The Usage of Dielectric Resonator Filters in Microwave Frequency Synthesizers

Hakan P. PartalAnaren Microwave, Inc., Syracuse, NY

E-mail. hpartal@ieee.org

Abstract

Microwave frequency synthesizers in the presence ofdielectric resonator filters are presented in this paper.Various types of microwave filters are discusseddepend on applications. Dielectric resonators areconsidered next to microstrip filters because of greatbenefits for the filter applications. A specific designexample is given to focus on the usage of the dielectricresonators andfilters.

1. Introduction

Frequency synthesizers find wide applications incommunications and radar systems. The vendorsmanufacture narrow or broadband, fair or fastsynthesizers. In a basic synthesizer, a main signal(s)usually generated by microwave oscillators at the inputis usually multiplied and certain harmonics are used byfiltering unwanted harmonics out (Figure 1). Anamplification may be needed at the end of thesynthesizer because of some circuit losses and/or gainrequirements.

Filtering is an important part of a synthesizer design.Passing a required band and cleaning up all spurs aremain issues in a synthesizer. When a spurious signalfalls close to a desired band, a steep response filterdesign is needed.

2. Filters and Dielectric Resonators (DRs)

Microwave filters can be designed in varioustechnologies. For lower frequencies than 2 GHzlumped component filters are commonly used. Sinceresonances of inductors could show up higher than 2.5GHz, distributed (microstrip or stripline) filtertechnologies are preferred at higher frequencies up to25 GHz. When pass band of a filter is higher than 25GHz, the distributed filters will have high insertionlosses mostly stems from material losses and be toosmall in size depend on the permittivity of the material.

Then cavity filters would be useful at very highfrequencies.

In a broadband synthesizer, we can take the advantageof distributed filters. However, conventionalmicrowave filters may not give sharp enough responseto reject unwanted harmonics; because of sizelimitations, low Q-factors or insertion lossrequirements. In this case DRs will be supportive tohelp reject the spurs close to a passband edge.

There is a large demand for DRs, which are compatiblewith the microstrip circuits in size and with theconventional waveguide and coaxial filters inperformance. They are commercially available frommany vendors and the manufacturers can provide themfor any resonant frequencies, at low cost. The unloadedQ value of the resonator element itself is extremelyhigh. Furthermore, tuning screws conveniently can beused without disturbing the outer surface of the circuithousing so as to tune the frequency of resonance withina reasonable range.

Clock Freq.

Logic MicrowaveControl Oscillator(s)

Multiplier

Filter

Out

Figure 1. Basic block diagram for a frequency synthesizer.

WAMICON 2006 1-4244-0849-0/06/$20.00 C 2006 IEEE

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These electroceramic materials with high relativepermittivity and good temperature stability are anattractive, low cost alternative to metallic resonantcavities where they have the advantage of sizereduction without reduction in performance. Relativepermittivity values for the dielectric resonatormaterials are in the range of 10-100. When thedielectric constant is high, an open-circuit boundarycondition applies at the small surfaces of the resonator.This is defined by

E.n = 0 (1)where E is the electric field vector, and n is the surfacenormal. A boundary with this property is often called amagnetic wall. An approximate solution for theresonance frequencies is obtained by extending themagnetic walls to an infinite tube. The problem is nowa straightforward waveguide problem. In the dielectricregion the guide is above cutoff and a standing waveexists at resonance, while in the air region the fieldsexhibit an exponential decay since the waveguide isbelow the cutoff.

A small radiating field from the resonator is usefullyemployed as a means of coupling, for instance, to amicrostrip line on a circuit board or probes within acavity (Figure 2).

DRs operate like a reaction cavity which reflects theRF energy at the resonance frequency. The derivationsfor the resonance frequency of dielectric resonators canbe found in the literature [1], [2]. However, here, weconsider an empirical formula to estimate thefrequency of resonance of a cylindrical puck resonator

fr= Kl[(D2h)13Er'12 (2)

where Er is the permittivity, fr is resonance frequency,D is diameter, h is thickness of the puck resonator, andK is a constant which is given as a function of thedistance to the ground, t, as shown in Figure 3 . Oncefris estimated using (2), then a proper cavity is selected,since the cavity dimensions are vitally important todetermine resonance frequency of operation.

Figure 2. H-field lines for a DR next to a microstrip line

1.2tlh 1

0.80.6 -

0.4 -

0.2 -

0-230 240 250 260

K (GHz mm)

Figure 3. The curve that defines the constant K given in (2).

3. Band Pass Filter Design

A filter design example will be presented here to showthe benefits of DRs.

Assume a frequency synthesizer design to producesignals between 9.8-11.8GHz with 200MHz steps.Employing multipliers to generate signals every200MHz, unwanted signals are going to appear close topass band edges at 9.6GHz and 12GHz, as well. Then asteep filter is required for this synthesizer. For thispurpose, let's design a band pass filter (BPF) with acenter frequency (f,) of 10.8GHz and a bandwidth of2GHz. Assume that a 60dB of rejection is needed forunwanted spurs.

First, an edge coupled microstrip filter to pass 9.8GHzto 11.8GHz band can easily be designed. A 13-sectionfilter is simulated and optimized using commercial RFsimulation software packages as shown in Figure 4.

Note that, almost all RF simulation software wouldexhibit a shift and/or a shrinkage in the frequency banddepending on the model generated. That is mostlybecause of channel properties, coupling effects, andsome software disabilities. Generally, anelectromagnetic simulation is recommended beforebuilding the filter. Normally, the electromagneticsimulation takes several hours. Having experience inestimating the approximate shift and shrinkage for eachsimulation software and skip the electromagneticsimulation will decrease the design time and cost.

With the estimated shift in frequency is 100 MHz, theband pass filter is designed from 9.7GHz to 11.7GHz.It should also be noted that simulated insertion loss isusually lower than the real circuit's, unless increasingloss tangent or copper losses while simulating.

H-field Cylindricallines dielectric

resonatorMicrostrip -,

trace -,

4

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Figure 5. Measured response of the microstrip BPF.

It is not possible to optimize this microstrip BPF toachieve 60 dB of rejection in the band-edge vicinity of200MHz. Using conventional microstrip bandstopresonators in cascade to this filter will hurt the passregion because of low Q-factor.

4. The Usage of Dielectric Resonators

Dielectric resonators that will provide extremely highQ-factor will be best solution to the type of problemsmentioned above. They can conveniently be placed incascade to the edge coupled microstrip line next to the50-ohm line by providing a proper channel. Magneticfield lines of cylindrical dielectric (puck) resonators areshown in Figure 2. The distances from microstrip line,walls, bottom conductor, and top conductor areimportant factors for frequency of resonance and Q-factor as they manipulate the H-field lines. Initially, thepuck can be located at least 2/10 away from the walls,where X is the wavelength, and tune the performanceand the frequency of resonance.

Figure 4. 9.8GHz-1 1.8GHz Edge coupled band pass filterand simulated response.

Measured results for the manufactured BPF is shownin (Figure 5).

In order to tune the performance and frequency ofresonance of a puck, tuning screws from top or sidescan be used. A screw from top is more effective fortuning, as it meets the field lines on a larger spot.However, a side screw can also be used for fine-tuning,if necessary. When a metallic tuning screw comescloser to a puck resonator, the resonant frequencyshifts up. On the other hand, a dielectric tuning screwshifts resonant frequency of the puck down as it comescloser to the puck. Typical tuning range of a dielectricresonator is up to 5°O of resonant frequency.

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Tuningscrews Microstrip

line

l-IL-\TTDielectric LT

resonators

Connectors M S21 LOG 1 dEIREF =G 6 E:-3.533 dD 11.950 e eo LHt

1-=1

Figure 6. Installing two puck resonators and tuning screws.

Using a 15 mil thick TLX9 dielectric substrate materialand an 8-mil spacer to locate two pucks in a test fixture(Figure 6), 70-80dB rejection is obtained as seen inFigure 7. When using multiple pucks, the spacingbetween them must be (2n-1)2/4, n=1,2,..., to obtainthe best Q-factor.

Then, we can locate and glue the two pucks in cascadeto the microstrip band pass filter properly. Tuningscrews are installed on the top cover, and the pucks arelocated inside their proper cavities.

Ei91 1.0i10Q@ dR PEF -20 X 4-754$4 dR ILM 8@04HO H

*S-'' ., .. ~~~~~~~~ ~ ~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~.... .......

Figure 7. Measured response for two DRs.

(b)

CHI. 1 ark e r:

1i.9Sfl BHz

1119520C LHz

4.- ..m 9i2.ies0 012

4.-38033 dB112.e:0 bHz

STOP i2.151 9DOO O9 SHiz

Figure 8. Measured response for the complete circuit(a) Measurement for 9GHz to 12.05GHz, (b) Zoom-in figurefor 11.95GHz to 12.05GHz.

5. Conclusion

Microwave filters are important parts of frequencysynthesizers. Various microwave filter technologiesand topologies can be used to achieve the responseneeded. Dielectric resonators are very beneficial forfilter applications because of their high Q factors, lowcosts, compatibility with microstrip circuits, and goodtemperature stabilities.

6. References

Final tuned response is shown in Figures 8. It is seen

that the rejection level at 12 GHz now is higher than 70dB. The insertion loss in band is less than 4dB. Thetransition lines between the microstrip bandpass filterand the dielectric resonator filter is causing extra lossesin addition to a very small contribution from the pucks.

[1] Darko Kajfez and Pierre Guillon, DielectricResonators, Scitech Publishing Company, 1998.

[2] Guillermo Gonzalez, Microwave TransistorAmplifiers, Prentice Hall, 1996.

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