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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
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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
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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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