Electromagnetic Simulation of Some Common Balun Structures

K.V. Puglia

Baluns serve many applications in RF and microwavecommunications, radar, and instrumentation equip-ment. The word balun is an acronym for bal-

anced-to-unbalanced converter, and the function is employedto change a single-ended signal, i.e., a signal that is referencedto ground, to a balanced signal with equal potentials with re-spect to ground but opposite polarity. These balun structuresare employed in such common RF and microwave componentsas mixers, antenna-feed networks, and frequency multipliers.

Within this technical memorandum, some common balunstructures are investigated using the electromagnetic simula-tion software package Microwave Office, available from Ap-plied Wave Research. The Microwave Office design suite is anintegrated software package that includes an object-orientedlinear and nonlinear circuit simulator as well as a full-wave,electromagnetic analysis of planar, physical structures thatmay be imported within the circuit simulator. Other ad-vanced features of the Microwave Office design suite areavailable directly from Applied Wave Research or from theirWeb-site at www.appwave.com.

Balun StructuresFigure 1 illustrates the equivalent circuit of a balun where asingle-ended source Es, with source impedance Zoa, is con-nected at the input, and balanced or unbalanced loads Zob areconnected at the output. The dotted lines connecting the indi-cated points to ground illustrate possible connections of bal-anced or unbalanced loads at the output of the balun.

If the balun is ideal, i.e., both loss-less and impedancematched at both the source and load, the following equationsmay be written

EZ

EZ

i

oa

o

ob

2 2

2= ⋅

and

NZZ

oa

ob

= .

For the special case of Zoa = Zob, the balun may be utilized asa power divider with 180° phase shift, and the followingequation may be written

EE

oi=2

.

The versatility of the balun as an impedance transformer isevident. Note that the total load at the output of the balun is2·Zob. This property is very useful in applications where a bal-anced load, e.g., mixer diodes or dipole antennas, is con-nected at the balun output. Note, also, that a voltage null is

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K.V. Puglia is with M/A-COM in Lowell, Massachusetts, USA. Figure 1. Balun equivalent circuit.

present at the center point of the load, another useful prop-erty, particularly for the case of sampling devices.

Three common balun structures are investigated: themicrostrip (MS) to slotline (SL) transition, the coplanar-wave-guide (CPWG) to SL transition, and, subsequently, two formsof the tapered balun are explored.

MS-to-SL TransitionFigure 2 illustrates a unique balun structure with many usefulapplications. The transition has been investigated using a de-rived equivalent circuit with some degree of theoretical andmeasurement accuracy [1]. An electromagnetic analysis ispreferred for operation of the transition over a wide fre-quency band. The equivalent circuit of Figure 2(b) has beenused in previous analysis and represents the circuit looking

back from the SL at the MS crossing. For circuit-simulationpurposes, the open-circuit MS stub and the short-circuit SLare represented as follows

( )Z jZoc mso= − mso cot φ

and

( )Z jZsc sls= + sls tan .φ

The balanced output is taken across the slot. An electro-magnetic simulation is conducted using EMSight softwarethat permits a graphical input of the structure or the import ofdrawing interchange format (DXF) or 2-D graphical designdata format (GDS-II) layout files.

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Figure 2. MS-to-SL transition. (a) Physical implementation. (b)Equivalent circuit.

Figure 3. MS-to-SL transition data.

Table 1.MS-to-SL structure parameters and dimensions.

Substrate material 25 mil Al2O3

MS width 24 mil

MS length @ open 80 mil

SL width 4 mil

SL width @ short 14 mil

SL length @ short 50 mil

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Figure 4. Back-to-back MS to SL transition data. Figure 5. CPWG-to-SL balun.

Figure 6. CPWG-to-SL balun data. (a) Single transition. (b) Back-to-back transition.

Figure 3 represents the return- and insertion-loss data ofthe MS-to-SL transition when each of the balanced ports is ter-minated in 50 Ω. Table 1 contains data pertinent to the struc-ture parameters and dimensions.

Often, these types of transitions are measured usingback-to-back circuits because the balanced ports present in-strumentation and measurement difficulties due to the equip-ment interface. An analysis has also been conducted usingtwo identical transition structures coupled at the balancedports. The results are presented in Figure 4.

The SL gap was changed to a value that produces an SL im-pedance of 50 Ω between the transitions. This was done be-

cause the load impedance at the output port is 50 Ω. TheMS-to-SL transition may be used in a single balanced mixerwhere two series connected diodes would result in a higherimpedance level. The SL is very useful in this application be-cause the higher terminal impedance may be more easilymatched over a broader range of frequencies.

CPWG-to-SL BalunAnother interesting balun structure is the CPWG-to-SL balun(Figure 5). This structure does not require a second plane;rather, bond wires or via holes are used to insure constant po-tential between various conductors.

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Figure 7. Tapered balun implementation. (a) In-line taperedbalun. (b) Marchand tapered balun.

Figure 8. Back-to-back tapered balun data.

Figure 9. Single-tapered balun input return loss.

Figure 10. Marchand balun using coaxial cable.

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Figure 11. Marchand balun input-return loss.

Figure 12. Folded Marchand balun.

Figure 13. Folded balun data.

Figure 14. CPWG Marchand balun.

This structure has been analyzed in both single andback-to-back configurations. The data is graphically dis-played in Figure 6.

Note that the amplitude balance at the balanced ports ofthe single is much better for the CPWG transition than for theMS transition. The back-to-back transition structure may beused as a broadband bandpass filter or broadband dc block.The bandwidth of the back-to-back transition structure isbroader than the single-transition structure due to the ap-proximate half wavelength SL between CPWG lines.

Tapered BalunThe tapered balun is one of the most popular types and iswidely employed as a basic element of single and double bal-anced mixers. Tapered baluns are inherently broadband de-vices, due primarily to the natural potential differencebetween signal and ground conductors. There are two basiccircuit implementations for the tapered balun (Figure 7).

The inline-tapered balun requires a gradual taper in boththe top and bottom conductor widths, and the signal potentialis always between the conductors. The Marchand balun re-quires a taper only in the ground conductors, and the signalpotential is developed across the gap in what is normally theground plane of a MS circuit. Both baluns require the indi-cated lengths for proper operation and possess imped-ance-transforming properties.

Input-return loss and insertion-loss data of back-to-backtapered baluns is illustrated in Figure 8. The sharp resonanceat the high end of the band is believed to be caused by pooreven-mode terminating impedance [2].

To examine the input-return loss of a single balun, the in-ternal port feature of EMSight will be utilized. The inter-nal-port feature allows the connection of certain lumped andactive elements internal to the layout structure. Specific rulesapply to the use of internal ports and should be well under-stood prior to simulation. The graphical data of Figure 9 wasobtained using the internal port between the end of the topconductor of the balun and a via inserted from the lower con-ductor to the top conductor.

Marchand BalunThe Marchand balun is a derivative of one of the first balunconfigurations that was physically realized in a coaxial con-figuration (Figure 10).

An MS implementation of the Marchand balun has beenanalyzed using the internal port feature of EMSight. The in-put-return loss is illustrated in Figure 11. The termination im-pedance at the internal port is 125 Ω.

As may be discerned from Figure 11, the Marchand balunhas a bandwidth of approximately one octave. A very usefulvariation of the Marchand balun encompasses folding the MSlines (Figure 12). This configuration is particularly attractivefor compact, double-balanced mixers with the local oscillatorand signal ports at opposite sides of the package.

The folded balun is beneficial in balanced mixers and inthe implementation of 180° hybrids, as the data in Figure 13indicates.

The single balun was terminated in 125 Ω at the balancedports.

Another variation of the Marchand balun in a more planar,i.e., CPWG, implementation is illustrated in Figure 14.

This particular configuration offers a broad range of im-pedance-transforming properties, using the center-conductorwidth, outer-conductor width and spacing, and theground-plane spacing. The input-return loss of this balunstructure is illustrated in Figure 15 for a balanced terminationof 100 Ω.

The input-return loss and insertion-loss data ofback-to-back CPWG Marchand baluns is also illustrated inFigure 15.

References[1] M.M. Radmanesh and B.W. Arnold, “Generalized microstrip-slotline tran-

sitions: Theory and simulation vs. experiment,” Microw. J., pp. 90-94, June1993.

[2] B. Climer, “Analysis of suspended microstrip taper baluns,” Proc. Inst.Elect. Eng., vol. 135, pt. H, No. 2, pp. 65-69, Apr. 1988.

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Figure 15. CPWG Marchand balun data. (a) Single balun with100 Ω. (b) Back-to-back baluns.