Download Variable Capacitor Suits Mixed NFC Specifications

TECH | FOCUS
Variable Capacitor Suits Mixed NFC
Specifications
(2) Performs F0 matching by
changing the capacitance upon
receiving the control voltage.
TX
I
n recent years, near field
communications (NFC)
Matching
NFC IC
Circuit
technology, which operates at frequencies
TX
of about 13.56MHz, has
IC
been introduced in a wider
product market, including
tablets, personal computers, and wearable devices
DAC
(1) Generates a
besides compact mobile decontrol voltage.
vices such as smartphones.
Furthermore, numerous Fig. 2: How to use a thin-film variable capacitor
NFC communication stan- (a) Capacitance adjustment using DAC
dards, which vary in accordance with the application and the place
NFC antennas. Because of such reasons,
where the product will be made availa growing demand for resonance freable, have been launched with varying
quency matching devices that would enoptimal resonance frequency range deable products to easily implement NFC
pending on the protocol. The optimal
functions, have been observed.
resonance frequency range also differs
To meet the abovementioned market
in relation to the NFC’s function, such
needs, Murata Manufacturing, Co., Ltd.
as card emulation and reader/writer
has introduced to the market a thinfunctions. In addition, it is necessary to
film variable capacitor (Fig. 1) that can
consider various inductance values of
control capacitance using control volt-
Fig. 1: Thin-film variable capacitor
(2) Performs F0 matching by changing the
capacitance upon receiving the control voltage.
Recommended resister value
Resister
Value
(1)
Number of GPIO port
3port
4port
5port
6port
TX
Matching
Circuit
Resister
GPIO0
240K
180K
180K
180K
510K
360K
360K
360K
R3
1M
750K
750K
750K
R4
-
1.5M
1.5M
1.5M
R5
-
-
3.0M
3.0M
R6
Step of
adjust
-
-
-
6.2M
8
16
32
64
TX
NFC
IC
IC
R1
R2
3.5
GPIO1
GPIO3
GPIO4
GPIO5
(1) Generates the control voltage by a resistor voltage divider
using the combination of a GPIO port (H or L output) and
resistor device.
Tuning Voltage [V]
3.0
GPIO2
2.5
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
step
Relationship between steps and control voltage
Fig. 2: How to use a thin-film variable capacitor
(b) Capacitance adjustment using the GPIO port
AEI April 2014
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TECH | FOCUS
(2) Performs F0 matching by
changing the capacitance upon
receiving the control voltage.
Fine tuning
The optimal resonance frequency adjustment can be performed for each terminal regardless of various inductance
values of antennas. This technique is used
for many FeliCa terminals at present.
TX
Matching
Circuit
TX
NFC
IC
IC
f=100Hz
3V
GPIO0
70
Capacitance [pF]
V_tune
36
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Initial value
65
60
Duty 100%
(=3V)
55
50
45
Dynamic matching
Resonance frequency matching adjustment can be performed by freely
changing control voltage as required,
regardless of NFC communication standards and functions, such as card emulation and reader/writer functions.
40
35
0
20
40
60
80
100
Duty[%]
Fig. 2: How to use a thin-film variable capacitor
(c) Capacitance adjustment using the PWM method
However, problems concerning high
dielectric constant materials, such as
displacement of capacitance values due
to repeated application of voltage (DC
bias aging characteristics) and hysteresis
characteristics of capacitance changes,
are also present. Murata has optimized
the composition of dielectric material to
minimize these problems and has confirmed that the thin-film variable capacitor can be used for the adjustment of the
resonance frequency of an NFC application without any trouble.
Key Advantages
The adjustment of resonance frequency for NFC as sample application
is described below. The ideal communication distance for NFC is achieved by
connecting the antenna and the capacitor
in parallel and adjusting the resonance
frequency to about 13.56MHz. Using a
thin-film variable capacitor for the capacitor to be connected in parallel provides the following advantages
Standardization of matching circuit
Matching circuits, which vary for individual terminals and antennas, can be
standardized by using a thin-film variable capacitor.
Sample Applications
Three examples of the methods for
adjusting the resonance frequency of an
NFC matching circuit using a thin-film
variable capacitor are introduced below.
The first example (Fig. 2-(a)) is a
method where the capacitance is adjusted using a voltage output digital-toanalog (D/A) converter that can generate
the control voltage.
The second example (Fig. 2-(b)) is
a method where the capacitance is adjusted by generating the control voltage
from the general-purpose input/output
(GPIO) port of the NFC IC or others.
GPIO is a type of output terminal provided in an IC chip. As a GPIO port is
not assigned with a specific application
or signal, its output can be changed as
desired using software instructions. Using this port, it is possible to perform operations, such as a 0V output when “0”
is input from the software and a 3V output when “1” is input from the software.
When the output terminal of the GPIO
70
35
65
60
Capacitance (pF)
Capacitance (pF)
Capacitor’s Formation
A thin-film variable capacitor is manufactured by forming a dielectric material
on a silicon substrate using the thin-film
forming technique and then packaged
using resin. The capacitor uses a barium
titanate-(BaTiO3) based material, which
is a high dielectric constant ceramic
material, for the dielectric thin film. Ceramic capacitors, which use ferroelectric
materials containing BaTiO3-based materials, possess the DC bias characteristic where the dielectric constant changes
or drops when an electric field is applied
to the dielectric material.
The thin-film variable capacitor utilizes
this DC bias characteristic and provides a
functionality to enable capacitance adjustment using a control voltage. Furthermore,
the dielectric material is constructed with
thin layers using the thin-film forming
technique. As a result, the electric field
strength applied to the dielectric material
increases and a higher capacitance change
rate is achieved with low voltage.
3V) f=100Hz
30
(1) Generates the control voltage
using the PWM method
age. To develop this variable capacitor,
Murata has taken full advantage of its
proprietary capacitor and thin-film micro-fabrication technologies, which the
company has nurtured over the years.
This thin-film variable capacitor is
already being used for FeliCa or mobile
wallet application in Japan, which is a
type of NFC. This product can be used
as a variable capacitor for wide range of
applications in addition to the NFC/FeliCa frequency adjustment application.
Square Signal (0
55
50
45
40
35
30
30
25
20
15
10
0
1
2
Tuning Voltage (V)
3
(a) 60pF-39pF variable capacitance type
Fig. 3: Variable capacitance characteristic
0
1
2
Tuning Voltage (V)
3
(b) 30pF-20pF variable capacitance type
Table 1: Product lineup of thin-film variable capacitors
port is used, a control voltage in steps
of the nth power of two be generated by
combining output signals of two values
(0V and 3V) and resistor devices with a
ratio of the nth power of two.
The third example (as shown in Fig.
2-(c)) is a method for generating a control
voltage by means of pulse width modulation (PWM) using a pulse wave voltage to
adjust the capacitance. Like the second example, this method uses the GPIO port of
NFC IC and can generate a pulse wave using the signals of two values (0V and 3V).
Product Lineup
Table 1 indicates the product lineup of
thin-film variable capacitors. Murata has
introduced the LXRW19V Series (1.3 ×
0.9mm) and LXRWJFV Series (1.95 ×
1.4mm) into the market.
Each series (Fig. 3) offers two models
with two types of capacitance variable
characteristics, namely a capacitance value that can be changed between 60 and
39pF and that between 30 and 20pF when
the control voltage of 0 to 3V is applied.
Also, the LXRWJFV Series is
equipped with a protective function
against any electrostatic discharge
(ESD) that enters NFC ICs from the
NFC antenna port or others.
Future Efforts
The substrate areas for mounting the
NFC function in mobile phones and smartphones are becoming increasingly smaller
because of larger battery compartment
areas and of higher number of mounted
components due to expanded functionality of these products. To cope with this
situation, Murata intends to work on the
further miniaturization of thin-film variable capacitors in order to form matching
circuits with even smaller footprints.
Furthermore, the matching circuits are
different depending on various types of
NFC ICs and NFC antenna requirements.
To enable Murata’s thin-film variable capacitors to meet the requirements of all
cases in optimal conditions, Murata will
expand the variable range of capacitance
values and enhance the product lineup with
a wider selection of capacitance values.
About This Article:
The author, Kenta Omori, is from the
Sales Engineering Section, Technology
Integrated Products Department, New
Products & Business Division, Murata
Manufacturing Co., Ltd.
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37