Balun Designs for Wireless-Mixers-Amplifiers and Antennas

8/10/2019 Balun Designs for Wireless-Mixers-Amplifiers and Antennas

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Balun Designs for Wireless

M ixers, Am plifiers and Antennas

BalunsJind w ide use in mixer antenna and balanced amplzJier circuits yet their

design is not widely practiced. The author shows a variety of designs that are simple

and yield go od first pass results.

Rick

Sturdivant

Hughes Aircraft C ompa ny

El Segundo California

aluns find wide use in mixer, antenn a and bal-

anced am plifierc ircuits. Yet balun design is still

regarded as if

t

were black magic by many

enginee rs, partly due to the fact that practical design

information on baluns can be difficult to find. How-

ever, many good baluns have been dev eloped over the

years and som c are surprisingly simple to design. In this

paper we will review somc good balun designs and

show that good first pass design results can be obtained .

Balun circ uits find wide application espe cially in new

wireless applications as they have customarily in

R F

and microw ave circuits. They a re used in circuits such

as mixers, push-pull amplifiers, antennas and other

applications requiring a conversion between unbal-

anced transmission line such as coa x, stripline or

microstrip) and a balanced line such as a two wire

transmission line of the type used for old television

antenna leads).

A

properly de signed balun is essential for these types

of circuits. In fact, it is often the perform anc e of the

balun which predominately determines the perfonn ance

of the overall circuit.

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Interestingly, the design of balun s is rarely covered in

most engineeringcurriculum. Furthermore, the design

information o n them is scattered throughout a num ber

of different articles. T he goal of this paper is to save the

circuit designer time by cataloging a few good balun

designs.

str p

Balun

Theory

housing

dielectric

supp rt

~h~

purpose of

a halull

i s

to forln good transition

Figure 2 Shielded parallel plate line is a balanced transmission

line

from an unbalanced transmission line into a balanced

transmission line. Figure illustrates coax, a familiar

unbalanced line. The current I exists on the center

conductor, I, is the ground return for

I

and Ie is the

externa l current induced on the outs ide of the shield.

The shield is considered tied to earth ground at the

input.

A shielded parallel plate line, on the other hand , has

equal potential and 180 degree s of phase difference

between the current on the two conductors. with no

curren t on the ground shield. Th is is illustrated in Figure

2. The currents are I,=-I,, and I, is the ground return.

I

exists only w hen the eclual am plitude and op posite phasc

relationship between

I

and I, is not maintained. Ic s thc

induced external current on ihe outside of the shield.

To speak of the potential at the balanced end of thc

circuit referenced to gr o~ lnd s unnecessary, since it is

the potential between the balanced pair of co~lductors

which is responsible for the transmission of power along

them.In fa ct, for the theoretically perfect balu n, an ideal

current sourc e placed betw een one of the balanced lines

and ground w ill produce zero current in that line.

How ever, for many mixe r and amplifier designs it is

not necessary to have a theoretically perfect balun. That

is, many circuits do not require the infinite ground

isolation property of the ideal balun. Rathe r, these

circuits only require that the voltages on the balanced

lines are of equal magnitu de and opposite phase , and

that the impedance to ground may be finite but equal

for both conductors). Baluns have been developed over

the years which meet this requirement. These quasi

baluns are the m ajor focu s of this article.

Half Wave

Tran,s.missionLine

Possibly the simplest balun is show n in Figure 3. The

phase sh ift of 180 degree s which is required for balun

operation is achieved by a half wavelength piece of

transmission line. Impedance tnatchin gcan be achieved

by properly choosing the impedance of the halfwave

line. This is an inherently narrow band design . A

quarter wave transformer can be added at the unbal-

anced input to improve thepe rfom ~an ce. his circuit is

designed by modifying the line impedance of the half

wave and quarter wave line i f used) to achieve the best

performance.

unb l nced =--

e

end view side view

Figure

I

Coaxial line is an unbalanced transmission line

Figure

3

Simple half wavelength balun

36 APPLIED MICROWAVE S~rrnrnr~.

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N

Sections

o Halj

Wave Lines

Lumped Element Balun

By interconnecting anusnber of halfwavelength trans-

mission lines with quarter wavelength lines (Figure

41

wide band baluns are possiblc[l]. The bandwidth can

be i~lcrcascci y adding more sections. The order,

n

o f

the balun depends upon the nurnber

of

halfwavelength

sections. The half wavelength lines provide the 180

degrees of phase shift while the quartel- wavelength

lines give the impedance transformations. This is a

balun design that is practical in tnicrostrip. Due to its

planar structure the circuit pattern can beetchedon only

one side of the substrate.

balanced

output

Figure 4 N section halfwave balun

An n=2 form of this balun has bccn dcsiglied and

tested. The cleinent lengths and isnpedances have been

optimized. The measured performance is shown in

Figure

5 .

Return Loss

4

Isotat~on

lnsertlon

Loss

lnse~t lon oss

Baluns can be do igncd by using lu~np ed lemenh.

Lumpcd clement baluns are well suited for low frc-

quency and

MMIC

applications[2],[3] for which dis-

tributed linc lengths would be too bulky. The theory of

lusnped clesne~lt aluns i5 based upon the familiar rat

race ring hybrid. The distributecl transini\\ion line4 of

the rat race ring are replaced by lu~npedelement quiva-

lent circuits. The quarter wave length lines of the ratrace

ring are replaced by lumped element lowpa5s f~ lt er

networks. The three quarter wavelength line is replaced

by a highpaus filter network. The resultant equivalent

lumped element circuit is shown in Figure

6. The

required design equations arc included.

where:

o

= I/(oC) = 1.4 4 K

R = Port Impedance

w = center frequency (rad/sec)

Figure

6

Lumped element balun

The theoretical pel-forniance of this balun is shown in

Figure 7. A 50 ohm load was assunled on all ports.

Therefore,

C

0.325pF and

L

1.125pF.

Parallel Plate Balun

Of all the baluns in use today possibly the most widely

used is the quarter wave coupled line balun. There are

many configurations for it. The broadside coupled

parallel plate balun is very common[4]

[ 5 ] , [ 6 ] .

t is

6

7 1 Sound in Inany mixel-designs and is shown in Figure 8

requency

[GHZJ

It consists of an input rnicrostrip line which transitions

into parallel plate line. The parallel plateline is adoublc

Figure

5

Measured performance of n=2 half

wave

balun

sided substrate.

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Return Loss

. . . . s x Insert on Loss

Lsolatlon

Insert lot Loss

a magnetic wall 15 placed between the balanced lincs.

Their neth hod, which

i

bawd on the variational tech-

nique, is 5traight forward and easy to pro gram .

9 1 2

1 3

Frequency

[GHz]

Figure 7. Theoretical perform anceofth e lumped element balun.

Figure

8.

Parallel plate balun. Backside contluctor shown as

dashed lines topside conduc tors as solid lines.

A taper is usually a dded to the backside g ro ~u id lane

to aid in the mo dc conversion from microstrip to parallel

platc. H owev er, relatively

good

performance is possible

without using a taper ifth e eve11mode

impedance

is very

large cornpared to the odd mod e impe danc e. Usually

soft sub strates a]-euseci for the sup port dielec tric and a re

usually very thin (5mil typically).

A s ide view of the balun is shown in F i p r c 9. The

parallel platc region is actua lly a pair of coupled line s.

T h c ev e n an d o dd ~ n o d c nalysis; ca n be do ne using th e

tcchniyue devclopcd by Bhat andK o~1 1[7 ]osthe analy-

s is o f br oads id e co ~ ~ p l e du sp end ed s ~ ~ b s t r a t etripline

(BCSS S) . The odd impedance,Zoo. is the charac teristic

impedance ofon e

of

the baliunced conducto rs to groun d

when shorting plane is placed at the symm etry line

between the balanced conductors . Thc even mode i n -

pedance is the character ist ic irnpcd~it lce ne ~z s i ~r edc-

tween on e of the balanced conductors and ground when

On e of the benefits of the parallel platc balun is that it

is possible to ach lev e grouncl isolation at the balanced

outpu ts. That is the balancccl lincs have the ir potential

refcrcnccd to each other, with near Lero potential be-

tween either balanced linc and ground.

Figure 9. Side view o f the parallel plate balun.

I lie actual parallel plate im ped anc e(Z op) is twice the

BCSSS odd mod e impedance. Therefore, the impcd-

ance u5edfor the design of the b alunis rclatcd totheocid

node BCSSS impedancc by

W he reZ op 1s the parallel platc impe dance . In order to

have a good match at the rnicrostrip inpu t, we must have

Equation 1 z


5/6

5 7 8

9

10 11

Frequency

[GHz]

Figure

10

Frequency response of the parallel plate balun

A parallel plate balun was used in the design of am ixe r

realized on 25 mil alumina (er=9.8). Th e performan ce

is good for a first cut design. The design called for a

mixer with a bandwidth covering 8-10 GH z with less

than 9dB conversion loss. The port

VSWR was less than

2:

1

using the balun configuration shown in Figure with

no taper to the trace on the back sides of the substrate.

The conversion loss as

a

function of the r.f. frequency is

shown in Figure I .

6 5 7 0 7 5 8 0 8 5 9 0 9 5 10 0 10 5 11 0

R F

frequency[GHz]

Figurc

11

Conversion loss of the singly balanced mixer

Microstrip Coupled Line Balun

Adesign similarto the parallel plate baluncan bem ade

using coupled microstrip. Thc design procedure uses

Equations 1-3. This type of balun has performance

similar to that shown in Figure 10. How ever, the

condition of Equation 3 can be difficult to satisfy in

microstrip.

Using only two coupled lines does not usually give

enough coup ling to achieve good performan ce, better to

employ three. A typical layout for this balun is shown

in Figure 12. T he two lines which couple to the center

strip are shorted at the unbalanced input and attached

periodically by jumper wires. This short circuit causes

the capacitance between these two strips and the ground

plane to be shorted out. Transform ing a quarter wave

length to the balanced o utput causes the im pedance to be

very large (theoretically an open circuit).

via

t

ground

unbalanced balanced

via t o

ground wires

Figure 12 Microstrip balun using coupled lines where the

coupled region is a quarter wave length long for the odd m ode

Marchand Balun

A wide band balun which has been used for years is the

Marchand balun[8J. Many exam ples of the Marchand

balun can be found in the literature [8,9,10]. Figu re 13

is an exam ple of a coaxial line Marchand balun.

balanced

Gutput

Figure 13 Marchand balun in coax

Published analyses suggest that bandwidths ap proach-

ing 50:

1

heoretically are possible. The author is aware

of realizations yielding bandwidths o f

:

1. The design is

amena ble to both coax and microstrip media.

A

quasi Marchand balun is shown in Figure 14. This

is not a true Marchand balun du e to the fact that the even

mode impedan ce between the coupled lines is defined

between eac h line and ground. With this embodiment

there is a finite (even m ode impe dance) between the

non-groun ded lines and ground. This is an additional

impedance not represented in Marchand s balun. The

predicted pe rformance is shown in Figu re

15

For Zoo

35 ohm and Zoe = 250 ohm and 5 0 ohm loads on the

ports.

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balanced

unb l nced

7 7

Zoe Zoo Zoe Zoo

-

Figure

14.

Quasi Marchan d balun u sing coupled lines.

-- eturn

LOSS

-.-- -

Insertion loss

4 8

1

1 2 4 1 6

8

Frequency [GHz ]

Figure

15.

Calculated response of the quasi M archand balun.

eferences

[ l ] B M ayer, R Knoechel, Biasable b alanced mixers and frequency

doublers using a new planar balun, Conference Proceeding: 20th

European M icrowave Con ference. V2, 1990 , pp. 1027- 1032.

121 S amue l P a r ~ s ~ ,Monolr th~c,umped e l ement , wg l e vdebnnd

modulator, 1992 IEk E M TT-S lnterrldl~ondlM l c r o ~ve Sqmpo-

~ l u m ~ ges t , p 1047-1050

[3] S. J Par~sr , 180 dcgrcc lumped elenlent Hy b~ld . 1989 IEEE

MTT-S Intcmat~ondlMicrownvc Syrnpos~umDrgest, Vol 3. pp.

1243-1246.

141

C. Y. Ho, New analysis tccllniquc builds bcttcr baluns .

Microwaves and RF, Aug., 1985. pp. 99-102.

[S]

B .

Clinier, Analysis of suspended microstrip taper baluns,

Proc. IEE. Vol. 135, Pt H. No. 2 April 1988, pp. 65-69 ,

161 S. Padin, B. Arend. G. Narayanan, A ~ [id cb an dSB mixcr using

high I'l-ecluency operatio nal amplifiers. Micro wav e Journ al. March

1992. pp. 131-133.

[7] B.

Bhat and S.K. Koul. Unified approach to solve a class of strip

and microstrip-like transmission lines, IEEE Trans. Microwave

Thcory Tcch.. vol. MTT-30, pp. 679.686, May 1982.

181

N .

Marchand, Transrnisiion line conversion transformers.

Electronics, 1944, 17. pp. 142- 145.

[9] J.W . McLauphlin.

R.W.

G r o ~ ,A w~ de-bandb,tlun. IRE

Trans.. 1958. MTT, pp.

3

14-316.

LIO] K

Rawer.

J .J .

Wolfe. Aprin tedcircuit balun for use with spiral

antennas, IKE trans., 1950, MTT , pp. 3 19-325.

Rick Sturdivant l.ec,ei~>ed

ile

BA D e-

Col orr~ioColleg e arrd tlle BS 6E L)e-

I [

L011,q HCLIC.IIll 1989. In 1992 he

1c.s.

111 1989 Irejo it~eiItk e atkrr Sys retn.~

G ~ , O L I I I

t

H ~ ~ g 1 ~ e . sir-~ .rtf t s u Me~rrl-

h e ri ft lr e ~ ' t ~ ~ ~ / l t l i c ~ u ~ S t u j j :r-eseirtly lr

i . \ ra.q~oi~sihlr

iw

tha de.\.ig/~f high

clc~nsit? ~ic.ro+r.u~.c,uc .kugc~sf ir p-

pIic.crtiot1 to ro ilu t.sy ~rc ~n7 s.VIlilr irt H u g h r ~7r 1zci s hrc~tl.c~.s~~orl.sihle

for IR &D pr.~jo(. ts 11 rnir~i(ltur-e~ir.(.lll(~tur..~r1i1di~~1eser . srld tlle

design

r?f kllC.s

or EEW otld ?wuTur- sj st en ls . H is ir1rerests tr1.c irl

mict.o\ta\'epocku,yin,q, nrlcl tkc desig n i~f fi'lter-s,ili.rer.s, ntld circlr la-

tor.s.

Rick is a rrlcnlhc/ of T a u Beta P i , aridE t i~ N O ~ NU .

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Balun Designs for Wireless-Mixers-Amplifiers and Antennas

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