Aperture Tuned Antennas for 3G-4G Applications Using MEMS
Digital Variable Capacitor
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Abstract — Industrial design constraints coupled with
increased frequency coverage required for today’s mobile Smart
Phone handsets, are making it difficult to develop highly efficient
broadband antenna solutions. This paper will show how antenna
aperture tuning using a high performance MEMS
(microelectromechanical system) Digital Variable Capacitor can
improve antenna performance across all frequency bands for 3G
and 4G LTE communications.
Index Terms — Antennas, 4G Mobile Communications,
Capacitors, Cellular Phones, Microelectromechanical Systems,
Mobile Communications.
I. INTRODUCTION
Today’s mobile devices exhibit a variety of challenges that
now demand a single antenna service multiple bands (multiband). Firstly, the need for multiple frequency bands to be
covered by mobile devices widened dramatically with the
introduction of 4G communication standards. Accounting for
the need for 2G-3G compatibility and for worldwide roaming,
the total number of frequency bands to be covered by one
device can easily exceed 10. On top of that, 4G LTE requires
two balanced channels for MIMO, ideally coupled to two
separate and uncorrelated antennas. The volume designated
for the antenna inside a phone is continuously decreasing, due
to the trend for slimmer devices with increased functionalities.
Moreover, display size increase has also been the trend in the
last few years, reaching up to 4.8” diagonal, creating a ground
plane very close to the antenna.
In order to meet wireless carrier performance requirements,
and improve the use experience, there is a recent trend by
handset OEM’s to use reconfigurable (tunable) antenna
technologies. There are two approaches for antenna tuning;
tunable impedance matching (TIM), and antenna aperture
tuning (AT). This paper compares TIM and AT, establishes
the component requirements for effective AT and gives
examples of AT using planar inverted F antenna (PIFA) type.
II. COMPARISON OF TIM AND AT SOLUTIONS
TIM and AT both aim at maximizing the total efficiency of
the antenna across a wide frequency range. Total efficiency is
equal to the intrinsic antenna radiation efficiency, multiplied
by the return loss. TIM only compensates for the return loss,
while AT further optimizes the intrinsic radiation efficiency
and achieves a higher total efficiency across the desired band.
The performance differential between TIM and AT can be
huge. In a typical situation, accounting for all previously
described constraints, the radiation efficiency of an antenna at
a desired frequency band which is not its intrinsic resonance
can be as low as -10dB (~10%) or lower. This means that TIM
can only target -10dB or worse total efficiency within the
tuning frequency range. On the other hand, AT can shift the
intrinsic resonance of the antenna to the desired frequency
band, therefore achieving total efficiency values better than
-5dB (~30%) throughout the full tuning frequency range. This
is a significant improvement which generates major benefits to
the TX-RX chain and network usage performance.
The clear advantage of AT compared to TIM is widely
accepted in the industry and it has been proven both
theoretically and with practical work in the past [1, 2].
However, when looking at practical implementations in
current mobile devices, AT is not as widely adopted as it
should be expected. The reasons for this relate to
shortcomings in the tunable component technologies available
for AT implementation: too much loss in the tunable
component; additional loss due to the implementation
parasitics; large size and cost of the tunable component.
The following sections show what level of performance is
required for a tunable component technology in order for AT
to show significant benefit and how the Cavendish Kinetics
digital variable capacitor (DVC) technology is able to deliver
such performance and make antenna tuning a reality.
III. CAVENDISH KINETICS DIGITAL VARIABLE CAPACITOR
The Cavendish Kinetics Nanomech™ process integrates
MEMS variable capacitors within the CMOS back end of line
metallization [3,4]. The fabrication is 100% CMOS
compatible and no special packaging processes are needed,
yielding a highly reliable and very low cost technology. The
currently available DVC product is a 5-bit (32 states) shunt
capacitor. Alternative capacitor sizes are available in the
product family, covering a total range of 0.35 – 5.0pF. The
device is delivered as a WLCSP solder bumped bare die of
~2mm2 size, with monolithically integrated CMOS serial
interface digital control and voltage up-converter.
While antenna tuning is the targeted application, the device
is also currently being adopted at prototype phase for other
reconfigurable front-end blocks such as tunable PA matching
and tunable filters. Nanomech™ technology is highly flexible
and allows the product to be customized for targeted
applications adapting specifications such as number of control
bits (i.e. step size), capacitance range and architecture.
Table 1 contains an overview of the key device performance
parameters of the Cavendish Kinetics DVC that directly relate
to antenna tuning.
978-1-4673-2141-9/13/$31.00 ©2013 IEEE
TABLE 1.
KEY PERFORMANCE PARAMETERS FOR EFFECTIVE APERTURE
TUNING
Antenna
Performance
Tuning
Range
Impact
Need
Band
Coverage
C-ratio > 3:1
Low
inductance
High SRF
Low
minimum C
Low ESR
High Q
High IP3
Low
harmonics
Low
parasitics
Low package
parasitics
Low cost
Antenna
Efficiency
TRP, TIS
Low Noise
TIS
Small Size
Cost,
Performance
location of tuning
impedance
antenna length
Cavendish
Kinetics DVC
C-ratio 5:1
SRF > 5GHz
Total tuner
impedance
Z=R + jωL -j/(ωC)
Cmin 0.6pF
Q>225 @
750MHz
ESR ~ 0.3Ω
IP3>65dBm
TX spurious < 85dBm
RX spurious < 120dBm
WLCSP base
die
2mm2
RF+GND
feed
Figure 2. Description of the PIFA tunable antenna model used for
performance analysis
Figure 1 shows what would be the realized Q factor of the
DVC when used in a typical low-band antenna tuning
application.
The PIFA antenna model shown in Figure 2 is used for this
analysis. It includes the tuner impedance (modeled as a series
RLC) located at a given distance from the feed. The antenna is
designed to work across the frequency band 700-1000MHz by
changing the tuner shunt capacitance in the 1-3.5pF range.
A. High Q Factor is the Antenna Tuning Enabler
4
3
375
2.5
2
250
1.5
1
125
DVC Capacitance [pF]
3.5
0.5
0
0
700
750
800
850
Target Frequency [MHz]
900
Figure 1. Measured quality factor (Q) of the Cavendish Kinetics
DVC used for tuning and antenna in the 700-900 MHz band
The PIFA model is used to analyze the effect of the
equivalent series resistance (ESR) on the tunable antenna
performance. In Figure 3 the antenna radiation efficiency is
plotted for the extreme values of the tuner capacitance and for
three alternative values of ESR.
0
Radiation Efficiency [dB]
500
DVC Q Factor at Target Frequency
platform is used in order to quantify the impact of the key
device parameter which is the Q factor.
C=1.0pF
C=3.5pF
-1
-2
R=0
R=0 C=1pF
-3
R=0.5 C=1pF
R=1Ω
-4
R=1.0 C=1pF
R=0 C=3.5pF
-5
R=0.5 C=3.5pF
-6
R=1.0 C=3.5pF
-7
Since loading an antenna with a shunt capacitance reduces
its resonance frequency, the Q factors remains very high
throughout the full tuning frequency range. The following
section focuses further on Q factor (ESR) since it is by far the
most important specification that enables antenna tuning to
work in real applications.
IV. MAKING ANTENNA TUNING WORK
Aperture tuning is a well-known methodology, but it
requires certain device characteristics to make it work in real
applications. In this section, a PIFA tunable antenna design
0.7
0.8
0.9
Frequency [GHz]
1
Figure 3. Radiation efficiency analyzed as a function of ESR at the
Cmin and Cmax extremes of the tuning range
An ideal capacitor with no loss would yield efficiency of
-1dB at 1000MHz at Cmin and of -3dB at 700MHz at Cmax.
But as the ESR starts to increase, while the Cmin performance
at 1000MHz is not affected, the efficiency at 700MHz at
Cmax quickly drops. In fact, for ESR > 0.5Ω the efficiency at
700MHz at Cmax is worse than the “un-tuned” Cmin case.
978-1-4673-2141-9/13/$31.00 ©2013 IEEE
This means that if the ESR is too high (in this case >0.5Ω)
there is no benefit in tuning the antenna at all.
This is a very important result since it quantifies the amount
of allowed loss of the tuner component in order for AT to be
effective. Common state of the art high Q SMT capacitors
have typical ESR values in the 0.1-0.15Ω range. Consider a
typical tuner implementation based on SPMT switch and a
bank of such fixed capacitors. Current state of the art solid
state switches have insertion loss of ~0.1dB or higher at these
frequencies. Even though this is acceptable for most switching
applications, the corresponding equivalent ESR of the switch
(assuming the closed switch is an ideal resistor) results to be
~1Ω! This ESR is far too high for antenna aperture tuning to
provide any benefit.
The Cavendish Kinetics DVC exhibits an ESR of ~0.3Ω at
Cmax, which is the critical tuner state as shown in Figure 3.
This makes this technology a critical advancement in antenna
tuning, together with all other important specifications shown
in Table 1.
based on hollow plastic carrier and copper patterns, mounted
on a PCB board which implements the mobile device ground
plane. The DVC is assembled on a small flex circuit which is
then soldered directly to the antenna copper pattern and to the
PCB ground plane. This allows for the best RF performance
since the tuning component is placed inside the antenna
aperture. In the model the flex circuit including the DVC is
implemented by using a via element with arbitrary impedance
set to match that of the DVC including the flex circuit
parasitics (equivalent LRC).
The design flow is as follows: 1) the antenna geometry
(within a maximum volume constraint) and location of the
DVC are optimized by maximizing the radiation efficiency at
two capacitance/frequency combinations: 0.6pF/900MHz and
2.1pF/700MHz; 2) a fixed matching network (shunt L + series
C) is designed to match input impedance of the antenna at
both capacitance/frequency combinations above; 3) the final
return loss and total efficiency are verified for the full DVC
capacitance range, final tuning if necessary.
B. Dual-band Tunable PIFA Design
V. DESIGN EXAMPLES OF TUNABLE PIFA ANTENNAS
This section describes two alternative tunable antenna
designs. The first is a simple case only focusing on the low
band radiation problem (700-900MHz range). The second is a
more complete antenna that aims at tuning the low band
resonance while at the same time maintaining a high band
resonance unaffected by the tuning. The first design has been
prototyped and test results will be shown in the next section.
(A)
The second design example still targets tuning the low band
across the same frequency range, but the antenna has also a
high band radiation which should be unaffected by the DVC
state. This reflects more closely a typical main antenna design
for a mobile device. The antenna implementation and DVC
integration approaches are the same as previous low band
design.
(B)
Figure 5. 3D model view of the dual-band PIFA antenna including
the DVC and a 6-elements fixed matching network.
(C)
Figure 4. Model of the low band PIFA tunable antenna design: (A)
top copper pattern showing location of DVC; (B) PCB pattern
showing ground plane clearance, feed and 2-elements matching
network; (C) full 3D model including PCB ground plane
(110x60mm).
A. Low Band Tunable PIFA Design
A PIFA antenna is designed targeting a radiation peak in the
low band to be tuned by a DVC shunt to ground. Figure 4
shows the model implemented in Sonnet ™. The antenna is
The 3D model is shown in Figure 5. The PIFA antenna
structure is made of 2 arms, targeted to low-band and highband respectively. The DVC is connected shunt along the lowband arm. The whole antenna is 0.6x9.5x55mm3 and it sticks
out of the PCB ground plane (90x60mm2). The design flow is
an extension of the previous low-band only case: 1) the
antenna geometry and location of the DVC are optimized by
maximizing
the
radiation
efficiency
at
three
capacitance/frequency
combinations:
0.6pF/900,
1.5pF/1900MHz, 2.1pF/700MHz; 2) the fixed 6-elements
matching network is designed to simultaneously match the
978-1-4673-2141-9/13/$31.00 ©2013 IEEE
input impedance presented by the antenna at all three
capacitance/frequency combinations above; 3) the final return
loss and total efficiency are verified for the full DVC
capacitance range, final tuning if necessary.
Figure 6 shows the total efficiency simulated for the two
extreme values of the DVC capacitor: the low band radiation
is tuned between 700 and 900MHz while the high band
radiation at 1900MHz remains unaffected. A third radiation in
the 1400-1600MHz range is also present and could be taken
advantage off (with further optimization) to fulfill global
roaming requirements.
Figure 7. Prototype reference PIFA tunable antenna for low-band
only operation
Starting from the design described in section V, a minor
final optimization is required on both the geometry of the
antenna and the values of the fixed impedance matching
network. The total antenna efficiency is finally obtained
across all control states, as shown in Figure 8. The envelope of
all total efficiency curves also shown represents the antenna
radiation efficiency envelope across all DVC states.
0
-2
A reference prototype antenna based on the PIFA design in
Figure 4 has been fabricated and tested. The PIFA tunable
antenna is created by assembling the DVC on a flex circuit
which is then directly soldered to the aperture of the antenna.
Figure 7 shows a picture of the reference antenna prototype.
Efficiency [dB]
VI. TUNABLE PIFA ANTENNA PROTOTYPE RESULTS
-4
C=1.2pF
-6
C=1.5pF
-8
C=1.8pF
-10
C=2.1pF
-12
C=2.4pF
-14
C=2.7pF
-16
C=3.0pF
-18
C=0.9pF
Radiation Efficiency
-20
0.7
0.75
0.8
0.85
Frequency [GHz]
0.9
Figure 8. Total efficiency of the PIFA reference tunable antenna
prototype, for a sub-group of the DVC control states
VII. CONCLUSION
Figure 6. Total efficiency of the dual-band PIFA antenna for the two
extreme DVC values
PIFA
110 mm
DVC on
a flex
circuit
SPI and
power
60 mm
It is well known that antenna aperture tuning provides better
antenna efficiency than feed point impedance match tuning for
the same antenna volume. In order to achieve high
performance using aperture tuning with capacitive loading, the
tuning component must have high Q in order to reduce
implementation losses. In addition, the component must have
low Cmin, acceptable tuning range, be physically small and
cost effective for implementation in consumer electronics
products. The CK DVC example exhibits these characteristics
and enables aperture tuned antennas in real applications. The
design examples shown in this paper are aimed at emphasizing
how the tunable antenna is to be considered as a complete
system: the antenna geometry, the tuning component and its
implementation parasitics are all part of the full design
optimization process.
REFERENCES
[1] Iellici, D., “New Developments in Embedded LTE Antennas for
4G Mobile Devices,” Antenna Systems 2010.
[2] Rowell, C.R.; Murch, R.D., “A capacitively loaded PIFA for
compact mobile telephone handsets,” Antennas and
Propagation, IEEE Transactions on, May 1997.
[3] www.cavendish-kinetics.com
[4] Gaddi R., et Al., “Reliability and performance characterization
of a MEMS-based non-volatile switch,” 2011 International
Reliability Physics Symposium.
978-1-4673-2141-9/13/$31.00 ©2013 IEEE