A lumped element circulator with semi-additivemetallized conductorsRobert Stonies, Daniel Teufer and Dirk Schulz

High Frequency InstituteUniversity of Dortmund

GermanyEmail: robert.stonies@udo.edu

Abstract— A symmetrical 3-port lumped element circulatorbuilt by the use of thin film technology including electroplatingis presented allowing very compact devices.

A semi-additive metallization process yields high accuracy inreproduction of the structure with small features while allowingthe implementation of via-holes. Two technological proceduresare presented and compared.

The lumped element circulator performance is numericallycalculated and optimized to some extend, providing valuableinformation about important device performances. A comparisonwith measured scattering parameters shows good agreement withthe calculation.

I. INTRODUCTION

Circulators and isolators are the most important non-reciprocal passive devices in microwave engineering. Theyhave been discussed in detail concerning their design, prin-ciples of operation and their performance.

Among the regarding publications, many kinds of circu-lators, in particular waveguide and stripline circulators, havebeen examined with numerical methods to characterize them.However, to the knowledge of the authors, lumped elementshave not been treated this way so far. The reason may bethat high aspect ratios and complex gyromagnetic media causesome difficulties in the numerical simulation.

This paper demonstrates the power of a numerical approachfor the design and optimization of compact lumped elementcirculators that among others yields inner quantities such asfield distributions and measurable outer quantities such asscattering parameters.

Further investigations and new designs that are reasonablefor industrial production processes are of great interest due tothe rising demand for mobile r.f. and microwave systems.

This work presents a small, fully symmetrical 3-port circu-lator designed for a popular ISM-frequency. A hybrid setupis chosen due to the advantage of using standard industrialprocesses for the production of printed circuit boards to realizethe coupling network and additional reactive lumped elements.Even though the isolation-bandwidth is about 70 MHz onlyfor the presented sample device, broadband types may bedeveloped through further design optimizations including ad-ditional matching elements and a careful selection of the ferritematerial.

Starting with a basic structure taken from [1], numericalsimulations are performed. Geometric parameters are modified

until the circulator works at the desired frequency and showsan acceptable performance.

Finally, a circulator is built and the simulations are com-pared with measurements to proof the results.

II. DESIGN CONSIDERATIONS

The most frequently used passive non-reciprocal device isa 3-port circulator or an isolator, respectively. The latter oneis typically obtained from the circulator by the termination ofone port.

In contrast to distributed devices such as the stripline-junction-circulator inter alia discussed by Bosma [2] and Fay[3], circulators built from lumped elements are comparablysmall. There is no need to satisfy the well known conditionkR = 1.84 [2], [4] for the normal mode excitation of a ferritedisc where R denotes the radius of the thin disc and

k2 = ω2εrε0µ0

[µ2 − κ2

µ

]. (1)

Herein k is the wavenumber and µ as well as κ are elementsof the Polder tensor appropriate to a normally magnetizedferrite disc.

←→µ =

µ −jκ 0jκ µ 00 0 1

(2)

For a lumped element circulator a size-reduction of 5-10[5] referring to distributed circulators can be achieved withadequate performance. Without loss of generality a 2.45 GHz(center frequency) device is exemplarily dealt with in thefollowing.

The most interesting part of a lumped element circulator isthe magnetic circuit, sometimes named isoductor. It consistsof a coupling network that is at least partly embedded intogyromagnetic media invoking the non-reciprocity. A simpleequivalent circuit for a circulator therefore shows three cou-pled inductors and an additional capacitor at each port for tun-ing (figure 1a). Performance is - amongst others - influencedby the coupling between the inductors L, which depends uponferrite properties and the geometry of the coupling network.

Details about general theory of lumped element circulatorsare discussed in some detail in [1], [4], [6]–[11].

L

L

1

2

3

L

CP

CP

CP

CS

CS

CS

Fig. 1. Equivalent circuit with additional capacitors for tuning and matching

The addition of series and parallel capacitors allows to tuneand match the circulator (figure 1 b). By adding more reactiveparts including inductors, the matching can be converted fromnarrow band to broadband. The elements of the matchingnetwork are to be placed carefully not to invoke unwantedinfluence to the non-reciprocal magnetic circuit.

In case of our small 2.45 GHz lumped element circulatorRG4 ferrite material from AFT company (Backnang, Ger-many) is used. The saturation magnetization of this materialis 80 kA·m−1 and the dielectric constant is 14.3 with a losstangent of 0.94·10−4. A magnetic dc bias field is used tosaturate the ferrite and results in above resonance condition.

Having started with some analytical calculations such asan estimation about the capacitance needed to resonate atthe desired frequency for a given inductor length, numericalsimulations are made now to include parasitics and obtaindetailed information about field quantities on the one handand performance on the other hand.

Two different technological concepts are investigated andcompared with respect to their suitability. The first one isa double-sided standard printed circuit board process withdrilled via-holes, typically used in modern fabrication. Thesecond one is a sequential layer build-up process facilitatinga very thin insulation layer between the conductor layers aswell as very fine structure features with high accuracy. Bothprocesses have in common that a semi-additive metallizationprocess is used to form the lumped elements.

III. SIMULATION AND OPTIMIZATION

Nowadays, high frequency circuits are most often designedusing numerical computations. Running the programs on usualpersonal computers is fast enough for almost any commonproblem to be solved. A computer program that suits tocalculate the characteristics of a lumped element circulator isMicrowave Studio c© (CST company, Germany), because thenecessary algorithms for gyromagnetic media are included.Furthermore, non-uniform meshing is implemented enablingto handle the high aspect ratios of these devices.

Based on the finite integration technique, the integral coun-terpart of the finite differences in time domain approach, thelumped element circulator is solved full three-dimensionally.The physical model is completely generated out of basic

shapes using a visual basic script. This gives the possibility forparametric sweeps of all model parameters that may becomeof interest.

As an important result of the calculation scattering pa-rameters are obtained. Assuming only linear behavior, theycharacterize the the 3-port completely.

Moreover, the numerical treatment of the lumped elementcirculator with the finite integration scheme gives valuableinsight into the device. Among others, critical electrical fieldstrengths can be found and places of high losses can bedetected. With the help of all this information, the structurecan be improved.

Parasitic elements are included a priori and there is no re-striction regarding the geometry in opposite to pure analyticalapproaches. An exemplary model is depicted in figure 2.

dielectric film

port 1

port 2 port 3

shielding

seriescapacitor

parallelcapacitors

magnetic couplingnetwork with insulated

crossovers

x

y

z

Fig. 2. Three-dimensional view of the simulation model’s geometry show-ing the interwoven conductor structure, ports and some additional lumpedcapacitors

The parasitics appear to influence the characteristics inparticular. Especially the crossover capacities are responsiblefor a significant decrease of the operating frequency.

The calculation time is within the range of several hours upto a week depending on the number of mesh cells necessaryto approximate the structures. Execution time also depends onthe frequency range of interest as the excitation function iscorrelated to it.

To speed up the calculation, the lumped capacitors maybe substituted through idealized capacitors first, virtually con-nected between two mesh points. Once the necessary capacityis evaluated, a separate parametric simulation model includinga single capacitor model can be used to obtain the exactgeometries. To be more exact with that capacitors, a fullthree-dimensional electrodynamic calculation is performed atthe design center frequency. Finally, the geometries of allsimulations are combined into one CAD-file and masks aregenerated.

As can be concluded from figure 3 and as expected, fringingfields tend to increase the capacity to an amount that doesnot allow to neglect them. It is supposed that electric fieldquantities resulting from an excitation with 1 W input powerat 2.45 GHz (figure 4) are well below the dielectric breakdownlimitations of the insulation.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.60.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

finite integration analytical

mm2

pF

capa

city

A0

Fig. 3. Comparison between analytically calculated capacity using theformula for an ideal parallel-plate capacitor and a full three-dimensionalelectrodynamically calculated capacity

Assuming linearity an estimation of the power handlingcapability is possible.

Fig. 4. Electric field (plane y=0 as indicated in figure 2) concentrating in thevicinity of crossing conductors, normalized to 1 W input power at 2.5 GHz

Another interesting observation, already predicted in [7],is that the r.f. magnetic field is not at all homogenouslydistributed in the ferrites. It is rather clearly concentratedaround the conductors (figure 5). The ferrites’ bulk shows amagnetic field that is less than 10 percent of the field close tothe conductor.

IV. TECHNOLOGICAL ASPECTS

At least two conductor layers with an insulation in be-tween are a prerequisite for the interwoven coupling network.Two different processes are evaluated, both involving a semi-additive metallization.

A. Common double-sided printed circuit board process

This manufacturing process is common in the electronicindustry for the production of two-sided microwave circuitboards based on PTFE or ceramic substrates.

Fig. 5. Magnetic field (plane y=0 as indicated in figure 2) concentrating inthe vicinity of conductors, normalized to 1 W input power at 2.5 GHz

The substrate materials used for in this case, FR4 glassreinforced epoxy (140 µm) and MylarTM (DuPont) polyesterfilm (175 µm), are drilled (250 µm diameter) to form thenecessary via-holes first. Sputtering thin layers of titanium andcopper on both sides gives adhesion and basic conductivity.Furthermore, the vias are activated for later electroplatingimplicitly. A 25 µm dry film photoresist FP325 (Elga Europe)is laminated on both sides and structured with a MA56 maskaligner (Suess). Electroplating is now used to grow copperinto the resist grooves and form the conductors. Because ofthe complicated structure with an island-like topology and thecombination of very small and big feature sizes this is a diffi-cult task. A simple acidic copper electroplating bath withoutspecial additives or widely used high speed electrolytes wouldnot yield good results because of a bad thickness distributionor very rough surfaces. A newly developed acidic copperelectrolyte system CVF-1 (Enthone) is successfully applied.It is especially intended to be used in via-fill applications andproduces very plane surfaces and shows a good homogeneityof thickness.

After electroplating, the resist is stripped and the seed layersare removed through chemical etching. The structures look likethe one depicted in figure 6 a.

B. Sequential layer build-up

The sequential layer build-up process does not need drillingand allows very thin insulation layers. Furthermore, onlysingle-sided processing takes place simplifying the handlingof the substrates. Even materials may be used that are difficultto machine such as ceramics.

First, the substrate is coated with titanium and copper onone side through sputtering to provide a seed layer for laterelectroplating. Then, the FP325 dry film resist is laminatedand structured. Copper is electroplated to a height of 15 µminto the grooves. The resist is stripped and the seed layers areetched. Now, a second layer of FP325 is laminated on top.It is structured with the via and feed-line pattern before thesurface is completely sputter-coated with titanium and copperagain. An additional electroplating process enhances the layerthickness to 15 µm. Finally, AZ-3210 (ALLRESIST), a thick

film positiv photo resist, is spin-coated onto the sample andstructured to protect the later bridges and vias of the top layerfrom being etched in the following.

a) b)

Fig. 6. Photo of two final substrates (10 mm diameter) carrying lumped ele-ment structures, a) two-sided isoductor with drilled via-holes , b) sequentiallybuilt structure with lumped capacitors and FP325 insulation layer

The layout used to structure the bottom conductors inthe second process includes additional interdigital lumpedcapacitors for matching and tuning. SEM pictures of twodifferent types are depicted in figure 7.

a) b)

Fig. 7. Scanning electron microscope pictures of the capacitors (metallizationthickness 15 µm), a) parallel capacitor with 30 µm wide, 713 µm long and17 µm spaced arms b) series capacitor with 30 µm wide, 470 µm long and30 µm spaced arms

The process described first does not include additionalcapacitors. Only the coupling network is realized to compareto the results of the simulation. However, this structure is in-cluded to offer an alternative and industrial conformal process.The sequential layer build-up appears to be more reliable andis therefore preferred and extended with additional elements.

V. COMPARISON WITH MEASURED DATA

To verify the calculated performance, the structures aremounted into a jig (figure 8) containing the ferrites andproviding the SMA connectors to interface to a vectorialnetwork analyzer.

It is noticeable from figure 8 that the active part of the cir-culator is small compared to the complete jig. More compactcases are currently under construction.

A 360B vectorial network analyzer (Wiltron) is used torecord scattering parameters. The dc magnetic field is gener-ated by a current-controlled electromagnet with a slittet yoke(figure 9).

For the double-sided processed structure including drilledvias, figure 10 shows a comparison between the calculated andmeasured performance. It should be recalled that the layoutfor the structure only consists of the isoductor conductors.No capacitors are included and hence there is only the smallparasitic capacity of the line-crossings that act as a tuning

Fig. 8. Jig with SMA-connectors and mounted lumped element substrate toconnect to a network analyzer

Fig. 9. Fully assembled circulator mounted into the electromagnet’s yokeslit, one port is terminated with 50 Ω

element for the device. This leads to a much higher resonancefrequency close to the gyromagnetic resonance frequency,reduces bandwidth and implies higher insertion losses. Theinclusion of explicit lumped tuning elements is no problem atall and similar to the sequential layer build-up.

A second structure, provided by the sequential layer build-up process, is also measured and the scattering parametersare compared with the calculated ones. As can be seen fromfigure 11, the measured data agree with the data resulting froma numerical calculation but the center frequency is slightlyshifted to lower frequencies. The authors assume that thereasons for this shift are inaccuracies in the calculation andin the technological process. First, the ferrite is treated as tobe magnetized completely homogenously, which is not truein reality. Magnetostatic calculations show, as expected, thatthe magnetic field is much stronger close to the circumferencethan it is in the middle of the thin disc. Second, thin layersare represented by one mesh-cell in height only to keep thetime for calculation short. This may lead to some inaccuracies,especially concerning the field distribution at edges.

Nevertheless, the comparison shows a good agreement andwith some correction and modifications it will be possible topredict the behavior of a lumped element circulator with highaccuracy by means of numerical calculations.

2,5 2,6 2,7 2,8 2,9 3,0 3,1 3,2 3,3 3,4 3,5 3,6 3,7

-20

-18

-16

-14

-12

-10

-8

-6

-4

-2

0

GHz

dB

S-p

ara

mete

r

frequency

S21

calculated

S12

calculated

S21

measured

S12

measured

Fig. 10. Calculated and measured S-parameters of a circulator with a structureprocessed double sided with drilled vias

2,2 2,3 2,4 2,5 2,6 2,7 2,8

-25

-20

-15

-10

-5

0

GHz

dB

S-p

ara

mete

r

frequency

S21

calculated

S12

calculated

S21

measured

S12

measured

Fig. 11. Calculated and measured S-parameters of a circulator with a structureprocessed with a sequential layer build-up

VI. CONCLUSION AND OUTLOOK

A very compact L-band lumped element circulator is devel-oped and built using industrial standard techniques. It is shownthat a numerical computation can be used efficiently to designthe device and to perform optimizations. Electroplating is usedto build three-dimensional structures such as an interwovenconductor network and interdigital capacitors. The hybridcharacter of the devices results in high flexibility as it ispossible to exchange the substrates carrying the lumped ele-ments independently. Field distributions are examined to gaininformation about critical parts and to optimize the magneticcoupling circuit. Further designs will be made incorporatingadditional elements for broadbandening [12] and an enhancedsimulation model will be setup to allow for better and fasterprediction of the device characteristics. Further miniaturizationand power handling capabilities will be examined as wellas the temperature dependency that may be compensated byadditional lumped elements [8].

ACKNOWLEDGMENT

The authors would like to thank M. Kilouli and H. Verbuntfrom Enthone for an excellent support and valuable discussionsabout the electroplating process. They would also like to thankW. Arnold and U. Hoeppe from AFT for supplying the ferritematerial.

REFERENCES

[1] R. Knerr, C. E. Barnes, and F. Bosch, “A compact broad-band thin-filmlumped-element l-band circulator,” IEEE Transactions on MicrowaveTheory and Techniques, vol. 18, no. 12, pp. 1100–1108, 12 1970.

[2] H. Bosma, “On stripline y-circulation at uhf,” IEEE Transactions onMicrowave Theory and Techniques, vol. 12, pp. 61–72, 1 1964.

[3] C. E. Fay and R. L. Comstock, “Operation of the ferrite junctioncirculator,” IEEE Transactions on Microwave Theory and Techniques,vol. 13, no. 1, pp. 15–27, Januar 1965.

[4] J. Helszajn, Principles of Microwave Ferrite Engineering. Wiley-Interscience, 1969.

[5] I. Ikushima and M. Maeda, “A temperature-stabilized broad-bandlumped-element circulator,” IEEE Transactions on Microwave Theoryand Techniques, vol. 22, no. 12, pp. 1220–1225, Dezember 1974.

[6] R. Knerr, “A lumped-element circulator without crossovers,” IEEETransactions on Microwave Theory and Techniques, pp. 544–548, 51974.

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[8] H. Katoh, “Temperature-stabilized 1.7-ghz broad-band lumped-elementcirculator,” IEEE Transactions on Microwave Theory and Techniques,vol. MTT-23, no. 8, pp. 689–696, 8 1975.

[9] J. Deutsch and B. Wieser, “Resonance isolator and y circulator withlumped elements at vhf,” IEEE Transactions on Magnetics, vol. 2, no. 3,pp. 278–282, 9 1966.

[10] E. Fliegler, “A 350-mhz broad-band lumped element circulator asa protective isolator,” IEEE Transactions on Microwave Theory andTechniques, 5 1969.

[11] I. Ikushima and M. Maeda, “A temperature-stabilized broad-bandlumped-element circulator,” IEEE Transactions on Microwave Theoryand Techniques, vol. 22, no. 12, pp. 1220–1225, 12 1974.

[12] R. H. Knerr, “A compact thin film lumped element circulator using acapacitor, common to all three arms, for broadbanding or switching,”International Microwave Symposium, vol. G, pp. 393–396, 5 1970.