“High CMRR in Reduced-Coupling Monolithic Baluns”
by
Robert C Frye, Phoo Hlaing* and Kai Liu**
RF Design Consulting, LLC, Berkeley Heights, NJ, 08854, USA *STATS ChipPAC,
Ltd Singapore 768442
**STATS ChipPAC, Inc, Tempe AZ, , 85284, USA
re 569059
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High CMRR in Reduced-Coupling Monolithic Baluns
Robert C Frye, Phoo Hlaing* and Kai Liu**
RF Design Consulting, LLC, Berkeley Heights, NJ, 08854, USA *STATS ChipPAC, Ltd Singapore 768442
**STATS ChipPAC, Inc, Tempe AZ, , 85284, USA
Abstract — Monolithic technologies (e.g. semiconductor-based
passive components) have small dimensions and tight fabrication
tolerances. For coupled-resonator baluns this results in superior
amplitude and phase balance characteristics, which are directly
related to the common-mode rejection. Achieving good CMRR
becomes more difficult at higher frequencies because of the effects
of capacitive coupling between the primary and secondary coils in
the circuit. In this paper a modified physical design is presented
that reduces the capacitive coupling and offers superior CMRR.
These designs also have reduced inductive coupling, so they have
a more narrow-band response. Measured performance characteristics of a practical example of one of these baluns are presented.
i1
iU
v1 +-
vU+i1
i2
v2 +i2
iU
3-port
network
Balun transformers are widely used in the front-end of wireless systems. Nearly all commonly used antenna configurations
are single-ended, and most RF integrated circuits use differential circuit architectures, so balun transformers are required to
interface the two. With growing emphasis on miniaturization
in wireless consumer products, there is a strong trend to integrate balun transformers into a common package with the
RFIC, either using on-chip passive components [1] or as separate integrated passive devices (IPDs).
Silicon IPDs are especially attractive for integration with
RFICs because of their common form factor and low cost [2],
and they have some performance advantages over on-chip implementation. In either kind of implementation the design
approach for the balun transformers is similar, and they share a
common planar monolithic structure.
The primary function of the balun is to provide mode conversion from a single-ended to differential port. Often, the
balun also transforms impedance between these two ports to
obtain a match. Figures of merit for the balun are the input and
output return loss, insertion loss and balance (amplitude and
phase) at the differential port.
Common-mode rejection in baluns is closely related to the
balance. In receiver circuits (LNAs) common-mode signals
can result in power supply modulation and self-mixing. In
transmitters, even-order harmonics are typically present in the
common-mode of their output, and it is desirable to block
these to maintain high linearity and electromagnetic compliance. For these reasons and others, CMRR is often a key
design consideration in baluns.
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iD
vD +-
3-port
network
iD
iC
i3
v3 +-
vC +iC
a: conventional S-parameter
representation
INTRODUCTION
978-1-4244-6057-1/10/$26.00 ©2010 IEEE
reference
plane
i3
Index terms – Balun, circuit topology, passive circuits.
I.
reference
plane
b: mixed-mode S-parameter
representation
Fig. 1: Conventional and mixed-mode S-parameters.
II. CMRR AND BALANCE
A balun is a 3-port device. Its S-parameters are measured
using the conventional arrangement shown in Fig. 1a. However, many of its key figures of merit are derived from the
mixed-mode S-parameter representation shown in Fig. 1b. In
the mixed-mode representation, the conventional S-parameters
for the two ports that make up the differential terminals of the
device are rearranged into differential and common-mode
components. These two components are linearly related to the
conventional signal components, and are an equally valid alternative representation [3, 4]. In Fig. 1, the conventional ports
1, 2 and 3 are transformed into unbalanced (U), differentialmode (D) and common-mode (C) ports. If port-1 is the unbalanced port, then the differential-mode and common-mode port
signals are derived from linear combinations of the signals at
ports 2 and 3.
In a balun, the “insertion loss” (or, more correctly, the differential-mode gain) is the power scattered from the unbalanced
port to the differential port. It is related to the conventional Sparameters by
DMG = S DU = (S 21 − S 31 ) / 2 .
(1)
Similarly, the common-mode gain (inverse of the CMRR) is
the power scattered from the unbalanced port into the com-
IMS 2010
mon-mode port. It is related to the conventional S-parameters
by
CMG = S CU = (S 21 + S 31 ) / 2 .
(2)
The balance characteristics of a balun are expressed in terms
of the conventional S-parameters. For signal input on the unbalanced port (port-1) the amplitude and phase differences are
measured between ports 2 and 3. These can be expressed in
terms of the common-mode and differential-mode gains, from
Eqns. 1 and 2:
S 21 = (CMG + DMG ) / 2
(3)
and
S 31 = (CMG − DMG ) / 2 .
(4)
From Eqns. 3 and 4 it can be seen that as the common-mode
gain approaches zero (i.e. very high CMRR),
S 21 ⇒ − S 31
(5)
i.e. the amplitude balance is perfect and the phase difference
between the two outputs is 180°. So, high CMRR is directly
associated with good amplitude and phase balance characteristics.
The balance characteristics of typical commercial discrete
baluns for use at GHz frequencies are ±1dB of phase imbalance and ±10° of departure from the ideal 180° phase
difference. These characteristics correspond roughly to 20dB
minimum CMRR.
to flow in the two halves of the center-tapped inductor. The net
result is that no signal is transferred to the unbalanced side. In
the ideal circuit, the CMRR is infinite and the balance is perfect.
In practical coupled-resonator baluns, in addition to the mutual inductive coupling between the two coils, there is
inescapably some parasitic capacitive coupling. A commonmode signal applied to the balanced terminals of the device
will not couple appreciable signal to the unbalanced port magnetically, but it will couple electrically through this parasitic
capacitance. So, even if great care is exercised to make the
inductor coils symmetric, ensuring that the tap point is at the
electrical center of the coil, the CMRR will not be perfect because of parasitic capacitive coupling.
III. COUPLED RESONATOR BALUNS
Fig. 2 shows a schematic diagram of a coupled-resonator balun. Its principles of operation are very similar to those of a
simple two-pole band-pass filter [5], with the exception of the
added center tap on the differential side of the coupled inductor. A key advantage of the coupled-resonator circuit is that
the center-tap (shown grounded) can be used to feed DC bias
into the terminals of the balanced port. This is especially useful in TX baluns, since it eliminates the need for bias tees in
the output transistors of the PA.
In the ideal circuit, the currents in the two resonators are
coupled by mutual inductance. So, an input to the unbalanced
side of the circuit induces current in the balanced side, and
vice-versa. A common-mode signal applied to the balanced
terminals of the device will cause equal and opposite currents
Fig. 2: Balun coil designs.
IV. COIL DESIGNS
For practical designs, these problems of capacitive coupling
become more severe at higher operating frequencies. In principle, the balun coil dimensions would shrink with increasing
frequency to maintain constant levels of coupling. However,
minimum design rules and resistive losses in narrow conductors place practical lower limits on the sizes of the coils.
Consequently, the parasitic capacitance becomes a greater
problem at higher frequencies.
Fig. 3 shows two alternative balun designs for operation in
the 802.11A band from 5.15 to 5.83GHz. For clarity, the primary and secondary windings of the coupled inductors are
shown in different colors, although except for small crossovers, they are implemented in the same metal layer. In these
examples the primary and secondary coils are edge-coupled.
Alternative designs using multilayer conductors can use broadside coupling, which usually results in a higher magnetic
coupling coefficient, but this also results in higher capacitive
coupling.
k
ZU
LU CU
CB
LB
ZB
Fig. 3: Coupled resonator balun circuit.
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For conventional coupled inductor baluns, coil designs are
chosen to make the coupling coefficient, k, between the two
coils as large as possible. Similar to coupled-resonator bandpass filters, in coupled-resonator baluns a larger coupling coefficient results in a larger bandwidth. This gives the balun
greater robustness to manufacturing variations and results in a
flatter passband response. To obtain high coupling coefficient,
the windings of the primary and secondary coils are interleaved to maximize the mutual inductance between the two
coils. Note, however, that this also results in higher capacitive
interaction between the two.
An alternative design is shown on the upper right. In this design, the primary and secondary coils are not interleaved.
Instead, one lies predominantly inside the other with some
small added space between. This results in greater average
separation between the two coils, reducing both the inductive
and capacitive coupling.
The schematic for these two designs is shown in the lower
part of the figure. Note that the capacitor on the balanced side,
CB, is divided into two parts connected to the center-tap. By
slightly unbalancing these two capacitors, some optimization
of the balance can be obtained, but large adjustments result in
degraded return loss
Table 1 compares simulated characteristics for these two baluns. There is a certain amount of arbitrariness in this
comparison, since it depends on the weight that is assigned to
different balun figures of merit during the optimization. Nonetheless, the results shown in the table illustrate the general
tradeoffs. Because of the reduced coupling coefficient, the
bandwidth (defined by the 0.5dB roll-off frequencies) for the
alternative design is significantly reduced, but still well in
excess of the application’s requirements. The insertion loss is
increased slightly. Most significantly, the CMRR is improved
by more than 10dB. This improvement is reflected in the improved balance characteristics.
The conventional approach to balun design seeks to maximize the coupling coefficient between the two coils. This
approach places emphasis on the bandwidth, match and insertion loss performance of the balun. However, this approach deemphasizes the importance of balance and CMRR, and especially at higher frequencies may degrade these figures of merit.
Table 1: COMPARISON OF SIMULATED CHARACTERISTICS
Parameter
Coupling coefficient
Bandwidth (GHz)
Insertion Loss (dB)
CMRR (dB)
Amplitude imbalance (dB)
Phase imbalance (deg)
Conventional
0.65
3.85
0.75
25.9
0.90
2.4
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Reduced
Coupling
0.44
1.90
0.94
36.8
0.12
1.8
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Fig. 4: Micrograph of a balun using reduced coupling.
Fig. 5: measured and simulated common-mode gain.
V. AN EXAMPLE
Figure 4 shows a micrograph of an example balun of this
more weakly coupled type used in a commercial application.
The actual coil size in this example is 0.25mm2, implemented
in IPD technology [6]. This balun is designed for operation in
the cellular GSM band centered at 1810MHz. It is part of a
multi-band system-in-package assembly containing several TX
baluns. In this system, the center-tap connection, which is used
to feed DC bias into the balanced port terminals, is shared with
a number of other baluns. Since the common-mode bias connection is not perfectly isolated, RF coupling to the bias
supply may introduce a common-mode component at the balanced terminals. At higher output powers, approaching the
compression point of the amplifier, even-harmonic components in the output spectrum are also introduced into the
common-mode. For both of these reasons good CMRR is important for this application to remove these common-mode
signals from the balun output .
Measured and simulated common-mode gain are shown in
Fig.5. The complicated structure exhibited by the characteristics as a function of frequency mainly arises from the
frequency-dependent impedance of the bias line external to the
balun (i.e. the center-tap of the balun is not ideally connected
to ac ground.) The key feature to note is the CMRR in the
pass-band, from 1710 to 1910MHz. At the band center the
IMS 2010
Acknowledgements
The authors wish to acknowledge the help of Guruprasad
Badakere, JaeHan Chung and Gwang Kim of STATS ChipPAC, Ltd. for help with device assembly and electrical
measurements.
REFERENCES
Figure 6: Amplitude balance and phase difference in the passband.
measured CMRR is 39.0dB, as compared with a simulated
value of 34.5dB. This is an exceptionally good CMRR for a
balun, and is a significant improvement over typical commercial discrete baluns. At the second harmonic the CMRR is
more than 35dB.
The measured and simulated amplitude and phase balance
performance in the pass-band is shown in Fig. 6. Over the
band, the worst-case amplitude imbalance is about 0.1dB, and
the maximum phase imbalance is 4°.
V. CONCLUSIONS
The conventional approach to the design of coupledresonator baluns is to maximize the coupling coefficient between the primary and secondary coils. This maximizes the
balun bandwidth and minimizes the loaded Q of the resonators, making them less sensitive to internal resistive losses.
The result is a wide-band balun with low insertion loss, which
is generally desirable.
However, this design approach does not take into account
the undesirable impact of increased coupling on the commonmode characteristics of the balun. For practical physical designs, higher magnetic coupling is invariably accompanied by
increased capacitive coupling. This capacitive coupling reduces the CMRR and degrades the balance characteristics of the
device.
Typical coupled-resonator baluns have bandwidth that is
well in excess of most application requirements. In many cases, the use of a more weakly coupled circuit results in only a
very modest reduction in the insertion loss, accompanied by
substantial improvements in CMRR.
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