диафрагмированные волноводные фильтры / db3556df-479d-4253-a966-17f35757b695

Proceedings ofAPMC2001, Taipei, Taiwan, R.O.C.

Millimeter Wave Filters for Low-Cost

Mass-Fabrication

R. Vahldieck and E. afli

(Invited Paper)

Swiss FederalInstitute a/Technology, IFH, ETH Zentrum,

Gloriastrasse 35, CH - 8092 Zurich, vah@ifh.ee.ethz.ch

This paper presents an overview of millimeter wave filters suitable for mass-fabrication. A new type of cross-coupled E-plane filters is introduced and compared with standard design approaches.

Qnasi-planar filters as stand-alone component or for integration with active devices are reviewed.

Cavity-type filters for TEIO-rnode or multimode operation with harmonic suppression are discussed and possibilities for plastic injection moulding mass-fabrication are described.

1Introduction

The pace of development of new systems incorporating satellite, optical cable and radio transmission technologies covering a wide range of frequencies, has put a lot of pressure on fast turn-around time for components, sub-systems and systems and, of course, on lower prices. This has created a number of new challenges not only in the lower microwave region but also and in particular at millimeter wave frequencies. The success of broadband wireless links at millimeter wave frequencies depends to a large extend on the availability of low cost components. Among them, filters and diplexers are of particular concern since they are in many cases the single most expensive component in a millimeter wave system. Their electrical performance is crucial for the overallsystem design. Low-insertion loss, high return loss, high slope selectivity, often also harmonic suppression as well as low-cost are simultaneously required for filters in millimeter wave systems.

Among the large variety of possible filter structures that potentially satisfy some or all of the above criteria, only few are really suitable for low-cost mass fabrication. Among them quasi­ planar filters, direct coupled cavity filters, E-plane metal insert filters or derivatives of it are very attractive. Quasi-planar filters utilize a supporting substrate on which the actual filter structure is printed. Here the repeatable accuracy is provided by etching technology, which is a great advantage in mass-production. However, the more the filter function is determined by the printed structure the lower the Q-factor becomes. In contrast, E-plane metal insert filters or iris-coupled cavity filters exhibit a moderatly high Q-factor, significantly higher than that of quasi-planar filtcrs, and are therefore more suitable for frequency selective front-end applications demanding low insertion loss. For high-volume application and if a sufficiently high tolerance margin between signal bandwidth and actual filter bandwidth is allowed, plastic injection moulding technique with subsequent surface metallization can be applied to make also these types of filters suitable for low-cost mass-production.

In the following, the above filter types are discussed with respect to performance and suitabilityfor mass-fabrication.

2Quasi-planar Filters

There are different classes of quasi-planar filters. They all have in common that the planar circuit is embedded in a waveguide housing. The differences between the various classes is that in some cases the planar structure is positioned parallel to the E-field of the TEw-mode of the waveguide and in some cases perpendicular to it. While in the first case, the planar structure affccts the fundamental mode of the waveguide such that the required filter effect is

achieved, the role of the waveguide housing in the second case is that of a shielding ground plane only. The filter function is solely perfonned by the printed circuit suspended in the waveguide. In this application, the waveguide dimensions must be chosen such that the operating frequency is always below the fundamental mode of the waveguide. These filters are based on TEM-mode operation. In contrast, the E-plane structures are based on HElOmode operation, show strongly dispersive behaviour and do not allow truly low-pass function. Finline filters made from bilateral, uniplanar or antipodal metallization are the most frequently used structures. Due to their higher unloaded Q-factor ( 1500) and the potential for integration with active devices, they are frequently used as bandpass filters in integrated front-end applications. An example for a bilateral large-gap finline filter fabricated on the same substrate as other circuits for a RX front-end is shown in Fig.3. An overview of quasi­ planar filters is given in [1]. A third class of quasi-planar filters is neither based on TEM­ mode propagation nor on HElO-mode operation, but utilize the HE 010 -mode. An interesting design which is suitable for mass-fabrication and at the same time allows for highly compact filters is given in [2]. Here the HEolO -mode is excited in a dielectric ring structure which is fanned on a high pennittivity substrate with bilateral metallization (see Fig.2). The ring resonator dimensions are obtained very accurately by etching techniques. The electromagnetic field is highly confined within the substrate area not covered by the metallization and thus very small filter components can be made. Although the unloaded Q of this solution is only in the range of about 1500, this is sufficient for a wide range of applications. Furthermore, the structure is largely independent of housing tolerances and thus especially suitable for mass­ fabrication. For drop-in applications the input/output coupling is realized through microstrip lines.

3E-Plane Metal Insert Filters

The perfonnance of these filters is essentially detennined by the metallization pattern of the insert. The insert thickness ranges from 50-100J..Lmand can be fabricated by photolithographic or electrofonning techniques with high accuracy and repeatability. For large volume application, several problems exist with this type of filter. For high slope-selectivity, in particular for diplexer application with sharp cutoff between the channels to avoid cross-talk, direct-coupled E-plane filters must consist of a large number of resonators to satisfy the specifications. This in turn increases the insertion loss, makes the filters quite long and increases the sensitivity with respect to manufacturing tolerances ofthe waveguide housing as well as the metal insert. An example for a 38GHz E-plane diplexer with quite demanding requirements is shown in Fig.5. Both 9-rcsonator filters were fabricated on the same metal insert made from a 50J..mL-thick silver-plated Nickel sheet using an electrofonning process for low tolerances « 3J..m)L. To reduce the number of resonators other E-plane filter solutions are currently implemented. One solution is to add an inductively coupled stopband section in front and behind the filter as shown in Fig.6a [3]. This increases the slope selectivity significantly without increasing the number of resonators. Another solution is to use folded E­ plane metal insert filters and then introduce cross-coupling between the resonators as shown in Fig.6b [4]. This solution has been developed recently by the author and his group and leads to very compact filter components. The transfer characteristics of these filters provide a much steeper slope selectivity than comparable direct coupled E-plane filters. Therefore, filter specifications can be met with a lower number of resonators and therefore with lower insertion loss. For low-cost mass-fabrication the separation wall between the two waveguides is made from a thin metal sheet so that the cross-coupling between the resonators can also be made with high-precision etching techniques. The positioning and opening of the cross­ couplings are calculated based on the mode matching technique, following a similar procedure as outlined in the design of E-plane filters (i.e. [5]). Fig.4 sketches the additional

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steps necessary. The cross coupling between the resonators consist of an opening in the waveguide wall extending over the full height of the waveguide. Thus only TEmo-modes need to be considered. The bifurcations left and right from region 5 are then described by generalized 3-port S-matrices connecting to the generalized S-matrices of the E-plane filters in the different waveguide channels. The overall filter design is then optimized to specifications.

4Cavity Filters

Among the large variety of cavity filters only few are suitable for mass-fabrication. Of particular interest are iris coupled cavity filters where the iris extends over thc full height of the waveguide (i.e. [6]). From the design point of view this allows accurate simulation of the structure based on the TEmo-mode approach. The MMT or CIET methods have been used with great success (i.e. [7]).To satisfy with these filters the requirements for close frequency spacing between Rx and Tx channels, asymmetric responses are needed. This can be achieved by additional attenuation poles at finite frequencies. To create such poles in direct-coupled waveguide filters, the use of strongly dispersive inverter sections is suggested in [8]. Results are shown in Fig.7c. This design principle has been incorporated into a diplexer at 28GHz [9]. The transfer characteristics of the diplexer, which is fabricated in plastic injection moulding technique, is shown in Fig. 7a. The excellent temperature performance is illustrated in Fig.7b. Another interesting application is the use of asymmetric iris-coupled structures as dual-mode filters with asymmetric transfer characteristic (i.e. [ l 0]). As shown in Fig. 8, a six-pole filter with three transmission zcros has been produced for 39GHz. Mode coupling takes place between the TE102and the TE2o)-mode. The input and output cavities will each produce two transmission poles and two transmission zeros with both transmission zeros located at the same frequency. Also for this application, excellent agreement between measured and calculated response can be stated, which makes also this configuration an excellent candidate for low-cost millimcter wave filters.

5Ileferences

[1]R. Vahldieck, "Quasi-planar filters for millimeter wave applications", Invited Paper, IEEE Trans. Microwave Theory, MTT-37, pp. 324-334, February 1989.

[2]Y. Ishikawa, T. Hiratsuka, S. Yamashita, and K.Iio, "Planar type dielectric resonator filter at millimeter wave frequency", IErCE Trans. , vol. E79-C, no. 5, pp. 679-684, 1996.

[3]J. Bornemann, "Selectivity-improved E-plane filter for millimetre-wave applications", Electron. Lett., vol.27, pp. 1891-1893, Oct. 1991.

[4]E. Ofli, R. Vahldieck, and S.Amari, " Analysis and design of mass-producible cross­ coupled, folded E-plane filters", in IEEE MTT-S Int. Microwave Symp. Dig., 2001, pp.

1775-1778.

[5]R. Vahldieck, J. Bornemann, F. Arndt, and D. Grauerholz, "Optimized waveguide E-plane metal insert filters for millimeter wave applications", IEEE Trans. Microwave Theory

Tech., vol. MTT-31, pp. 65-69, Jan. 1983.

[6]U. Rosenberg, "New planar waveguide cavity elliptic function filters", Proc. 25th EuMC,

pp.524-527, Sept. 1995.

[7]J. Bornemann, S. Amari and R. Vahldicck, "A combined mode-matching and coupled­

integral-equations technique for the design of narrow-band H-plane waveguide diplexers",

IEEE AP-S Int. Symp. Dig., 1999, pp. 950-953.

[8] S.Amari, J. Bornemann, W. Menzel and F. Alessandri, "Diplexer design using pre­ synthcsizcd waveguide filters with strongly dispersive inverters", IEEE MTT-S Int. Microwave Symp. Dig., 2001, pp. 1627-1630.

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[9]W. Menzel, F. Alessandri, A. Plattner and J. Bornemann, "Planar integrated waveguide diplexer for low-loss millimeter wave applications", Proc. 27th European Microwave Canf., pp. 676-680, Jerusalem, Sep. 1997.

[10]M. Guglielmi, O. Roquebrun, P. Jarry, E. Kerherve, M. Capurso, M. Piloni, "Low-cost dual-mode asymmetric filters in rectangular waveguide", IEEE MTT-S Int. Microwave

Symp. Dig., 2001, pp. 1787-1790.

Fig.1 Quasi-planar filters. a) Bilateral finline structure, printed circuit parallel to E­ field; b) suspended microstrip structure, printed circuit perpendicular to E-field and waveguide dimensions below cutoff; c) TEolO-mode resonator and b­ dimensions below cutoff.

Fig. 2 Example of a TEolo-mode filter, according to [ 2).

5]6

Fig. 5 E-plane metal insert diplexer for LMDS application in the 38GHz range (courtesv Huber+Suhner AG. Switzerland).

Fig.6Two different solutions to improve the skirt selectivity of E-plane filters.

a) Utilizing inductively coupled stopband section [3] and b) utilizing cross­ coupled resonators [4J.

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Tuning screw positions

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