Current Techniques for Tuning Dielectric Resonators

Current Techniques for Tuning Dielectric Resonators

Electronic tuning of dielectric resonators over a relatively wide range while also preserving their characteristics is highly desirable, especially for use in nonaccessable systems such as airborne or spaceborne systems or in areas of high sophistication such as cellular radio systems. This article reviews the advantages and disadvantages of the various electronic tuning techniques available currently.

B.S. Virdee
University of North London, School of Communications Technology & Mathematical Sciences
London, UK

A survey of communication systems, particularly those with working frequencies ranging from 0.5 to 20 GHz, reveals that an urgent need exists for good quality, small, competitive and, more importantly, tunable resonant devices. Examples include UHF mobile radio systems (toward the lower frequencies of this range) and radar and satellite communications systems (toward the higher frequencies). It is evident that the requirements of such a device depend on the type of application. For example, the important prerequisite for a cell site base station power combiner used in a radio mobile station is a high Q filter capable of handling power in the range of tens of watts. For airborne and spaceborne systems such as satellite communications, the most important factors usually are a heavy constraint on both size and weight and strict requirements on the device tuning speed for frequency hopping or rapidly tuning the receiver front-end filters/preselectors.

Except for tunability, most of these requirements are fulfilled by a resonant ceramic material known as a dielectric resonator (DR). Although initially developed in 1939,¹ these ceramics did not find applications in microwave circuits until the late 1960s^2,3 because of the uncontrolled temperature coefficient of the resonant frequency. The first application appeared in 1968 when the DR was used as a microwave filter element. Since then, utilization of DRs in both filters and oscillators has proliferated.

Currently, DRs are considered one of the more important microwave devices. Besides being versatile and adaptable to various microwave structures and configurations, they fill a gap that exists between waveguide and stripline technologies. The ceramics provide quality factors and temperature stability better than those of invar cavity resonators along with an integration capability approaching that of stripline resonators. DRs are also progressively replacing the large and costly metal cavity resonator in almost all applications.

Commonly cylindrical in shape, DRs sustain the same electromagnetic field modes as those of a conventional metallic resonator cavity and behave in a similar way, except that the electromagnetic fields extend beyond the bulk of the device.⁴ For this reason, DRs are usually placed inside a metal shield to prevent external interactions.

Why is Electronic Tuning of DRs Needed?

Most DRs implemented in microwave circuits (such as filters and VCOs) are tuned or adjusted mechanically. This tuning is achieved by perturbing the field distribution surrounding the DR through the movement of metallic or dielectric objects (either manually or electrically) by using, for example, a step motor.⁵

It is obvious that mechanical tuning involves mechanical parts in motion. Therefore, the process is slow and inaccurate and, hence, not highly reliable. Even if this approach is satisfactory in some cases, it is certainly not suitable for applications such as frequency hopping or frequency modulation where high tuning speed is needed. Moreover, some skill is needed on the part of the engineer or technician making the adjustment on site; this requirement can make the tuning expensive.

Electronic tuning of DRs over a wide range while preserving their characteristics is highly desirable, especially for use in inaccessible systems such as airborne or spaceborne systems or in areas of high sophistication such as cellular radio systems. Automatic adjustment or tuning that is controlled, if necessary, from a distance via a communication link then must be considered.

Tuning Techniques

Essentially, DR tuning can be accomplished by modifying the various electromagnetic fields supported by the resonator. Currently, three principal nonmechanical techniques are available that are capable of achieving this tuning: varactor, ferrite and optical. However, from a practical point of view, ferrite and optical tuning have a number of difficulties associated with them, which are discussed in this article. Consequently, the approach most commonly adopted relies on the electronic tuning afforded by coupling varactor diodes to the DR.

A DR can be tuned over an approximate 10 percent bandwidth by perturbing the resonator's magnetic field. This tuning can be achieved by varying the air gap between the ceramic disc and metal enclosure with, for example, a movable metal plate or screw coaxial with the ceramic disc. The tuning mechanism involved here can be explained by the perturbational principle.⁶ Namely, when a metal wall of any resonant cavity is moved inward, the change in resonant frequency is proportional to the difference in stored magnetic and electric energies with the displaced volume.

With electronic tuning, since there is no volume displaced, the frequency variation is related to the change in the energy stored in the reactance of the system. In both cases, it is evident that the adjustment of the resonant frequency to a prescribed value is based on the perturbation of the fringing fields outside the resonator. For a particular mode, the tuning range is determined by the amount of this perturbation. For example, if the tuning mechanism increases the stored magnetic energy with respect to the electric energy, the resonant frequency will shift toward higher frequencies⁴ with a deviation proportional to the energy difference. If the tuning mechanism decreases, the shift will be toward lower frequencies. Thus, a wide tuning bandwidth requires a strong perturbation, which may degrade the system quality drastically. A compromise between the tuning bandwidth and DR qualities is necessary.

DR frequency control may be accomplished either by solid-state devices such as varactors or PIN diodes, or low loss magnetic materials such as a single-crystal yttrium iron garnet sphere or disc ferrite. Disc ferrite materials are based on the control of the magnetic field and lead to a substantial tuning range of 250 MHz at 8 GHz while maintaining a high resonator unloaded Q (greater than 1000).⁷ However, like most of the mechanisms based on magnetic materials, methods utilizing ferrite materials usually require a large and cumbersome electromagnet for producing the controlling magnetic field. Furthermore, because they are current driven, the materials' tuning speed usually is very slow (» 0.5 MHz/s) and subject to phase noise. In addition, because of the nature of the ferrite material, the electronic tuning characteristic usually has hysteresis associated with it. These features negate some of the advantages offered by DR circuits, particularly in view of the stringent power and volume requirements of many modern systems. Figure 1 shows the configuration used for this type of tuning.

Faster tuning can be achieved by solid-state devices, which usually are varactor diodes if continuous tuning is desired or PIN diodes in the case of discrete tuning. The four main methods of coupling varactor diodes to the DR are described. In each case, a TE_{01 d} mode is employed because of the ease with which the resonant frequency, Q factor and coupling coefficient can be calculated using simple models.⁸ Moreover, the structure of the electromagnetic fields of this particular mode allows the realization of small, compact circuits.

The first method, shown in Figure 2 , involves placing the varactors on the mechanical tuning plate, which is located directly above the resonator. The principal advantage of this approach is that, in order to obtain an appreciable amount of electronic tuning, the varactors must be coupled tightly to the circuit by reducing the separation between the top of the resonator and the tuning plate. This configuration can seriously degrade the resonators' unloaded Q factor. However, the technique has been used to produce 4 MHz of electronic tuning in a 14 GHz dielectric resonator oscillator (DRO).⁹

The second method of coupling varactors to the electromagnetic field of an excited resonator is to connect the varactors directly to two resonant ring-shaped conductive tracks fabricated on a quartz disc that is placed on top of the resonator, as shown in Figure 3 . The degree of coupling is controlled by the thickness of the disc. Although it is possible to produce as much as 56 MHz of electronic tuning in a X-band DR with an unloaded Q greater than 1000,^7,10 this method is particularly cumbersome to implement. Furthermore, severe problems can arise when these two techniques are used in applications that are susceptible to prolonged periods of intense vibration.

Figure 4 shows the third technique for coupling a varactor diode to the resonator. Basically, the configuration consists of two semicircular annular conductive tracks loaded with a pair of varactor diodes. The resonator is mounted directly above the tuning circuit and is supported by a low loss spacer. In this arrangement, the tuning configuration can be realized on the microstrip substrate and the magnetic coupling between the DR and tuning circuit can be adjusted by altering the height H of the spacer that is supporting the resonator. The very nature of the structure indicates that the configuration performs well under vibration. This technique can provide a tuning range of 44 MHz at S-band while maintaining a high unloaded Q above 1000.¹¹ Additionally, the configuration is simple to construct and lends itself to an inexpensive and reproducible manufacturing process.

An unusual technique known as invasive tuning has also been employed.¹² Here, the term invasive refers to the fact that the tuning mechanism is incorporated within the body of the DR, as shown in Figure 5 . The varactor diode is placed inside the slot that has been cut out of the DR. The purpose of the metallized sides of the slot is to ensure that the perturbed field appears uniformly at the sides of the slot. This field then is displaced by the varactor or other conducting device across the slot. Variation of the slot capacitance perturbs the electromagnetic field associated with the DR and, hence, affects its resonant frequency. The configuration can provide a very large tuning bandwidth of approximately 150 MHz at 5 GHz with an insertion loss variation of only 2 dB. This tuning bandwidth exceeds any tuning method reported to date with the exception of ferrite tuning. However, its loaded Q factor has varied from 160 to 500 across the tuning range.

In a DRO tuned using PIN diodes,¹³ the tuning element consists of a conducting disc divided into four segments interconnected by PIN diodes and placed at a distance d above the DR, as shown in Figure 6 . Depending upon the conducting state of the diodes, the current flow lines induced in the disc by the electrical field of the TE_{01 d} mode may or may not be cut. Perturbation results yielding a jump in frequency of approximately 40 MHz at a center frequency of 16 GHz have been reported. No variation in the value of the unloaded Q due to this tuning system has been given.

Optical tuning of microwave devices, in general, and DRs, in particular, is highly desirable mainly for its high tuning speed. Interest in such a tuning method was aroused by the advent of new electro-optic devices such as diode lasers and optic switches. A straightforward way to tune the DR optically is to sensitize it to light. This sensitization can be accomplished either by, for example, simply adding a photosensitive material on top of the DR or spattering a very thin layer of photoconductor materials. Basically, there are two types of photoconductor materials: semiconductor (Si, Ge, PbS and InSb) with relatively high dark current and small response time (10 m s) and insulator (CdS, CdSe and Tl₂ S) with low dark current but relatively long response time (1 ms).¹⁴ Hence, illumination of the photosensitive material produces a change in conductivity, which, in turn, perturbs the field around the DR and leads to a shift of the resonant frequency. In this case, an appreciable opening must be made in the housing for good coupling between the light source and the sensitizer, rendering the device vulnerable to external influences.

Optical tuning of DRs has been reported previously.^15,16 The tuning was achieved by changing the conductivity of a layer of a photosensitive material deposited on the top surface of the DR by means of light from a laser or light-emitting diode (LED), as shown in Figure 7 . Using this technique, the device was determined to be optically tunable up to 15 MHz at X-band.^15,16 However, a major disadvantage of this method is that the external Q of the circuit degrades by approximately 30 percent. Moreover, light sources employed for fast modulation, such as an He-Ne gas laser or GaAs LED, produce a small frequency change (typically 0.4 to 0.5 MHz at 10 GHz). A comparison of the various tuning mechanisms is listed in Table 1 .

Applications

As may be expected, DRs tuned with semiconductor devices (in particular, varactor diodes) have intrinsic power/intermodulation limitations. Therefore, varactor-tuned DRs are most suitable for lower power operations (less than 100 mW). Some of the applications include VCOs (varactor-tuned DRs are currently used extensively in VCOs in many communication systems) and filters (DRs are used as elements of a filter where high discrimination and fine tunability are required, for example, in satellite communications front-end filters/preselectors). In addition, varactor-tuned DRs are used as devices for frequency modulation, as elements for frequency hopping in spread spectrum techniques (for example, in airborne and spaceborne communication, radar sets and ship-to-shore communications) and as devices for data scrambling (such as in satellite TV broadcasting).

Conclusion

A careful comparative study of the numerous tuning techniques available has shown that the varactor diode currently is the most suitable tuning mechanism for low power requirements since if offers low loss and high tuning speed. Furthermore, varactors also offer continuous frequency control and potential advantages in size, weight and cost. The main advantage of employing a resonant loop structure using varactor diodes constructed directly onto a microstrip substrate is the ease with which the coupled resonator can be tuned through a planar network. This type of tuning configuration greatly simplifies the implementation of DRs in high volume production circuits by eliminating the need for a complex mechanical tuning structure.

References

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16. P.R. Herczfeld et al., "Optically Tuned and FM Modulated X-band Dielectric Resonator Oscillator," Proceedings of the 14th European Microwave Conference, Liege, 1984, pp. 268-273.