da64ba55

Roberto Sorrentino, Luca Pelliccia

DIEI, University of Perugia

Via G. Duranti 93, 06125, Perugia, Italy

Abstract— Innovative concepts are presented for both miniaturized high-selectivity and MEMS-based reconfigurable waveguide filters recently developed. The aim is to reduce size and weight of satellite telecommunication systems without compromising the high unloaded Q required in such applications. A first class of new miniaturized filters is based on ridges arbitrarily located and oriented within a waveguide. The ridges behave as quasi-TEM resonators and allow for pseudo-elliptic filter responses. A second class of very compact waveguide filters employs TM dual-mode cavities as building blocks to generate Nth order filters with N transmission zeros. Two new concepts have then been developed for high-Q bandpass filters with tunable central frequency or bandwidth using RF MEMS. Cantilever MEMS switches are suitably arranged in a rectangular waveguide so as to change either the TE101 mode resonant frequency or the coupling between E-plane filter resonators. Both approaches are shown to yield higher unloaded Q (~1000) than previous approaches involving RF MEMS along with ridged or conductor-loaded cavities.

Keywordswaveguide filter; high-Q; miniaturized, space application

I.INTRODUCTION

Advanced communication systems, both terrestrial and satellite, involve very sophisticated apparatuses operating with multi-standard and multi-band components. In this context, miniaturisation and reconfigurability are key features to reduce volume, mass and, ultimately, costs of the apparatuses. This paper focuses on some recent advances in the miniaturisation and reconfigurability of waveguide filters, which are still a notable interest for high-Q applications.

Reconfigurable and miniaturized waveguide filters indeed will be key elements in future satellite telecommunication systems which will require low size and weight as well as flexibility. Despite the numerous advantages of printed circuit technology, waveguide filters are still necessary in applications such as front-end receivers or the last stage of transmitting apparatuses, where low loss and high power handling capability are of great importance. On the other hand, waveguide filters have the notable drawback, particularly for satellite equipments, of large size and significant weight. Therefore, many efforts have been devoted to reduce the size of such filters without degrading their electrical performance. To this purpose, a large number of configurations have been

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developed and proposed in the literature, such as dualor multi-mode filters exploiting multiple resonant modes within a single physical cavity [1-3], or elliptic and pseudo-elliptic filters realising very sharp frequency responses with relatively low order functions [4-6].

Two new classes of compact waveguide pseudo-elliptic filters have recently been developed. The first class is based on the ridge resonators [7-9], whereas the second class of very compact filters is based on a TM dual-mode cavity [10-11] allowing also the propagation of non-resonating modes. In the first part of this paper, the basic properties of such filter classes are illustrated along with the experimental results of some prototypes.

On the other hand, size and weight of tunable and multistandard satellite front-ends can be substantially reduced by employing tuneable and reconfigurable bandpass filters with fast tuning speed, high unloaded Q (> 500), wide tuning range and low manufacturing cost. Innovative programmable and reconfigurable RF-systems can be developed provided that efficient and reliable solutions for electronically reconfigurable filters are found. So far it has been difficult to achieve an acceptable compromise among broad band tuning, high Q, low-cost and small size. While the unloaded Q of tunable or reconfigurable filters in planar technology is limited by the planar nature of the resonators (Q<300) [12], higher Qs can be obtained by employing waveguide resonators. Concerning the tuning elements, as is well known RF MEMS are an attractive technology thanks to their low insertion loss, low power consumption, compact size and high integrability [12]. By combining RF MEMS with high Q resonators it is possible to obtain high Q (>500) tunable and reconfigurable filters [13-14].

Two innovative concepts allowing for very high unloaded Q in reconfigurable bandpass filters employing cantilever MEMS switches were developed in [15] and in [16]. These two approaches are illustrated in the second part of this paper. The former allows for filter central frequency change, whereas the latter for filter bandwidth variation. To validate the new concepts, bandpass filter prototypes using equivalent hardwired connections emulating the RF MEMS states have been fabricated and measured. Experimental results confirm

the theoretical expectations showing unloaded Qs of the order of 1000.

II.RIDGE RESONATOR FILTERS

The basic element of the ridge resonator filter is a rectangular ridge arbitrarily located and oriented within a

rectangular waveguide [7]. The ridge sketched in Fig. 1 can be realised in slant, transverse or shifted positions with respect to the waveguide axis.

Fig. 1. Rectangular ridge resonator arbitrarily located and oriented within a waveguide.

When w<<l, in a first approximation, i.e. neglecting the fringing field at the ridge ends, the ridge section resonates when l is half a wavelength. In fact, since the ridge gap g is considerably smaller than the waveguide height b, the phase velocity is approximately independent of the ridge position

and can be assumed to be that of a TEM mode.

The slant angle and the offset between the ridge and the waveguide axis give additional degrees of freedom. The ridge position in fact affects the coupling between the dominant TE10 mode and the resonant mode of the ridge, while an additional input-to-output coupling occurs through higher order modes excited by the ridge itself. Therefore it possible to generate both a reflection and a transmission zero.

Fig. 2. Waveguide filter using rectangular ridge resonators: (a) measurement (solid lines) and full-wave simulation (dashed lines); (b) manufactured prototype.

Various filter responses can be obtained by simply varying

the ridge position and orientation within the waveguide [7].

As an example, Fig. 2a shows the response of a 5th order filter (photo in Fig. 2b). The pass-band is 10.6-11 GHz. Two slant ridges are used as first and fifth resonators to realise two upper stop-band transmission zeros; a transverse offset ridge is used as a third resonator generating a lower transmission zero. The total filter length is 55mm and the measured in-band insertion loss is 0.25 dB.

Other pseudo-elliptic filter responses have been obtained using nearly-square dual-mode ridge resonators for which l~w [9].

III.TM DUAL-MODE FILTERS

The TM dual-mode cavity combines the advantages of the dual-mode concept with the functionalities provided by the use of non-resonating modes [10-11].

The structure of the TM dual-mode cavity is depicted in Fig. 3. The degenerate modes resonating in the cavity are the TM120 and the TM210.

Fig. 3. TM dual-mode cavity.

Since both modes are independent of the longitudinal direction, the cavity length l can be chosen as small as possible provided that the cavity unloaded Q is not compromised. A good choice is l~1/8 g: in this case, the Q decrease with respect to a conventional TE dual-mode cavity is less than 50%, while the length reduction is about 75%.

Besides the resonating TM120 and TM210 modes, nonresonating modes, i.e. modes that resonate far away from the

operating frequency (TM110, TE111), can also be excited. Such non-resonating modes can be exploited to create an additional input-to-output path. Such a topology provides two symmetric

transmission zeros at real or imaginary frequencies [10-11].

By properly cascading multiple TM dual-mode cavities as building blocks, Nth order filters with N transmission zeros can be realised. As an example, Fig. 4a shows the response of a 4th order filter (photo in Fig. 4b) obtained by combining two TM dual-mode cavities throughout a thick coupling slot. The passband is 9.9-10.1 GHz. Apart from the feeding waveguide, the filter length is only 12 mm. The measured insertion loss for this aluminium prototype is 0.65 dB.

Fig. 4. Two-cavity TM dual-mode filter: (a) measurement (solid lines) and full-wave simulation (dashed lines); (b) manufactured prototype.

IV. HIGH-Q RECONFIGURABLE WAVEGUIDE FILTERS

An innovative concept providing frequency-reconfigurable bandpass waveguide filters with high unloaded Q in has been proposed in [15] employing cantilever MEMS switches [12].

Fig. 5. Proposed solution of a generic one-cavity reconfigurable bandpass filter employing three lines with three cantilever MEMS switches: structure and parameters (a), side view of a cantilever MEMS switch (b), top view of a cantilever MEMS switch (c).

Let us consider the structure depicted in Fig. 5, where the new tuning principle is applied to a TE101 mode rectangular cavity. Ohmic cantilever MEMS switches [12] are cascaded along metallic lines connecting the broad waveguide walls. The low-loss substrate supporting the MEMS lines is located close to the cavity side wall. The effective width a of the cavity can be varied by opening (off-state) or closing (on-state) the MEMS switches of one or more lines.

Two main configurations can be identified. When all MEMS are simultaneously in the on-state the effective cavity width a is reduced so as to increase the resonant frequency of the TE101 mode. The opposite occurs when all MEMS are in the off-state, because the E-field can pass through the interrupted lines. Intermediate resonant frequencies can be also obtained by activating only some of the MEMS lines.

The device with real MEMS can be manufactured by FBK (Fondazione Bruno Kessler, Trento) on a 500 μm thick quartz. Thin biasing lines for MEMS control (~40V) can be led outside the cavity by small slots in the side cavity walls. Undesired radiation through the slots is avoid by cutting them parallel to the current flowing on the waveguide side wall.

In order to validate the concept, a 4th order bandpass filter prototype at X-band has been designed, manufactured and tested. Ansoft HFSS simulations have been carried out by modelling off-state MEMS as its real series capacitance (10fF) and on-state MEMS as its real series resistance (0.9ohm). A photograph of the disassembled measured filter is shown in Fig. 6a. In this first prototype, MEMS switches have been replaced by hardwired connections emulating the two MEMS states, which can alternately be assembled in the filter: continuous copper lines are used for the on-state MEMS, while gaps along the lines are used for the off-state MEMS.

Fig. 6. Hardwired prototype of the 10 GHz 4th order reconfigurable bandpass filter: (a) picture of the disassembled structure with hardwired connections; (b) its measured |s21|. Channel 1: all MEMS lines are in the onstate (solid blue line). Channel 2: the central MEMS line is in the on-state (dashed red line). Channel 3: all MEMS lines are in the off-state (dotted green line).

A total central frequency shift of 400 MHz (4%) has been measured and the unloaded Q is above 900 in all cases. Such results agree with Ansoft HFSS simulations.

A similar tuning concept has been developed in [16] to realise a bandwidth-reconfigurable E-plane filter. The E-plane one-pole filter in Fig. 7 uses ohmic cantilever MEMS switches arranged at both sides of the metallic E-plane coupling septa. The effective length d of each septum can be changed by activating or deactivating the MEMS switches, so as to control the filter couplings and so the bandwidth.

Fig. 7. One-resonator BW-reconfigurable filter employing two lines for each septum side with two cantilever MEMS switches: structure and parameters (a), side (b) and top (c) view of a cantilever MEMS switch (b).

Measurements of a 3rd order bandpass filter at 10 GHz using hardwired connections instead of real MEMS (in Fig. 8) show a relative bandwidth change from 3.4% to 4.8% (~140% tuning) with an unchanged central frequency and a Q above 1100.

Fig. 8. Hardwired prototype of the 10 GHz 3rd order reconfigurable bandpass filter: (a) picture of the disassembled structure with hardwired connections; (b) measured |s21| and |s11| of the filter prototype: narrow BW state (|s21| solid blue line, |s11| dashed red line); wide BW state (|s21| dotted green line, |s11| dot-dashed black line).

V.CONCLUSIONS

Innovative solutions for microwave waveguide filters have recently been developed in order to alleviate the problem of size and weight inherent to waveguide technology. The use of the ridge resonator concept and the TM dual-mode cavity allow for the realisation of a variety of pseudo-elliptic filters with reduced size compared to the conventional counterpart without compromising the waveguide filter low-loss performance (Q>3000).

Two new concepts have been developed for very high-Q reconfigurable rectangular waveguide filters using RF MEMS.

Cantilever MEMS switches can be sequentially cascaded along lines connecting the broad waveguide walls so as to realise either a virtually movable side wall or a variable-length E-plane septum and thus changing the filter central frequency (4%) or the filter bandwidth (140% tuning) respectively. Measurements of two preliminary prototypes using hardwired connections show Q of the order of 1000. Forthcoming work includes the realisation and test of a solution employing real cantilever MEMS.

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