Single Mode DR Filters for Wireless Base Stations

1 Introduction
    Dielectric resonator (DR) cavity filters have been used for satellite communications since the early 1980s because of their high Q (>10,000) and compactness. Temperature stability [1]-[3] and HEH11 dual mode are regarded as the major breakthroughs in DR cavity filter technology [4],[5]. In the early days, single TE01 mode cavities did not attract much attention for satellite applications because they provided no significant advantage over air-filled cylindrical dual-mode cavities [6] if transmission zeros could not be implemented in the stop band.

    A TE01 mode filter with planar layout offers many advantages over an in-line configuration. The performance requirements for filters and multiplexer networks of wireless base stations and satellite applications are quite different. The cost of each filter and the issue of mass production are much more important than volume and weight in wireless base station applications. In the authors’ opinions, the electrical performance of a state-of-the-art TE01 mode DR filter almost matches the performance of a HE11 dual-mode DR filter because the cross-coupling techniques have been developed for quasi-elliptic function filters. Also, asymmetric filter response with multiple transmission zeros in the stop band have been successfully implemented. TE01 single-mode filters have simple design, flexible layout options, and are cheaper than HE11 dual-mode filters to manufacture. However, they are bigger and heavier. Much literature on the TE01 mode DR cavity filter for wireless base stations can be found in the public domain [7]-[10].

    Another three single-mode DR cavity filters have emerged in the last decade [11]-[14]: half-wave TM mode, dielectric combline, and TM01 mode with both ends shorted. The half-wave has a smaller footprint; the dielectric combline is a pretty good solution in the middle range of cavity Q;  and the TM01 mode with both ends shorted (denoted as TM010) has excellent volume efficiency to deliver cavity Q. However, the difficulty of shortening both ends of a dielectric rod limits the applications of TM010.

    This paper summarizes the technological innovation of single-mode DR cavity filters for wireless base station applications. Section 2 summarizes aspects of cavity performance and design.  Section 3 summarizes configurations with a variety of cross-coupling schemes that can be implemented in single-mode DR cavity filters. Section 4 presents several design examples. Section 5 concludes the paper.

2 Cavity Design and Performance

2.1 TE01 Mode Cavity
    For high Q microwave filters, cavity electrical performance, and size and weight should be assessed simultaneously. This is because high Q microwave filters always occupy a significant amount of space in a transceiver subsystem, especially in L-band.

    Fig. 1 shows a basic configuration of a TE01 mode in a DR cavity. The conductive enclosure can be a circular or rectangular cavity. In order to limit the loss from the conductive enclosure, the cavity diameter (CD) is usually greater than 1.5 times the DR diameter (OD), and the height of the cavity (L) is about 3 times that of DR thickness (t). 

    Typical high Q dielectric material that is commercially available is listed in Table 1. Material with a dielectric constant of 30 may yield the highest dielectric Q. However, it is very expensive and not practical for most applications below 2.5 GHz.

    The design of a TE01 mode DR cavity should take into account cavity Qu, size, and spurious responses simultaneously. These can be computed using a rigorous radial mode-matching technique [15], [16] or with generalized 3-D EM wave simulators such as HFSS or CTS. Cavity Qu, size, and spurious responses are dictated by the DR aspect ratio, which is defined as the ratio of DR diameter (OD) to DR thickness (t) (Fig. 1). The aspect ratio of the DR cavity should be properly chosen; otherwise, the high-order modes may be too close to the working mode. Mode charts [7], [15]-[17] have been proposed for the design of DR cavities. The mode chart of a solid DR of 1.9 GHz TE01 mode with dielectric constant of 44 is shown in Fig. 2 (a). It is also well known that opening a hole in the center of TE01 mode DR can increase the spurious-free region of the cavity (Fig. 2 (b)). This is because the TE01 mode DR cavity has a minimum electric field at the DR center, and all other closer spurious modes have a maximum electric field. The results in Fig. 2 suggest that the aspect ratio of the TE01 mode DR (εr = 44.0) cavity is around 2.5, and the diameter of the center hole can be opened up to 35% of the DR diameter. The relative mode locations in the frequency spectrum are not a strong function of the conductive enclosure, except in the case of TM mode. The mode charts in Fig. 2 [7] suggest that HE11 (HEH11) and HE12 (HEE11) modes are the closer spurious frequencies, but TM01 mode is usually the one that causes interference in pass band if the cavity is not tuned properly. There are three characters and two numbers used for the index of the DR cavity mode. The first two characters comprise TE, TM or HE—which represent transverse E-field, transverse H-field, and hybrid mode. The first number represents the azimuth variation, and the second number represents the order of the modes in the frequency domain according to the condition defined by the previous characters and number. A typical E-field (on the x-y plane) and H-field (on the x-z plane) of TE01 mode are computed by HFSS, as shown in Fig. 3.  These are important for input/output and inter-cavity design for a DR cavity filter.

2.2 Dielectric Combline and TM Mode Cavity
    The high Q TE01 mode dielectric resonator cavity usually has an aspect ratio of the DR puck range from 2.0 to 3.5 to open up a spurious-free region for TEH01 or HEH01 mode.  This aspect ratio ranges from 0.15 to 0.5 for medium Q TM mode, and it looks like a rod rather than a disk. Another point of view is that the dielectric rod plays a similar role to a metal rod in a coaxial resonator. So the basic properties of the TM01 mode, such as field distribution and cavity Q, is similar to a coaxial resonator except that Q degradation in the dielectric rod, which is dominated by  dielectric loss, is much smaller than in the metal rod. Also, the spurious performance can be quite different.

    There are three operating conditions for the DR TM mode filter: with half-wave resonators, quarter-wave resonators, and with both sides of the dielectric rod shorted—TM010 mode (the third number denotes no field variation in the z-direction), as shown in Fig. 4(a), (b) and (c). The characteristics of a 2 GHz cavity configured as shown in Fig. 4(a), (b), and (c) are designed by simulation using HFSS. The performance results using TE01 mode, dual mode HE11 mode, and metallic coaxial resonator are also included for comparison (Table 2). The dielectric constant of the ceramic puck and rod is taken as 45.0, and the loss tangent is 4e-5 (i.e. dielectric Q, Qd=25,000). For the results in Table 2, the metal is assumed to be silver plated. Fig. 5 (a) and (b) show the field distributions of the TM mode cavities; Fig. 5 (c) and (d) are the magnitudes of E- and H-fields of the x-z plane of the dielectric combline;  and Fig. 5 (e), (f) are the TM010 mode. Dielectric combline has a field variation along the z-axis (Fig. 5 (c) and (d)) and its length is pretty much fixed by resonant frequency. The TM010 mode has a uniform field distribution along the z-axis (Fig. 5 (e) and (f)), which means that cavity high can be reduced without affecting resonant frequency. This provides an additional dimension to the trade-off between cavity Q and size.

    The half-wave resonator provides high Q, which can be as high as in TE01 mode.  However, the cavity is much longer than in TE01 mode.  Although the half-wave resonator cavity has a smaller footprint than the TE01 cavity, its overall size is still bigger. The dielectric combline works as a quarter-wave resonator, creating smaller volume and medium-range Q for the DR cavity technology. The TM01 mode resonator with both ends shorted has lower Q but smaller DR volume. For this reason, the cost of ceramic material is much cheaper. When the TM01 mode resonator is the same size as the metallic combline resonator, the metallic coaxial resonator has the smallest cavity Q.

    Table 2 shows high Q design of different modes. With variations in size for each design, the results are shown in Fig. 6. TE01 mode has better Q and volume efficiency than other modes, and it is suitable for high Q applications (cavity > 10,000).  Between 8000 and 10,000, TE01 efficiency drops significantly; thus, TE01 is not practical. Because it has a smaller than TE01, and less ceramic material is needed, dielectric combline fills the gap in this range of applications. Below a cavity Q of 8000, a dielectric combline cavity becomes impractical because the diameter of the dielectric rod needs to be significantly increased.  A bigger puck volume means higher cost. Both-ends shorted TM010 mode also needs to be used. Between cavity Q of 4000 and 8000, TM010 mode DR cavity provides better volume efficiency than a metal coaxial resonator (Fig. 6). Qu and Qu/volume of the combline and TM010 mode DR cavity in Fig. 6 form a continuous performance spectrum as a function of volume and cavity Q. 

3 Filter Topologies with A Variety of Cross-Coupling Schemes
    There is a variety of cross-coupling schemes for microwave filter design that can be used to meet the filter rejection requirement with reduced filter order. The available forms for symmetric responses are: canonical [16]-[18], longitudinal [19], and cascaded quadruplet [20] (Fig.7). For asymmetric responses, cascaded tri-sections [21],[22] and cascaded canonical asymmetric building blocks [23] can be used. The building blocks of asymmetric filter response are shown in Fig. 8.

    Using the building blocks in Fig. 8 for low order filters and cascading two of them for higher order filters (Fig. 9) provides microwave filter designers with alternatives for filter topology and implementation. In Tables 3 and 4, the maximum transmission zeros that can be implemented with this approach are compared with other options and summarized for symmetric and asymmetric filter responses, respectively. The realizable transmission zeros of the approach in Fig. 9 is two less than or the same as the canonical approach for symmetric filter response. It is one less than the canonical approach for asymmetric filters. However, in most filter applications, it is not practical to implement the maximum zeros.  As well as having an advantage over longitudinal and cascaded tri-section in terms of possible zeros, the approach in Fig. 9 also offers a tuning mechanism for independent placement of transmission zeros.

4 Design Examples
    (1) Example 1: 6-pole and 8-pole quasi-elliptic-function filters
    The physical layout and measured results of a 6-pole (5 MHz bandwidth) and 8-pole (15 MHz bandwidth) quasi-elliptic-function filter at PCS frequency are shown in Fig. 10 and
Fig. 11. The 6-pole filter has one 2-5 cross coupling, and the 8-pole filter has 3-6 and 2-7 cross couplings. The 6-pole filter is realized by high Q DR puck with dielectric constant of 29, and the effective-filter unloaded Q is 24,500.

    (2) Example 2: Very high Q (Qu =29,000), 8-pole quasi-elliptic-function filter
In this example, an 8-pole filter with two transmission zeros in each side of the stop band is implemented by cascading two generalized quadruplets. Compared with the symmetric canonical filter in Fig. 11, this filter better balances the transmission zeros on both sides of stop band. The schematic, layout, and measured response of the 8-pole filter are shown in Fig. 12.

    (3) Example 3: 5-pole TE01 mode canonical asymmetric filter 
    The TE01 mode DR cavity is a very interesting microwave filter technology, especially because of its asymmetric filter responses. In Fig. 13, the coupled magnetic field runs perpendicular to the plane of the page when looking at the top view of the planar mechanical layout. More physical detail can be seen in the modal field distribution in Fig. 3. For the cross-coupling scheme of the 5-pole filter in Fig. 13(a), the physical layout and orientation of the magnetic field of the cavity sidewall are shown in Fig. 13 (b). M24 (-), M25 (+) and M15 (-) are implemented by coupling irises. The measured results are shown in Fig. 13(c). Two negative cross-couplings are implemented by an iris that is inductive.  Detailed description and analysis of this filter can be found in [7] and [23].

    (4) Example 4: A 5-pole TM mode canonical asymmetric filter
    Compared with a TE01 mode DR cavity, the TM mode’s filter has a quite different characteristic. For the cross-coupling scheme in a 5-pole filter with cross-couplings M24, M25 and M15 (Fig. 14(a)), the couplings are also implemented by an iris. But they are all positive, which yields three transmission zeros on the high-side stop band. The picture of the filter is Fig. 14(c) and the measured response is shown in Fig. 14(d).

    (5) Example 5: 3-pole elliptic function TE01 mode DR cavity filter and combiner
    A true odd-order elliptic function filter requires non-adjacent coupling between the source or load and an internal resonator. The schematic and measured response of a 3-pole elliptic function filter is shown in Figs. 15(a) and (b). The physical layout and measured response of a combiner based on (a) is shown in Figs. 15(c) and (d). The non-adjacent coupling M03 and input coupling RA are realized by inductive loops to construct proper phase between cavity number one and three for positive cross-coupling.

5 Conclusions
    This paper reviews state-of-the-art single-mode DR cavity filters for wireless base stations.  The cavity characteristics of three TM modes at 2 GHz are analyzed using HFSS. The design of DR cavity filter for high, medium, and low Q operation is compared with the design for TE01 DR cavity modes. TE01 mode is suitable for high Q (>12,000) applications, dielectric combline is suitable for medium Q (8000-12,000) applications, and TM010 mode is suitable for lower Q (<8000) applications. A variety of cross-coupling schemes for implementing symmetric and asymmetric transmission zeros are presented, and topology based on cascading canonical asymmetric building blocks is discussed. 

    Designs with excellent measured performance are presented, with 6 and 8-pole quasi-elliptic-function filters taken as examples. An 8-pole filter with two transmission zeros on each stop band is an example of how, by cascading asymmetric building blocks, symmetric transmission zeros can be better balanced.  This kind of 8-pole filter is high Q (around 30,000), and represents state-of-the-art high Q dielectric material technology. A 5-pole DR TE01 mode canonical asymmetric filter is an example of how three low-side transmission zeros can be implemented by three non-adjacent coupling irises. For a DR TM010 mode cavity filter, three non-adjacent coupling irises can yield three high-side transmission zeros. The combining of a DR TM010 mode cavity filter with a 3-pole elliptic function filter highlights the progress that has been made in single mode DR cavity filters for wireless base stations.

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[Abstract] This paper presents state-of-the-art high Q single-mode dielectric resonator (DR) cavity filters for PCS wireless base stations. DR cavity filters shrink the cavity size significantly more than waveguide cavity filters and offer about twice higher Q than coaxial resonators. Thus, they have important applications in wireless base stations operating below 2.5 GHz. Dual-mode and triple-mode DR cavity filters have existed for a while; however, single-mode DR cavity filters are predominant because they are cheaper to manufacture. This paper summarizes the main characteristics of TE01 mode DR cavities, including mode chart and field distribution, and compares cavity Q with waveguide and combline (coaxial) cavities. Dielectric combline and TM010 mode DR cavities are analyzed and compared to TE01 mode DR cavities. General filter design techniques are discussed, and several design examples are given to show how filter technology has developed.

[Keywords] microwave resonator; cavity filter; DR cavity; dielectric resonator loaded cavity