диафрагмированные волноводные фильтры / 3c672105-bb2c-43fd-8338-e392bdd5f156

Abstract–New concepts are presented for both miniaturized and MEMS-based reconfigurable waveguide filters. The ultimate goal is to reduce size of telecommunication systems without compromising the high unloaded Q required in such applications. One class of miniaturized filters is based on ridges arbitrarily located and oriented within a waveguide allowing for pseudo-elliptic filter responses; the other one is based on cavities employing TM modes as building blocks to obtain Nth order filters with N transmission zeros. Three new concepts have then been developed for high-Q (>1000) bandpass waveguide filters with tunable central frequency or bandwidth using RF MEMS switches.

1. INTRODUCTION

With the advent of multi-standard and multiband communication systems, reconfigurability and miniaturisation are key features for reducing volume, mass and cost in both terrestrial and satellite systems. Despite the numerous advantages of printed circuit technology, waveguide filters are still necessary in applications where low loss and high power handling capability are of great importance [1]. Waveguide filters have however the considerable drawbacks of large size and significant weight. Many efforts have thus been devoted to reduce the size of such filters without degrading their electrical performance. In the last decades, numerous solutions have been developed and proposed in the literature, such as dualor multimode filters exploiting multiple resonant modes within a single physical cavity [2-4], or pseudoelliptic filters realising very sharp frequency responses with relatively low order functions [5-7].

On the other hand, the complexity of multistandard and multi-frequency front-ends can be dramatically reduced by employing tuneable and reconfigurable devices so as to achieve a substantial saving in terms of volume, mass and cost. To this end, tuneable filters with fast tuning

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speed, high unloaded Q (> 500), wide tuning range and low manufacturing cost are required. Tuneable filters are most often based on planar technology, but the unloaded Q is limited by the planar nature of the resonators (Q<300) [8]. Higher Qs, instead, can be obtained by employing waveguide resonators. As is well known, RF MEMS is a very attractive enabling technology for tuneable components thanks to the low insertion loss, low power consumption, compact size, linearity and high integrability [8]. RF MEMS switches allow indeed for high-Q (>500) tunable and reconfigurable filters [9-10].

The basic properties of two innovative waveguide filter classes recently developed are illustrated along with experimental results [11-16] in the first part of this paper. In the second part, new approaches are presented for MEMS-based reconfigurable bandpass filters allowing for very high unloaded Q (> 1000) [17-19].

2. COMPACT WAVEGUIDE FILTERS

This section illustrates two solutions recently developed for compact waveguide pseudo-elliptic filters. The first class is based on ridge resonators [11-13], whereas the second class is based on a TM dual-mode cavity [14-16].

2.1. Ridge Resonator Filters

In Fig. 1 the basic element of the class of ridge resonator filters is depicted: it consists of a rectangular ridge arbitrarily located and oriented within a rectangular waveguide section [11].

The ridge can be realised in slant, transverse or shifted positions with respect to the waveguide longitudinal axis.

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

When w<<l, by neglecting the fringing fields at the ridge ends, the ridge section resonates when l is half a free space 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, in a first approximation, to be that of a TEM mode.

The slant angle and the offset between the ridge and the waveguide axis provide additional degrees of freedom for obtaining pseudo-elliptic filter responses. The ridge position affects the coupling between the dominant TE10 waveguide 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. It is therefore possible to generate both a reflection and a transmission zero (i.e. a singlet), so achieving several filter responses by simply varying the ridge position and orientation within the waveguide [11].

As an example, Fig. 2b shows the response of the 5th order filter of Fig. 2a. The pass-band is 10.1-10.35 GHz. Two transverse offset ridges are used as first and fifth resonators to realise two poles and two lower stop-band transmission zeros; an additional offset ridge is used as the central filter resonator generating a pole and a third lower stop-band transmission zero. Two TE101 mode resonant cavities provide the other two filter poles. The total filter length is 65mm and the measured in-band IL for this aluminium prototype is 0.25 dB.

Nearly-square dual-mode ridge resonators (i.e. where l~w) can be used for other pseudoelliptic filter responses [13].

2.2. TM Dual Mode Filters

In [14-16] the combination of TM dual-mode cavities and non-resonating modes to obtain ultra-compact Nth order waveguide filters with N

Fig. 2. Waveguide filter using rectangular ridge resonators:

(a) manufactured prototype; (b) measurement (solid lines) and full-wave simulation (dashed lines).

Fig. 3. TM dual-mode cavity.

The degenerate modes resonating in the cavity

are the TM120 and the TM210. 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 fulfils the IL requirement.

Besides the resonating TM120 and TM210 modes, non-resonating modes, i.e. modes that

resonate far away from the operating frequency

(TM110, TE111), can also be excited. Such nonresonating 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 [14-15].

By properly cascading multiple TM dualmode cavities as building blocks, Nth order filters with N transmission zeros can be realised.

As an example, Fig. 4b shows the response of a 4th order filter (Fig. 4a) obtained by combining two TM dual-mode cavities through a thick coupling slot. The pass-band is 9.9-10.1 GHz. Without including the feeding waveguides, the filter length is only 12 mm. The measured IL for this aluminium prototype is 0.65 dB.

values of the unloaded Q has been proposed by the authors of this paper in [17]. This solution employs cantilever MEMS switches [8].

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

In [16] an X-band 8th order filter with 8 symmetrical transmission zeros and 1.5% FBW has been manufactured and tested showing an IL of and 0.6 dB.

3. HIGH-Q MEMS-BASED RECONFIGURABLE FILTERS

New solutions for reconfigurable waveguide filters using RF MEMS switches are described in this section. Two solutions allow for frequency tuning [17-18] while a third one is used for reconfiguration of the fractional bandwidth [19]. In order to validate the new tuning approaches, filter prototypes using equivalent hardwired connections emulating the RF MEMS states have been fabricated and measured.

3.1. Virtual Movable Wall

in TE101 Mode Cavity

A new way to provide the central frequency reconfigurability in waveguide filters with high

Fig. 5. TE101 mode reconfigurable waveguide cavity employing three lines with three cantilever MEMS switches.

Let us consider Fig. 5, where the new approach is applied to a rectangular cavity where the fundamental TE101 mode is excited. Ohmic cantilever MEMS switches [8] are cascaded along metallic lines connecting the broad waveguide walls and supported on a low-loss substrate located close to the cavity side wall. Depending on the state of the MEMS switches, the effective width a of the cavity can be varied by opening (off-state) or closing (on-state) the MEMS switches of the lines.

Two main configurations can be identified: when all MEMS are simultaneously in the onstate 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, since in this case the E-field can pass through the interrupted lines.

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

Fig. 6. Hardwired prototype of the X-band 4th order reconfigurable bandpass filter: (a) picture of the disassembled structure; (b) measured |s21|: all MEMS lines are in the on-state (red solid line), all MEMS lines are in the off-state (black dashed line).

the resonant frequency of the fundamental TE101 mode can be changed depending on the MEMS state.

The proposed tuning principle provides a frequency tuning up to 10% and unloaded Q- factors above 1000.

Fig. 7. MEMS-based reconfigurable strip-loaded E-plane resonator.

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.

The device using real MEMS is to 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 avoided by cutting them parallel to the current flowing on the waveguide side wall.

3.2. Reconfigurable Strip-loaded

E-plane Resonators

Another new concept for reconfigurable bandpass filters leading to very high Q-factors (>1000) is proposed in [18]. The reconfigurable filter is based on a rectangular waveguide resonator loaded with an E-plane metal strip patterned on a low-loss substrate.

As depicted in Fig. 7, the conductive strip is connected to both E-plane coupling septa of the resonator by thin conducting lines that can be switched on and off by RF-MEMS. Therefore,

Fig. 8. Hardwired prototype of the 3rd order reconfigurable strip-loaded E-plane bandpass filter: (a) picture of the disassembled structure; (b) measured |s11|: all MEMS lines are in the on-state (red line), all MEMS lines are in the offstate (black line).

In the final version of the filter that is presently under fabrication at FBK, the cantilever MEMS are realised on 500µm thick quartz dies which are mounted on the conducting line lines using flipchip technology. The quartz dies will protrude beyond the waveguide broad wall in order to

allow the connection of the MEMS bias lines out of the guide. Thin slots opened in the waveguide broad do not to interrupt the flowing currents, thus minimising undesired radiation.

The new tuning concept has been validated by fabricating and measuring a 10 GHz 3rd order bandpass filter where MEMS switches have been replaced by hardwired connections (Fig. 8a). A 6.25% frequency shift and unloaded Q-factors above 1050 have been measured in agreement with the HFSS simulations. RL variation due to the tuning is negligible (Fig. 8b).

3.3. BW-reconfigurable E-plane Filters

A similar tuning concept using RF MEMS has been developed to realise a bandwidthreconfigurable waveguide E-plane filter [19].

The rectangular waveguide E-plane resonator of Fig. 9 uses ohmic cantilever MEMS switches arranged at both sides of the metallic E-plane coupling septa of a rectangular waveguide E- plane resonator.

outside of the guide by thin longitudinal slots in the guide broad wall centre in order to avoid undesired radiation.

Fig. 9. BW-reconfigurable E-plane resonator employing two lines for each septum side with two cantilever MEMS switches.

The effective length d of each septum can be changed by activating or deactivating the MEMS switches without changing the resonator length l, thus controlling the filter couplings and so the bandwidth but avoiding frequency shifts of the passband.

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

In the forthcoming implementation of the filter using real MEMS, the switches will be realised on quartz dies and their bias line will be led

Fig. 10. Hardwired prototype of the 10 GHz 3rd order reconfigurable bandpass filter: (a) picture of the disassembled structure; (b) measured |s21|: wide BW state (solid red line); narrow BW state (dashed black line).

4. CONCLUSIONS

Two new solutions for compact 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 counterparts without compromising low-loss performance of the waveguide filter (unloaded Q >3000).

Three new concepts have been recently developed to achieve very high-Q reconfigurable waveguide filters using RF MEMS. Depending on MEMS arrangement, position and state, the bandpass filter central frequencies or the filter bandwidths can be changed. All the proposed solutions allow for very high unloaded Q (>1000). Cantilever MEMS switches realised at FBK (Fondazione Bruno Kessler) in Trento, are employed and are fabricated using a well established eight-mask micro-machined process.

Measurements of preliminary filter prototypes using hardwired connections instead of real MEMS show unloaded Qs of the order of 1000. Forthcoming work includes the realisation and test of devices employing real cantilever MEMS.

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