Web page of project TEC2007-65376

PROJECT TEC2007-65376

  1. Some essential data.
  2. Introduction to the project and main goals.
  3. Results.
  4. Associated Ph.D. Thesis.

Some essential data.

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2.- Introduction to the project and main goals.

SUMMARY:

This proposal is, partially, the continuation of our previous research project (TEC2004-03214). In that project many tasks were part of certain research lines that are expected to have a natural prolongation in the present proposal. Thus, we pretend to continue with several of our traditional general research lines although moving to new directions involving more advanced problems and more practically oriented applications. In particular our investigation will focus on three general subjects:

  1. Excitation problems in planar guiding structures,
  2. Planar periodic structures, and
  3. High performance filters.

MAIN GOALS:

The present proposal can be framed in the analysis of planar passive structures for microwave integrated circuits. It covers theoretical, numerical and experimental aspects of a variety of elements ranging from the basic printed circuit lines to more complex filters, periodic structures, and antennas. More specifically, three different topics will be dealt with in this research project, namely,

  1. Excitation problems in planar guiding systems,
  2. Periodic structures, and
  3. High-performance filters.
Next, each of these topics will be separately considered.

1. EXCITATION PROBLEMS IN PLANAR GUIDING SYSTEMS

The guidance of electromagnetic fields in microwave printed circuits has been a subject of considerable theoretical and experimental interest since long ago. A common simplified way of studying complex system, when possible, is by cascading the responses of individual segments of the systems, such as the guiding elements and discontinuities. Thus many CAD tools of transmission systems are based on approximate models (for instance, a quasi-TEM approach) or they assumed that the only effect of increasing the frequency comes from the dispersive nature of the modes. These approximations are certainly limited in frequency, and for that reason a rigorous treatment of the printed circuit requires the use of very powerful EM simulators, where the guiding elements are also included as part of the geometry to be solved. Clearly the inclusion of these "extended" metallic regions usually involves the use of enormous computational resources, and finally it will be only obtained some specific answer to our particular problem in terms of "numbers" and/or "plots", without too much information about the "physics" of the problem. Thus, it is very convenient to have a deep physical insight into the EM propagation phenomena in order to know the limitation of the approximate models as well as to get a more complete understanding of the EM propagation in planar guiding structures. Despite the maturity of this research area and the many efforts already carried out, there still persist some challenging topics that have not been sufficiently investigated, especially at relatively high frequencies; namely, those frequencies for which the ratio between the height of the line substrate and the free-space wavelength is about or beyond one tenth (see, for example [J. Zehentner at al., Proc. of IEEE MTT-S, pp. 507-510, 2004]). The Microwave Group of the University of Seville has been very active in this subject, mainly by means of a very fruitful collaboration with the Applied Electromagnetic Group of University of Houston, and in particular with Prof. David R. Jackson.

Our common approach to advance in this investigation was to include the effect on a realistic source in a translational-symmetry guiding system (for example, the inclusion of a delta-gap voltage source in an infinite microstrip line). This study has given many fruits, and in particular it should be emphasized that the translational nature of the guiding system has allowed us to deal the problem with quasi-analytical tools, which has made it possible the obtaining of many analytical results physically meaningful [Mesa et al, IEEE-MTT vol.47, pp. 207-215, 1999]. One of the most important contributions is the identification of two different in-nature EM fields excited by the source, namely, the discrete spectrum (DS)and the continuous spectrum (CS). The DS can be associated with the bound propagating modes of the guiding structure (for example the fundamental and higher-order modes of a microstrip line) whereas the CS can be associated with unintended but unavoidable radiation effects (leaky modes and the so-called residual waves).

Further interesting results product of this research line have been systematically presented in IEEE T-MTT and other relevant forums, and have constituted an important part of the investigation supported by previous proposals (see for example [F. Mesa and D.R. Jackson, Wiley Enc. of RF and Microw. Engin. vol.3, pp. 2268-2293, 2005] and references therein).

Nevertheless this subject is far from being closed, and thus there are very important practical issues that have been dealt with in the past and whose investigation should be continued in the future. As an example it could be highlighted the appearance of spurious effects due to the unavoidable excitation of the CS part of the total field [F. Mesa et al. IEEE-MTT vol.50, pp. 2267-2275, 2002]. The spurious effects can make the transmission system response be very different from that expected from standard transmission-line or waveguide theory, and they can take the form of unexpected decays in the energy transmission, unanticipated high levels of coupling with adjacent lines, changes in the expected input impedance, etc. These effects can be caused by the CS field itself and/or by its interference with the DS field. The existence of these effects makes that a proper design of the guiding systems should take into account the conditions that minimize these phenomena. Another topic that has recently been considered is how practical values of losses affect the DS and the CS [J. Bernal et al, Proc. of IEEE MTT-S, pp. 1307-1310, 2006]. It has been found that the CS component seems to be less affected by losses than its corresponding DS counterpart, thus causing that, far from the source, the propagation characteristics of the fields are dominated by the unwanted and "not well controlled" CS components.

Other important questions that are still open and that are proposed as parts of the future research supported by this proposal are the following:

1.1. Probe-fed excitation:

(A) Investigate the equivalent circuit representation for a probe-fed microstrip line (as was already done for the gap feed). This new situation can be more interesting because of the extended use of the probe feeding in printed-circuit lines.

(B) Extend the probe-fed microstrip investigation to the problem of a via interconnect that connects two microstrip lines on different sides of the ground plane (a very practical problem in packaging).

(C) Compare the relative level of high-frequency spurious effects from different types of feeds, such as the gap source, the via-probe feeding, the aperture-coupled feed, etc.

1.2. EMC problems and pulse propagation:

(A) Investigate time-domain current excitation on a lossy microstrip line (a combination of two previous topics dealt with by our group: the lossy microstrip line and time-domain pulse propagation [W. Langston et al, Proc. of IEEE MTT-S, pp. 1311-1314, 2006]). The loss might have interesting effects on the pulse shape far away form the source.

(B) Investigate pulse propagation on coupled lines. This is a problem of interest to the packaging community, who are interested in "far-end crosstalk" and "near-end crosstalk." The "eye diagram" should be obtained to study the level of signal distortion.

(C) Investigate the nature of the field surrounding a microstrip line or covered microstrip line when it is excited by a pulse. This could be extended to the field on a coupled line in order to get a physical insight into the crosstalk fields.

(D) Investigate the nature of the field in the cross section of a microstrip line (constant z) as z changes, to see how the nature of the field changes as the character of the current changes. This could be particularly interesting if loss is assumed, so that the bound mode decays for large distances, leaving eventually the RW current. What do the corresponding fields look like?.

1.3. Sources in complex structures:

(A) Investigate the effects of lateral sidewalls on the current excited by a gap voltage source, and also when there is a slotted upper metallic wall. This latter structure has been proposed as a very efficient leaky-wave antenna,and its rigorous study can bring more practical applications.

(B) Include a realistic source in a periodic 1D/2D structure. This situation is very challenging because of the inclusion of an aperiodic source in a periodic environment. In fact, this sort of problem has hardly been treated in the literature, probably because of its considerable theoretical complexity (in the past, most works dealt only with the computation of the Floquet modal solutions [P. Baccarelli et al., IEEE-MTT, vol. 54, pp. 1350-1362, 2006]).

Some relevant national and international groups related to the above topics are:

2. PLANAR PERIODIC STRUCTURES

Another objective of the current proposal is the implementation of efficient numerical codes for the electromagnetic analysis of planar periodic structures embedded in multilayered dielectric media. In particular, our aim will be focused to implement specific numerical codes for the analysis of planar periodic structures with one-dimensional (1D) periodicity, and additional codes for the analysis of planar periodic structures with two dimensional (2D) periodicity.

2.1. 1-D periodic structures

The codes for the structures with 1D periodicity will be mainly oriented to the computation of the complex wavenumbers and the complex Bloch impedances of the unit cell of planar periodic transmission lines (PPTL) [F.J. Glandorf et al., IEEE-MTT, vol. 35, pp. 336-343, 1987; P. Baccarelli et al., IEEE-MTT, vol. 54, pp. 1350-1362, 2006]. The ABCD matrix of the unit cell can be obtained in terms of these wavenumbers and impedances, and once the ABCD matrix of the unit cell is known, one may easily compute the scattering matrix of a section of PPTL with a finite number of cells without requiring the electromagnetic analysis of the whole structure [C. Y. Ong et al., IEEE Microwave Wireless Components Lett., vol. 12, pp. 264-266, 2002; L. Zhu, IEEE-MTT, vol. 51, pp. 2133-2138, 2003]. Since PPTL show forbidden bands of propagation over certain frequency ranges (in fact, PPTL are a particular case of the so-called electromagnetic bandgap structures), the numerical codes for the characterization of finite sections of PPTL are intended to be used in the design, fabrication and measurement of band-stop and low-pass filters [V. Radisic et al., IEEE Microwave Guided Wave Lett., vol. 8, pp. 69-71, 1998; T. Kim et al., IEEE Microwave Guided Wave Lett., vol. 10, pp. 13-15, 2000]. The slow-wave characteristics exhibited by PPTL are also expected to be exploited to reduce the size of distributed circuit components fabricated with this type of lines [F. R. Yang et al., IEEE-MTT, vol. 47, pp. 1509-1514, 1999; J. Sor et al., IEEE-MTT, vol. 49, pp. 2336-2341, 2006]. Certain PPTL leak power along its length due to the n=-1 space harmonic, which makes them suitable for the fabrication of backward leaky-wave antennas [A. A. Oliner and R. C. Johnson, Leaky Wave Antennas, Antenna Engineering Handbook, 3rd ed. New York: McGraw-Hill, 1993, ch. 10]. These antennas show frequency scanning capability, high directivity and large radiation bandwidths, their fundamental radiation properties (main beam direction and beamwidth) being controlled by the values of the complex wavenumbers of the associated PPTL [K. Potharazu et al., IEEE-AP, vol. 40, pp. 950-958, 1992; A. A. Oliner and R. C. Johnson, Leaky Wave Antennas, Antenna Engineering Handbook, 3rd ed. New York: McGraw-Hill, 1993, ch. 10; J. L. Gómez-Tornero et al., IEEE-AP, vol. 53, pp. 2834-2842, 2005]. In accordance with this, the code for the analysis of PPTL is expected to be used for the design, fabrication and measurement of novel prototypes of planar periodic leaky-wave antennas.

It should be pointed out that the codes for the analysis of planar structures with 1D periodicity can be alternatively used for the analysis of frequency selective surfaces (FSS) that are infinite in one dimension and finite in the other. These FSS support array current surface waves that have not been detected in the analysis of FSS that are infinite in two dimensions [B. A. Munk et al., IEEE-AP, vol. 49, pp. 1782-1793, 2001].2.1 2D periodic structures The numerical codes for the structures with 2D periodicity will be mainly oriented to the determination of the reflection and transmission parameters of FSS that are infinite in two dimensions [R. Mittra et al., Proc. IEEE, vol. 76, pp. 1593-1615, 1988; B. A. Munk, "Frequency selective surfaces", Wiley Interscience, New York, 2000], and to the computation of the dispersion diagram of the modes supported by these structures [M. Bozzi et al., IEEE-AP, vol. 53, pp. 29-35, 2005]. Traditionally, FSS have been applied to the design of dichroic subreflectors of reflector antennas with different feeds working at different frequencies, to the design of hybrid radomes for military platforms, and to the design of polarizers [R. Mittra et al., Proc. IEEE, vol. 76, pp. 1593-1615, 1988]. However, in the last few years, FSS have found new applications in the field of antennas. For instance, conductor backed FSS behave as high-impedance surfaces (or artificial magnetic conductors) that can be used as ground planes of low-profile antennas in order to improve their radiation efficiency [C. R. Simovski et al., IEEE-AP, vol. 53, pp. 908-914, 2005; G. Goussetis et al., IEEE-AP, vol. 54, pp. 82-89, 2006]. Conductor backed FSS can also be used as substrates of printed antennas when the bandgap of the FSS coincides with the operating frequency band of the antennas, which reduces the excitation of surface waves and increases the gain of the antennas [R. Coccioli et al., IEEE-MTT, vol. 47, pp. 2131-2138, 1999], and to model the performance of planar reflectarrays [D.M. Pozar et al., IEEE-AP, vol. 45, pp. 287-296, 1997]. This latter type of antennas is a potential alternative to reflector antennas. In fact, reflectarrays are easier to manufacture than reflector antennas and present less distortion and cross-polarization at the cost of a narrower bandwidth. Bearing in mind the interest recently arisen in this type of antennas, our FSS code will be applied to explore new topologies of reflectarrays. In Spain, Prof. J. A. Encinar from the Polytechnic University of Madrid is a recognized expert in the field of reflectarrays [J. A. Encinar, IEEE-AP, vol. 49, pp. 1403-1410, 2001; J. A. Encinar et al., IEEE-AP, vol. 52, pp. 1138-1148, 2004], and it is expected to establish a collaboration with his group in the frame of this research project, which hopefully will help us to gain experience in the design, fabrication and measurement of reflectarrays.

Finally, it should be mentioned that the code for the analysis of FSS can be easily adapted to the computation of the dispersion diagram of rectangular waveguides periodically loaded with strips or slots. The fundamental mode of these periodically loaded waveguide presents lower and upper cutoff frequencies, which indicates that this type of periodic guiding structure shows potential band-pass properties [M. N. M. Kehn et al., IEEE-AP, vol. 54, pp. 2275-2282, 2006]. Our research group has experimentally verified this behaviour in the case of rectangular waveguides loaded with split ring resonators [R. Marqués et al., Phys. Rev. Lett., vol. 89, pp. 183901-1/4, 2002], and it is intended to check this result with the numerical code developed for the analysis of FSS.

Some research groups that have made relevant contributions in the electromagnetic analysis of 1D/2D planar periodic structures, leaky-wave antennas, and FSS are

3. HIGH PERFORMACE FILTERS.

Filters are essential components of communications systems. At RF and microwave frequencies, the implementation of the required filtering functions can be made by using both lumped parameters and distributed networks, and in particular in modern high-frequency devices, this filtering function is carried out by a wide variety of devices made in planar technology. However, the emerging applications of RF and microwave technologies are becoming more and more stringent about overall filter performance, size, weight and cost, which has pushed the microwave community to explore new ways of realizing the filtering function [J.S. Hong, M.J. Lancaster, Microstrip Filters for RF/Microwave Applications, John Wiley & Sons, New York, 2001]. To have an idea of the explosive increasing of this generic research topic in the last few years, one only needs to look over the recent scientific and technical literature about planar filters. Considering only a few of the leading journals in the microwave area, more than 350 papers have been published since year 2002 on different implementations of microstrip and coplanar waveguide filters to solve a large variety of practical problems. Lots of innovative ideas have recently been reported to improve the above mentioned desired features of many types of filters.

In the past, before the appearance in the market of the nowadays widespread used commercial electromagnetic simulators, filter designers were limited to use architectures and geometries for which tabulated data or analytical formulas were available. Now that this restriction has been removed, a world of new possibilities is open. Nevertheless, simulation capability is not the Holy Grail, and new concepts together with the development of simple circuit models are necessary. Moreover, these models and electromagnetic full-wave simulations can be combined with optimization algorithms to achieve the final goal of designing new competitive filters for the new applications of RF and microwave technologies.

The Microwaves Group of the University of Seville has been working for many years in the fundamental and applied problems behind the development of simulation tools, in particular, those especially conceived to deal with planar structures. Analytical and numerical techniques to characterize the behaviour of planar transmission lines, resonators, radiating patches, and full circuits have been provided in the past by members of this group, whose results can be consulted in more than one hundred of papers published in the most important journals in the field of microwave theories and techniques. These research lines have a place in the frame of the present proposal, as it has been exposed in previous sections. Nevertheless, some members of the group have also been involved in the design of innovative planar passive devices (including filters, of course), and it is expected to continue on this topic in the present proposal. Although commercial simulators will be necessarily used to develop this task, home-made codes will be also very useful for this purpose, as has been happening in recent years. The research field of planar filters is a very wide field, and thus our aim will focused on the following particular topics:

3.1. Compact filters

Miniaturized filters can be used in many applications where dissipative losses are not the bottle-neck of the system requirements (note that small-size microstrip filters used to be relatively lossy when compared with large size distributed circuit implementations). Reducing the size of a microstrip filter can be done in a rather trivial way just choosing a high dielectric constant substrate. However the choice of the substrate may be imposed by other considerations, and thus the circuit size reduction must then be achieved by using appropriately designed building block components. Small size resonators and sections have been proposed in the last few years by many authors (see, for instance, [P. Akkaraekthalin et al., ETRI Journal, vol. 28, n. 5, pp. 607-614, 2006; P.-H. Deng et al., IEEE-MTT, vol. 54, n. 2, pp. 533-539, 2006; M.K. Mandal et al., IEEE-MWCL, vol. 16, n. 1, pp. 46-48, 2006; H. Gan et al., IEEE-MWCL, vol. 16, n. 3, pp. 107-109, 2006] for bandpass filters or [W.-H. Tu et al., IEEE-MTT, vol. 54, n. 10, pp. 3786-3792, 2006]). It is expected to contribute to this research line with new designs improving filter selectivity, band-pass losses and stop-band rejection level and bandwidth.

3.2. Filters using the combination of microstrip and CPW structures

The use of the bottom side of the substrate (commonly used as ground plane) to insert filter components should allow us to increase the design flexibility in order to reduce the overall size of the filter or to eliminate spurious frequencies (the most recent paper using this idea is [P.-H. Deng et al., IEEE-MTT vol. 54, n. 10, pp. 3746-3750, 2006]). Quasi-elliptic filters with very good selectivity and wideband rejection can be fabricated using this methodology. Spurious bands suppression can also be attained in this way. In fact, researcher of the present proposals have already made some contributions in this field [M.C. Velázquez-Ahumada et al., IEEE-MTT, vol. 52, n. 3, pp. 1082-1086, 2004; IEEE-MTT, vol. 53, n. 5, pp. 1823-1828, 2005; IEEE-MTT vol. 55, n. 1, 2007, to appear]. It is believed that this line of reasoning can still give place to additional improvements in filter design.

3.3. Dual band filters

Dual-band filters are key components in dual-band wireless communication systems. Stepped impedance resonators based [Y.P. Zhang et al., IEEE-MTT, vol. 54, n. 10, pp. 3746-3750, 2006], compact dual-band resonator based [M.-L. Lai et al., IEEE-MTT, vol. 54, n. 1, pp. 160-168, 2006] and dual-band filters with embedded low-pass filters embedded [A. Manchec et al., IEEE-MWCL, vol. 16, n. 1, pp. 4-6, 2006] are recently proposed solutions. It is expected to explore in depth these lines of research making use of novel dual band resonators and employing double sided technology. Some selected groups working in the field: there are many groups making a very good work on the topic. A few selected national and international groups close to our interests are

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3.- Results.

In this section we will include those book chapters, journal and conference papers reporting original results of the project.