Wide Tuning Range Dielectric Resonator by Optimizing the Tuning Stub Characteristic Impedance

Amir Effendy#, Widad Ismail*

#Agilent Technologies, Bayan Lepas FIZ Phase 3, Penang, Malaysia *School of Electrical &Electronic Engineering, Universiti Sains Malaysia, Nibong Tebal, Penang,

Malaysia

Abstract — A practical approach of getting a wide tuning range dielectric resonator given a small tuning voltage is presented. For this work, a dielectric resonator with resonant frequency of 3.6 GHz was used. The dielectric resonator is placed on a low dielectric constant ceramic standoff. A circular tuning stub coupled to a varactor at one end, is situated underneath the dielectric resonator. Two microstrip stubs are introduced tangential to the dielectric resonator to couple the response out. The tuning stub electrical length and characteristic impedance were varied and the varactor capacitance was swept using 0 – 10 V bias to find the widest tuning range. A tuning stub with an electrical length of λ/2 and a characteristic impedance of 28 Ω yielded 0.42% of tuning bandwidth or 15 MHz at 3.6 GHz.

Index Terms — Ceramics, dielectric resonators, microwave oscillators, resonator filters.

I. INTRODUCTION

In microwave fixed frequency oscillators as well as microwave filters, some limited tuning capability is necessary for center frequency alignment. Dielectric resonator, DR is tuned effectively by perturbing its electromagnetic fields in the excited resonant mode. Several widely known electrical tuning techniques by means of magnetic, varactor, and optic are discussed in these articles by Virdee [1] and Farr et al [2].

As for band pass filters, having an electronic tuning capability allows for bands selection and may also improve adjacent channel rejection. The filter can be controlled to ‘pass’ only the channels it is operating.

Among techniques employed for electronically tuned DR are optical tuning using laser [3], magnetic tuning with ferrite rod [4] and varactor tuning. Many varactor tuned DRs require wide tuning voltage range [5-7] i.e. more than 10 V. Alternatively, multiple DR’s are used to achieve a broadband tuning [8]. A method proposed in this work uses 10 V to achieve wide tuning range DR. It should be applicable either in dielectric resonator oscillators and filters.

II. DR CIRCUIT DESIGN

The DR was excited in TE01δ resonant mode and has a nominal resonant frequency around 3.6 GHz. A low dielectric constant, εr standoff is used to elevate the DR from the printed circuit board. A varactor is coupled via a circular tuning stub printed underneath the DR. A long, thin line feeds the DC tuning voltage to the varactor. Two λ/4 (at 3.6 GHz)

Fig. 1: DR resonant circuit, DR position is marked by dotted circle.

open ended microstrips are placed tangential to the DR to couple the response out via SMA connectors on the other ends. Figure 1 shows the top view of the circuit. The circuit will be placed in a metal enclosure for isolation and to minimize radiation loss.

A. TE01δ Magnetic Field Behavior in the Resonant Circuit

The tuning stub width, which corresponds to the characteristic impedance, Z0 are arbitrarily chosen, while the length is maintained at λ/2. As shown in Fig. 2 (a), (b) and (c) the tuning stubs Z0 are 50 Ω, 34 Ω and 28 Ω respectively. These resonant circuits with different tuning stub Z0 were put into three dimensional electromagnetic field (3D-EM) simulation to analyze the field coupling behavior under different tuning stub widths. In TE01δ resonant mode, the magnetic field (H-field) is exploited for coupling to the transmission lines. Thus, the H-field behavior is of particular interest in the simulation. The resonant circuit model used for the simulation is shown in Fig. 3. The model is simplified to reduce simulation complexity, it excluded the SMA connectors. The SMA connectors are assumed as perfect 50 Ω ports.

The simulation results are presented as the snapshots of H-field pattern in the metal enclosure on the vertical plane crossing the center of the DR and looking towards the output ports, as depicted in Fig. 4. Fig. 4 (a), (b) and (c) show the H-field pattern of the resonant circuit corresponds to circuit in Fig. 3 (a), (b) and (c) respectively. The H-field strength is represented by the contour lines colour shades, the denser the lines and the more towards the red colour the shade

varactor

Tuning stub

Coupling stub

Dielectric resonator

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(following the rainbow spectrum), the stronger the field is. Absence of line and dark blue shade means the weakest H-

Fig. 4: H-field patterns of the resonant circuits; (a), (b), (c) corresponds to circuits in Fig. 2 (a), (b), (c) respectively. field. From Fig. 4, the simulation shows that the resonant circuit of Fig. 2 (c) has the strongest H-field concentration between the DR and the tuning stub – the contour lines and green- yellowish shade covering the region that stretches from the DR to the tuning stub. However, the H-field coupling to the microstrip is the weakest, with hardly noticeable contour lines. On the contrary, the resonant circuit with the tuning stub of Z0 = 50 Ω, the H-field is the weakest in the vicinity of the DR and weak coupling to the tuning stub, but there is a strong H-field presence in the microstrip lines, indicating strong signal.

A. Response of the Resonant Circuits

Each circuit in Fig. 2 was then measured for the tuning bandwidth and insertion loss, IL. The varactor was swept from 0 – 10 V at 1 V interval and the frequency ranges covered were observed. From their frequency tuning plots in Fig. 5, from circuits of Fig. 2 (a) to (b) to (c) the tuning bandwidth increases with circuit of Fig. 2 (c) having 0.42% while still maintaining an approximately linear curve. However as the tuning stubs Z0 decrease, the IL also increases. For the circuit of Fig. 2 (a) the IL is about 3.5 dB;

(a)

(b)

(c)

Fig. 2: DR resonant circuits with different tuning stub Z0

Fig. 3: Resonant circuit model for 3D-EM simulations.

(a) circuit with tuning stub of Z0 = 50 Ω

(b) circuit with tuning stub of Z0 = 34 Ω

(c) circuit with tuning stub of Z0 = 28 Ω

Output ports Dielectric resonator

Metal enclosure b d

Tuning stub Low εr support

Coupling microstrip

Substrate The plane the H-field is observed

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the circuit of Fig. 2 (b), the IL ranging from 4.0 to 4.7 dB and circuit of Fig. 2 (c) from 5.2 to 6.5 dB.

III. CONCLUSSION

Comparing the H-field distributions from the simulations and the measurement results, there is a good correlation between H-field distribution and the tuning bandwidths and insertion losses measurements. For the tuning stub with the lowest Z0 i.e. circuit of Fig. 2 (c) where there is a strong H-field coupling between the DR and the tuning stub, the tuning bandwidth is the biggest i.e. 0.42% or 15 MHz; it also has the highest IL at resonance. With the tuning stub of Z0 = 50 Ω where the simulation shows the weakest field coupling

between the DR and the tuning stub, the measurement shows it has the smallest tuning bandwidth but on the other hand, it has the least IL at resonance as expected since the simulation show strong H-field around the microstrip lines that couple the signal to the output.

The unloaded Q factors, QU of the resonant circuits are plotted in Fig. 6. The Q factor of the structure with tuning stub Z0 = 34 Ω is close to that of 28 Ω, so it is not plotted. As the tuning bandwidth increases QU degrades because the wide tuning stub (i.e. Z0 = 28 Ω) that gives strong coupling to the resonator also incurs higher losses due to dissipation in a wider conductor – the wide tuning stub.

ACKNOWLEDGEMENT

The authors wish to acknowledge the support and funding from Agilent Technologies, in particular Lim Kok Keong of Microwave Modules and Accessories (MMA) and Kang Chia Chiek of Microwave and Communication Division (MCD). The author also wishes to extend the appreciation to Dr. Mandeep Singh who was in Universiti Sains Malaysia for his advice and supervision.

REFERENCES

[1] B. S. Virdee, "Current Techniques For Tuning Dielectric Resonators," in Microwave Journal Norwood, MA: Horizon House Publications Inc, 1998, pp. 130-138.

[2] A. N. Farr, G. N. Blackie, and D. Williams, "Novel Techniques for Electronic Tuning of Dielectric Resonators," in Microwave Conference, 1983. 13th European, 1983, pp. 791-796.

[3] G. Jianping and X. Deming, "Technique for optically tuning dielectric resonators," Electronics Letters, vol. 34, pp. 2137-2138, 1998.

[4] J. Krupka, "Magnetic tuning of cylindrical H01δ-mode dielectric resonators," Microwave Theory and Techniques, IEEE Transactions on, vol. 37, pp. 743-747, 1989.

[5] A. El-Moussaoui, S. Kazeminejad, and D. P. Howson, "Microwave dielectric resonator tuning with balanced loop-varactor circuit," Electronics Letters, vol. 25, pp. 1314-1315, 1989.

[6] B. S. Virdee, "Effective technique for electronically tuning a dielectric resonator," Electronics Letters, vol. 33, pp. 301-302, 1997.

[7] X. Xiaoming and R. Sloan, "Novel varactor tuning of dielectric resonator circuits," Microwave and Guided Wave Letters, IEEE, vol. 9, pp. 105-107, 1999.

[8] D. Paunovic and A. Nesic, "Multichannel Ku-band oscillator with dielectric resonators," Electronics Letters, vol. 25, pp. 1308-1309, 1989.

Fig. 5: Frequency sweep (solid lines) and insertion loss (dotted lines) versus vtune measurements for circuits with tuning stub Z0 (50 Ω) –

, (34 Ω) – and (28 Ω) – , tuning bandwidths shown in percentage.

Fig. 6: Unloaded Q factor, QU of the resonators, 50 Ω tuning stub – and 28 Ω tuning stub – .

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