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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 Copyright©2014 Dempa Publications, Inc. 35 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 AEI April 2014 Copyright©2014 Dempa Publications, Inc. 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. AEI April 2014 Copyright©2014 Dempa Publications, Inc. 37