Review on Dielectric Resonator Filter

Home , Dielectric resonator

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

Charu Dubey1, Md Rashid Mahmood2 1 Department of Electronics and Communication Engineering, CET IILM AHL, Greater Noida 2 Department of Electronics and Communication Engineering, I.T.S Engineering College, Greater Noida [email protected], [email protected]

Abstract:- This article describes some types of Dielectric Resonator Filter Design and the procedure which can be applied to design the filters. Section I describes the Transversal Dielectric Resonator Filter Design, Section II describes the Ferrite Tuned Dual-Mode Dielectric Resonator Filter, Section III describes the Dielectric Resonator Filters Fabricated from High-K Ceramic Substrates, Section IV describes the Dual-Mode Half-Cut Dielectric Resonator Filters.

Keywords – Dielectric resonator filter, microstrip filter, resonator filter

I. INTRODUCTION In this section, a procedure to synthesize a new transversal filter has been presented. The C.A.D. design and F.E.M. shows that a small modification of the external quality factor permits to optimize the transmission coefficient response. Considering the transversal network representation (figure 4), where D.R. are independent, the analysis is subsequently different than the previous one. Equivalents network 1 (figure 1) and 2 (figure 2) are two different representation of a conventional filter. We propose to determine a relation between the equivalent circuit elements[1].

Fig 1: Conventional three poles filter equivalent circuit

Fig. 2: Equivalent transversal representation of a conventional three pole filter.

To study the transversal representation, which is a parallel association of resonant structures, the admittance parameters matrix can be employed. Subsequently, admittance parameters matrix is also calculated for equivalent network as shown in figure 1. Comparing the admittance matrix, we obtain relation (1) between the different resonant circuits:

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

2 2 2 2 Z (Z + M ω ) = Z1.Z2.Z3 (1) Where, Z is the equivalent impedance of the classical filter D.R.

Z = R+j(ωL- 1/ωC) (2)

Zi, represented the equivalent impedance of the transversal filter D.R.

Zi = Ri+j(ωLi- 1/ωCi) (3)

Note that resistances R and Ri, express filter insertion loss. The determination of the zero location of (1) gives the resonant frequencies f l . f2 md f3 from the following relations. R = Ri C=Ci

L1 = L-√2M, f2=f0 (4) L2=L

L3=L+√2M

The three D.R. introduced in the transversal structure are now characterized. Expressions (4) must be introduced into the admittance matrix to give a relation between transformation ratios NI. We obtain: N3=N4=N7=N6=N1/2 N5=-N6=N1/√2 A minus sign added to N6, express that the phase difference between the second D R. and other is 180 degrees[2].

SECTION II- Dual-mode filter in which two resonances are coupled in a single structure, have significant advantages of weight and size. Dual-mode dielectric resonator filters have been reported by several authors. For this work a ferrite tuned dual-mode filter is designed and constructed which offers excellent temperature and electrical characteristics. The mode of operation is the degenerate hybrid mode. The input/output coupling is achieved by using two SMA type coaxial connectors in which the centre pin is extended into the cavity, positioned at 900 to each other in the maximum radial electric field. Coupling between the two modes is achieved by means of a coupling screw oriented at 450 with respect to the electrical fields of the two orthogonal modes as shown in Figure 3. For independent tuning, two tuning screws are mounted at the direction of maximum electric field of each mode, being perpendicular to each other and each facing to one of the coaxial probes[3].

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

Figure 3. Dual-mode ferrite tuned DR

Experiments have been performed which show that dual-mode as well as the previously reported single mode filters can be magnetically tuned using ferrite. This is accomplished by inserting a thin ferrite rod through an axial hole in the dielectric resonator. The magnetic field controls the permeability of the ferrite and hence the resonant frequency of the system. The tuning range is a function of the ferrite properties and also the magnetic field applied. In this experiment two different types of ferrite have been investigated. In both cases the tuning of the dual mode filter is achieved either at below resonance or above resonance state of the ferrite[4]. The tuning in the resonance state of the ferrite is strongly affected by the spin wave properties of the ferrite. In the first part a Nickel ferrite rod (Trans-Tech. TT2.113) of dimension 4 mm diameter x 16 mm height was a close fit inside a ring dielectric resonator of 35.5 mm diameter (Trans- Tech.C88(10)1400(Y)630(B)(162)) housed inside a circular metal cavity as shown in Figure 4. The presence of ferrite does not alters the dual mode coupling and the loss introduced by the ferrite is negligible. A variable electromagnet provided the magnetic field necessary to tune the filter. Tuning range of 0.7 MHz and 5 MHz are obtained for the cases of below resonance and above resonance respectively. In the second experiment the Nickel ferrite is replaced by an Aluminum Doped Yttrium Garnet (Trans-Tech G-1009) which has a lower saturation magnetization of 175. The tuning range obtained below resonance is about 1.5 MHz and furthermore above resonance, the tuning range is similar to the Nickel ferrite. This is shown in Figure 5. The main difference of the two types of ferrites is in their saturation magnetization. This has a direct effect on the amount of necessary DC magnetic field required to tune the dual-mode filter. It is observed that the Yttrium Garnet in a comparison to the Nickel ferrite, offers more tuning range in the below resonance state, but more importantly due to its lower saturation magnetization (4 ;T Ms) requires less magnetic field to reach above resonance state. It is noteworthy that the dual-mode response is preserved over a reasonable frequency range as the magnetic field is altered[5].

Fig 4: Sorted Ring Filter

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

Fig 5: Dual-mode frequency response SECTION III- In this section, a new configuration of DR filters is presented that is suitable for low-cost mass production. The dielectric resonators are constructed as one piece from a substrate of high-K materials. All the resonators are connected one to another by the same high-K dielectric material, resulting in a whole piece which can be efficiently and accurately cut to the desired dimensions. The substrate can be easily cut using low cost water jet machining technology [6]. The one piece dielectric resonator filter is assembled inside the filter housing using a support made of a low-K substrate such as Teflon. Thus, the assembly of an N-pole filter reduces from the assembly of N individual resonators to the assembly of one dielectric substrate. This makes the assembly, integration and alignment of the resonators very simple so that the time and cost can be greatly reduced. Two example filters have been designed, fabricated and measured. The measured results exhibit an excellent RF performance verifying the proposed unique concept. The inter-resonator coupling and input/output coupling can be synthesized from the elements of the filter coupling matrix. The coupling coefficient between the two resonators depends on the spacing and the shape of the gap. In order to calculate the coupling, EM HFSS simulations of two identical coupled resonators have been carried out for various values of the inter-resonator distance D shown in Fig. 6, with the calculation of the even and odd resonance frequencies fe and fm. 2 2 2 2 The normalized coupling K is then given by(fm -fe )/(fm ±fe ) A plot of the calculated coupling coefficient versus D is given in Fig. 7. With the knowledge of the required coupling value the spacing D can be determined [7].

Fig 6: Simulation model of inter-resonator coupling coefficient (Top view).

Fig 7: Calculated coupling coefficient.

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

The probes are placed co-directional with the E-field of I/0 resonators to excite the TE01 mode inside the resonators. The desired external Q is obtained by varying the I/O coupling between the probes and the end resonators, which is controlled by changing the length of the probes shown in Fig. 8. The calculated external quality factor Q, versus the probe length is presented in Fig. 9.

Fig 8:Simulation model of external quality factor

Fig 9: Calculated external quality factor SECTION IV- Dielectric Resonators (DR) are traditionally employed as single-mode resonators, dual-mode resonators or triple- mode resonators. There has been limited interest to investigate the possibility of realizing quadruple-mode dielectric resonators, mainly because quadruple-mode filters are not easy to tune. The coupling and tuning mechanisms must permit an independent control of the four operating modes. In practice, having many screws in one cavity increases complexity. This problem is more pronounced in quadruple mode filters but also exists in triple mode and traditional dual-mode DR filters. Dual-mode operation in dielectric resonators is not restricted to traditional cylindrical resonators. A half cylinder resonator (half-cut) can also support dual-mode operation. The fabricated half-cut dual mode filter measurements verify the designs. A considerable size reduction is achieved in comparison with traditional dual mode DR filters [8].

A. Dual Mode Cavity- To design a dual mode cavity, the operating modes must resonate at the same frequency equal to the centre frequency of the filter. Given a half cylinder with dielectric material of  r in free space, there exists one unique value for D and L that results in the desired dual mode resonator. Since the two modes are Eigen modes of the structure, they are orthogonal, and can coexist at the same frequency without coupling [9].

B. Intra-cavity coupling- The cavity supports various coupling amounts between the two orthogonal modes inside the cavity. An effective coupling mechanism is a metal screw or rod placed as shown in Fig.10 designated by Screw 1. Various parameters such as distance of the screw from the edge of the resonator, length and diameter of screw, have effect on the coupling amount.

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

Fig 10: (left) front view and (right) top view of the dual mode cavity with coupling and tuning mechanisms Using an eigenmode solver, the cavity, DR and coupling screw are simulated together, and the first two resonant frequencies are found. The coupling coefficient, k, is

- (5)

By sweeping the design variable (e.g. screws size), the desired coupling is interpolated.

C. Inter-cavity coupling- An effective method for inter-cavity coupling is usage of polarization discriminate iris in cavity walls. For coupling of ½HEH11 mode, a vertical iris is required, as shown in Fig. 11. A horizontal iris however couples the ½HEE11 mode. This is somewhat intuitive when considering the iris as a waveguide under cutoff, passing mainly one polarization of field. In either case, parameters such as width, thickness and length of iris determine amount of coupling.

Fig11: Orthogonal coupling irises for HEE and HEH modes The coupling coefficient can be computed using Sparameter method, finding eigenmodes and using, or alternatively using a symmetry plane placed in the middle of the iris, finding the even and odd ( fe , fm ) resonances of the mode coupled, and using the formula [10]

2 2 2 2 K=(fe -fm )/(fe +fm )

As shown in Fig. 12, simultaneous cross couplings between non-adjacent resonators are achievable to realize advanced filtering functions.

Fig12: Advanced coupling methods between two cavities and S/L. D. Input coupling-

Proceedings of National Conference on Recent Advances in Electronics and Communication Engineering (RACE-2014), 28-29 March 2014

Input coupling to the filter is realized using either coaxial probes or waveguides. Length of probe and its distance from the resonator, allow for realization of various input coupling values. A probe placed as in Fig.10 would only couple energy to ½HEH11 mode, as the other mode is orthogonal. For waveguide coupling either a horizontal or vertical iris is used to couple to the desired mode, with appropriate iris size. Tuning of center frequencies of resonators is also essential, as exact sizes with ceramic resonators are normally hard to achieve, as well as deviations in supplier’s dielectric constant, and other fabrication tolerances [11]. Metal screws parallel to the main sides of the resonator provide considerable tuning. With the configuration shown in Fig.10, Screw 2 tunes ½HEH11 and Screw 3 tunes ½HEE11. They tune the modes almost independently, due to orthogonality of modes [12].

II CONCLUSION

In the past four decades, tremendous advances in microwave filter technology have been made. There are filter types for almost every usage: waveguide filters for low-loss characteristics, transmission-line filters for compactness, ferromagnetic resonators for filters that can be magnetically tuned, and many other filter types for special purposes. In spite of all these advances, there are still applications requiring newer filter types. For example, in 1- 2-GHz radio systems, waveguide filters are physically too cumbersome to use, and transmission-line filters are too Iossy for narrow-band applications. Also, low-loss and integrable filters are needed to replace the integrable but lossy transmission-line filters at higher frequencies. Dielectric resonators hold promise to fulfill these demands. In fact, dielectric resonator filters have already been designed and used in radio systems ranging from 1.7 to 7 GHz, and their applications are expected to grow rapidly in the future. An important feature of dielectric resonators is the design variety that they offer because they are small and are easily coupled to microstrip or stripline, coaxial probes, or waveguide.

III REFRENCES

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