WEIF-21
40 TO 90 GHz IMPEDANCE-TRANSFORMING
CPW MARCHAND BALUN
K. S. Ang, I. D. Robertson, K. Elgaid”, and I. G. Thayne*
Microwave and Systems Research Group
Department of Electronic and Electrical Engineering
University of Surrey
Guildford, Surrey GU2 5XH, UK
*Nanoelectronics Research Centre
University of Glasgow
simulations using fill-wave electramagnetic analysis
[6] or lumped element models [12]. Various
synthesis techniques have also been reported. Tsai
and Gupta [13] use the design curves of the origmal
Machand balun [141to synthesis a planar Marchand
balun based on coupled line equivalent circuit
models. Schwindt and Nguyen [15] apply a circuit
ABSTRACT
It is shown analytically that impedancetransforming planar Marchand baluns can be
designed. A GaAs monolithic CPW balun,
transforming between a 50!2 source impedance
and 160Q load terminations has been realised
to demonstrate the technique. The balun
operates from 40 to 904%~ with excellent
performance.
synthesis technique based on the analytically derived
INTRODUCTION
Baluns are key components in balanced circuit
topologies such as double balanced mixers, pushpull amplifier^ and “CY
doublers [1]-[4].
Various balun digurations have been reported for
applications in microwave integrated circuits
(MIC’s) and microwave monolithic integrated
circuits (MMIC’s) [5]-[7]. Among them, the planar
version of the Marchand balun [8] is perhaps one of
the most attsactve due to its planar structure and
wideband performance.
The planar Marchand balun consists of two coupled
sections, which may be realised using microstrip
coupled lines [9], Lange couplers [7], multi-layer
coupled structures [lo], or spiral coils [ll], [12].
These baluns are usually designed thmugh circuit
scattering parameters of the multilayer planar
Marchand balun. In this paper, the planar Marchand
balun is analysed as a combination of two identical
coupled sections. This shnplities the balun design to
designing couplers with the approPriate coupling
factor. This approach also provides valuable insight
into the balun o p t i o n , relating the coupling factor
of the coupler to the balun input and output
impedances.This results m an effective technique for
designing impedance-transforming baluns.
ANALYSIS
Fig. 1 shows the design graph and a schematic
diagram with the port definitions of the planar
Marchand balun. It provides balanced outputs to
load terminations Z,, h m an unbalanced input with
source impedance Z,,. It consists of two coupled
sections, each of which is one quarter-wavelength
long at the center fhquency of operation. The
derivation of the design graph is now descn’bed.
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2000 IEEE MlT-S Digest
-
mu
.I
When the balun output load termhations are
changed from 2, to 21, the balun S matrix of the
balun has to be mormalised and is refmed to as
[q'h,un, and it can be shown that the S-parameters
become:-
[?
[5
1-c2-+1
Sh.11
-14
4
I
1
I
I
1
0
2
4
6
8
10
=
1+ c2
1
- 1)
Zdzo
Figure 1:Reqyiredcouplingvs. impedance ratio
For ideal couplers with infinite directivity and
coupling f w , C, the S parameters are given by:
sc.11
s
~
sc.21
=, -~ j ~ J Z
s~..,~
=o
=
(44
l+C2[?4)
=c
=0
1 -C2
Sb.22
(1)
For a specific coupling f m r , the design equations
for the coupled line are given in terms of its even
Sh.23
=
(44
and odd mode impedances [16]:
z,
= z,
J
E
I-c
The S parameters of the balun can then be obtained
by relating the voltage waves at the three ports,
with the known port terminations. The balun S
matrixhas the form:
r
[Slhalun
=
l
l
sh.21
'*?I]
sh.21
sh,22
sh,23
',,I
',23
'h,22
(3)
Equation (4b) shows that the use of identical
coupled sections results in balun outputs of equal
amplitude and opposite phase, regardless of the
coupling factor and port terminations. To achieve
optimum power transfer of -3dB to each port, we
require
Equating (4b) and (5), the required couprmg factor
for optimum b t z pe&rmance is given by:
(6)
,
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To enable accurate charactensat
*
ion usmg the
HP85lOXF RFOW system at the University of
Surrey, three b a l m were fabricated and measured
as twoports with each spare port terminated onchip m turn. The balun insertion loss and phase error
performance are shown in Fig. 3, and the circuit
exhibits excellent bandwidth (40to 9OGHz).
Figure 3: Measured loss and phase error
Eqn (6), results m the graph of required coupling
fixtor in dB vs the chosen impedance-transfdg
ratio in Fig. 1. It is mtemting to note that when all
the ports are terminated with the same impedance
like 5052, where the impedance transforming ratio
is unity, the required coupling factor is 4 . 8 d B and
not -3dB. Based on (4), the use of commonly
ass~med -3dB couplers [7] will result in an
insertion loss and output isolation Of -3.5dB and
input, output retum loss of -9.5dB at the centre
frequency.
EXPERIMENETAL RESULTS
To validate the analyticalresults and d e "
the
design approach, an impedance transforming mmwave balun has been designed. The balun performs
transformation to a higher impedance of 16022. This
may reptesent requirements in balance diode mixers
and multiplim [17], [4] for transforming between a
50Q source impedance and the diode impedances.
The circuits are fabricated on a 400 micron GaAs
substrate using the mm-wave IC fabration facilities
at the University of Glasgow.
The required coupling factor for this chosen
impedance transformation, as given by (9, is 8.7dB. This loose coupling can be easily achieved
using simple parallel coupled lines. Fig. 2 shows a
photograph of the fabricated circuit(s):
0.045
Frequency IGHr
120
The mput and output matching is shown in Fig. 4.
The data illustrated has not been r e n d i s e d , but
the output impedance has been determined to be
160 Ohms.
0*04'
Frequency /GHz 12
Figure 4: Measured port matches
Figure 2: Photograph of fabricated circuits
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CONCLUSIONS
This paper has demonstrated that impedance"sfoming bahns can be realised using planar
Marchand bahxns with two identical coupled
sections. Analytical results relating the impedancemf&g
ratio to the coupling factor of the
couplers have been presented. The technique has
been verified through experimental results of a mmwave GaAs monolithic planar Marchand baluns
using CPW coupled lines. This impedancetmsfinming balun could p v e mvaluable in the
design of balanced microwave circuits such as
mixem, pushpull amplifiersand frequency doublers.
[8]
N. Marchand, "Transmission line conversion
transformers," Electron., vol. 17, no. 12, pp.142-145,
Dec.1944.
[9]
W. R Brinlee, A. M. Pavio and K. R. Varian, "A
novel planar double-balanced 6-18GHz MMIC mixer"
in 1994 IEEE Microwave and Millimetre Wave
Monolithic Circuit Symp. Dig., pp. 139-142
[lo]
K. Nishkawa, I. Toyoda and T. Tokumitsu,
"Compact and broad-band three-dimensional MMIC
balun," IEEE Trans. Microwave Theory Tech., vol.
MTT-47, pp. 96-98, Jan. 1999.
[Ill
T. Gokdemir, S. B. Economides, A. Khalid, A. A
Rezazadeh and I. D. Robertson, "Design and
performance of GaAs MMIC CPW baluns using
over-laid andspiral couplers," in 1997 IEEE Int.
Microwave Symp. Dig., pp. 401-404.
[12]
Y. 1. Yoon et al., "Design and characterization of
multilayer spiral transmissiodine baluns," IEEE
Trans. Microwave Theory Tech., vol. MTT-47, pp.
1841-1847, Sept. 1999.
REFERENCES
[11 S. A. Maas, "A broadband, planar, doubly balanced
monolithic Ka-band diode mixer," IEEE Trans.
Microwave Theory Tech., vol. MlT-41, pp. 23302335, Dec. 1993.
[2]
S. A. Maas,F. M. Yamada, A. K. Oki, N. Matovelle
and C. Hochuli, "An 18-40 GHz monolithic ring
mixer," in 1998 IEEE Radio Frequency Integrated
Circuits Dig., pp. 29-32.
[3]
P. C. Hsu, C. Nguyen and M. Kintis, 'Uniplanar
broad-band push-pull FET amplifiers," IEEE Trans.
Microwave Theory Tech., vol. MTT-45, pp. 21502152,Dec. 1997.
[4]
S. A. Maas and Y. Ryu, "A broadband, planar,
[5]
[q
[I
[131 C. M. Tsai and K.C. Gupta, "A generalized model for
coupled lines and its applications to two-layer planar
circuits," IEEE Trans. Microwave Theory Tech., vol.
MTT-40, pp. 2190-2099, DIX 1992.
[14]
J. H. Cloete, "Graphs of circuit elements for the
Marchand balun," Microwave J., vol. 24, pp. 125-128,
May 1981.
monolithic resistive frequency doubler," in 1994
IEEE Int. Microwave Symp. Dig., pp. 44346.
[I51 R Schwindt and C. Nguyen, "Computer-aided
analysis and design of a planar multilayer Marchand
balun," IEEE Trans. Microwave Theory Tech., vol.
MTT-42, pp. 1429-1434, July 1994.
A. M. Pavio and A. Kikel, "A monolithic or hybrid
broadband compensated balun," in 1990 IEEE Int.
Microwave Symp. Dig., pp. 483-486.
[16]
T. Chen et al. "Broadband monolithic passive baluns
and monolithic double balanced mixer", IEEE Trans.
Microwave Theory Tech., vol. MTT-39, pp. 19801986,Dec. 1991.
Matthai, G. L., L. Young and E. M. T. Jones,
Microwave Filters, Impedance Matching Networks,
and Coupling Structures, Dedham, MA: Artech
House, 1980.
I171
S. A. Maas, Microwave Mixers, (2nd ed.) Norwood,
MA: Artech House, 1992.
M.C.Tsai,"A new compact wideband balun," in
1993 IEEE Microwave and Millimetre Wave
Monolithic Circuit Symp. Dig., pp. 123-125.
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