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EM-BASED DESIGN

SOFTWARE FOR

SPIRAL INDUCTORS

AND TRANSFORMERS

Aspiral is a very important form of planaror multilayer inductor or transformer.Previous designs of spiral components,

however, have relied heavily on approximatequasi-static models and verification using elec-tromagnetic field solvers. Recently, an applica-tion-specific graphical user interface has beencreated for use with the IE3D high perfor-mance EM Simulation and Optimizationsoftware to facilitate the design of spiral cir-cuit components.

SpiralSyn is a spiral analysis and synthesis

module integrated with the IE3D Library. It

performs electromagnetic analysis and synthe-sis using the IE3D field solver, with a graphi-cal user interface. SpiralSyn creates spiral ob-jects and exports them to IE3D. It can also beinvoked from a spiral component layout.

SpiralSyn currently handles seven spiralstructures from inductors to transformers,as well as interdigital and metal-insulator-metal (MIM) capacitors. The list of induc-tors and transformers includes single-endedrectangular and differential rectangular spi-ral inductors, single-ended octagonal and

differential octagonal spiral inductors,double-layer rectangular spiral induc-tors, partly bifilar rectangular spiraltransformers, and entirely bifilar rec-tangular spiral transformers.

EXPERIMENTAL VERIFICATIONSpiralSyn was used to analyze several

spiral structures that had available mea-sured data to verify its accuracy. Figure1 shows the layout and comparison ofsimulated and measured S-parameters1

of a rectangular spiral inductor. Figure

ZELAND SOFTWARE INC.Fremont, CA

|

S1

1

|

,

|

S21

|

(dB)

FREQUENCY(GHz)

0

4

8

120 0.5 1.0 1.5 2.0 2.5

IE3D|S11|Measured|S11|IE3D|S21|Measured|S21|

Fig. 1 Simulated andmeasured response of arectangular spiral

inductor.w

Reprinted with permission ofMICROWAVE JOURNAL from the April 2005 issue.2005 Horizon House Publications, Inc.

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2 shows the physical layout and com-pares simulated S-parameters with mea-sured data2 of a rectangular spiral trans-former. It can easily be seen that theSpiralSyn simulations are in excellentagreement with the corresponding mea-surements in the two examples.

A DESIGN EXAMPLEA design example of a rectangular spi-

ral inductor using SpiralSyn is presentedto demonstrate the softwares usefulness.The case for analysis is a 6.875-turn spiralon 4.5 m thick SiO2 on a 500 m thicksilicon substrate. The graphical user in-terface of SpiralSyn is so simple that en-tering the dimensions and parameters ofthe spiral, selecting the options and start-ing the simulation is all that is necessaryto perform the analysis. In this case, Spi-ralSyn completed the analysis in approxi-mately one minute using a 2.5 GHz Pen-tium 4 machine. The analysis results areshown inFigure 3.

The synthesis example uses the same layer structureand trace parameters, although the synthesis target is a 5nH inductance at 1.5 GHz. SpiralSyn determined it to bea 5.75-turn spiral with an inductance value of 5.01 nH.

Figure 4 shows the results of that synthesis.

CONCLUSIONAn IE3D Library module for spirals has been de-

scribed and verified through comparisons with measure-ments. SpiralSyn performs spiral synthesis based on accu-rate electromagnetic simulation and is a useful tool inRFIC design.

References1. D.M. Krafcsik and D.E. Dawson, A Closed-form Expression for Rep-

resenting the Distributed Nature of Spiral Inductors, IEEE Mi-crowave and Millimeter-wave Monolithic Circuits Symposium Digest,Baltimore, MD, June 45, 1986, pp. 8792.

2. E. Frlan, et al., Computer-aided Design of Square Spiral Trans-formers and Inductors, IEEE International Microwave SymposiumDigest, Long Beach, CA, June 1315, 1989, pp. 661664.

Zeland Software Inc.,

Fremont, CA (510) 623-7162, www.zeland.com.

PRODUCT FEATURE

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S22

|

,

|

S42

|

(dB)

FREQUENCY(GHz)

0

5

10

152 4 6 8 10

IE3D|S22|Measured|S22|IE3D|S42|Measured|S42|

v Fig. 2 Simulated and measured response of a rectangular spiral transformer.

v Fig. 3 SpiralSyn analysis of a spiral inductor on silicon.

v Fig. 4 SpiralSyn synthesis of a spiral inductor on silicon.