To guarantee interoperability between contactless card readers and transponders, international standards specify the properties of the air interface. For example, ISO/IEC 14443 is the fundamental international standard for proximity cards, ISO/IEC 10373-6 is the test standard for proximity systems, EMVCo is the industry standard for payment and ECMA 340 is the Near Field Communication (NFC) interface and protocol. Conformance of the contactless card readers and transponders to these standards is typically essential and in some instances needs to be certified by an accredited test laboratory. A number of properties are specified for the air interface of contactless products by the international standards. One property is the so-called Load Modulation Amplitude (LMA).
For example, in the communication link from a device in card mode (hereinafter referred to as the transponder device) to a device in contactless reader mode (hereinafter referred to as the contactless reader), the information is communicated using load modulation. Due to the inductive proximity coupling between the loop antenna circuit of the reader and the loop antenna circuit of the transponder device, the presence of the transponder device affects the contactless reader and is typically referred to as the “card loading effect”. From the perspective of the contactless reader, a change in resonance frequency and a decrease in the Quality (Q) factor of the resonant circuit occurs. If the contactless reader/transponder device coupling system is viewed as a transformer, the transponder device represents a load to the contactless reader. Modulating the frequency and Q of the transponder loop antenna circuit produces a modulation of the load on the contactless reader. The contactless reader detects this load modulation at the reader antenna as an AC voltage. For systems compliant with ISO/IEC 14443, for example, the load modulation is applied to a sub-carrier frequency (e.g. 0.8475 MHz) of the 13.56 MHz carrier frequency specified by the standard or the 13.56 carrier frequency is directly modulated by the encoded signal for systems compliant with FeliCa, a contactless RFID smartcard system developed by Sony in Japan.
Each standard typically specifies a minimum limit for the load modulation amplitude that needs to be achieved by the transponder device in card mode.
Typically, restrictions such as available space or cost place strict limits on the antenna size. Furthermore, the presence of other components in close proximity to the contactless reader antenna circuit or transponder device antenna circuit effect the antenna circuit resonance properties, typically producing a shift in resonance frequency and decreasing the Q-factor. To address this issue, typically ferrite materials such as sintered or polymer ferrite foils are used for one layer of the construction of transponder and reader antennas. For example, see US Patent Publication 201100068178 A1 incorporated by reference herein.
For transponder devices that are powered only by the contactless reader device, there is typically a physical limitation on the load modulation that may be achieved using conventional methods such as passive switching of a resistor or capacitor to modulate the frequency or Q-factor of the antenna resonance circuit. The physical limitation typically depends on antenna size of the transponder device, the coupling between transponder and reader, the Q-factor of the resonant circuit, the switching time and other parameters. Note, the switching time is fixed for the 847.5 kHz subcarrier frequency in context of the ISO/IEC 14443 standard. These physical limitations allow the generation of a limit curve for the minimum antenna area that can achieve compliance with the minimum load modulation specified by the standards.
The minimum load modulation required can be achieved using a smaller planar loop antenna if the card mode communication is transmitted actively into the contactless reader antenna. Options exist which can induce the same voltage into the contactless reader antenna as is possible using conventional passive amplitude load modulation. For example, one option is to transmit a 13.56 MHz carrier signal that is modulated by the 847.5 kHz subcarrier frequency which is in turn modulated using the encoded data operating in card mode.
However, for active load modulation to work, the active load modulation of the transponder device typically needs to be in phase with the, for example, 13.56 MHz alternating magnetic field emitted by the contactless reader. The contactless reader typically provides the time reference for communication using the contactless interface. Typical transponder devices derive the clock frequency from the exemplary 13.56 MHz carrier signal provided by the contactless reader. Therefore, the signal typically used for the communication link from the transponder device to the contactless reader is in phase with the carrier signal emitted by the contactless reader. For a transponder device actively emitting in card mode with only one antenna, however, it is typically not possible to obtain the time reference from the contactless reader carrier signal.
In accordance with the invention, a special antenna geometry (e.g. a planar loop, but three dimensional embodiments are also possible) together with a special receiver and driver allow a transponder device to receive the exemplary 13.56 MHz signal from the contactless reader at the same time as the transponder device is transmitting in active card mode. This allows synchronization of the active load modulation signal with the carrier signal transmitted by a contactless reader (not shown) as is shown in
The size for planar loop antenna 110 typically depends on the contactless performance that is desired. For interoperability with products that meet the ISO/IEC14443 standard, geometric size classes are defined. Typically, the largest size is the card format which is specified in ISO/IEC7810 as the ID-1 format which is about 86 mm by about 55 mm. For certain applications, the size may need to be considerably smaller, typically the smallest size would be about 5 mm by about 5 mm in accordance with the invention.
Typically, the width of conductors 101 of coils 115 and 125 is in the rang of about 0.1 mm to about 3 mm for embodiments in accordance with the invention. For typical commercial processes, 0.1 mm is the lower limit on the width resolution. For etching processes, some copper thicknesses are typical. Typically 35 μm, 18 μm and 12 μm are commercially available thicknesses for conductors 110 using an etching process. Electroplating or galvanic processes allow thicknesses on the order of about 1 μm. Thickness is also dependent on the design requirements for the environment where planar loop antenna 110 will be used.
The amount of current typically flowing in conductors 101 of coils 115 and 125 typically requires a certain conductor volume to avoid thermally overloading conductors 101. Typical currents in conductors 101 range from about 10 mA to about 1 A at the exemplary frequency of 13.56 MHz. The skin effect, where only the outer part of the conductor 101 contributes to current conduction, typically operates to increase resistance for high frequency currents. Smaller cross-sectional area for conductors 101 results in higher specific resistance thereby increasing the resistance losses in coils 115 and 125. Typically, a higher resistance for a given inductance lowers the quality factor (Q) of an antenna circuit. Typical values for Q for exemplary embodiments in accordance with the invention are in the range from about 10 to about 40. However, the width of conductors 101 for a given area for planar loop antenna 110 is limited by the requirement that the middle of coils 115 and 125 be conductor free for effective H-field transmission or reception.
The spacing between conductors 101 of coils 115 and 125 is typically determined by the commercially available process which typically results in a spacing between conductors 101 on the order of about 0.1 mm in an embodiment in accordance with the invention. There is a proximity effect between conductors 101 when carrying an AC current. Each trace of conductor 101 produces an H-field which reduces the useable cross-section of conductors 101 for carrying current and increases the effective resistance. The proximity effect increases with frequency and decreases with increased spacing between conductors 101. Hence, a closer spacing of conductors 101 increases the resistance of planar loop antenna 110.
If an AC current is driven in coil 115, coil 115 emits an H-field. For illustrative purposes,
r(x,y,z,θ)=√{square root over ((a cos θ−x)2+(a sin θ−y)2+z2)}{square root over ((a cos θ−x)2+(a sin θ−y)2+z2)} (1)
where a is the radius of circular loop antenna 215 and θ is the angle between the radius and the x-axis. The z component of the H-field, Hz, can be calculated at any point (x,y,z) using the following equation:
where β is the phase constant 2πfc/c and IA is the current in the antenna.
For coils 115 and 125 of planar loop antenna 110 that have a rectangular shape in an embodiment in accordance with the invention, the H-field is typically computed using High Frequency Structural Simulator (HFSS) available from ANSYS Corporation. Typical operating voltages for the contactless reader antenna are typically in the range of about 30 volts to about 40 volts with a current on the order of several 100 mA.
In a plane parallel and below coil 115, the magnetic flux in the plane under the center of coil 115 has one direction while the magnetic flux in the plane outside of coil 115 points in the opposite direction (e.g. see direction for H-field of circular loop antenna 215 in
The coupling coefficient k between coils 115 and 125 may be estimated as follows. A constant AC voltage U1 is applied to coil 115 having an inductance L1 and the induced voltage U2 is measured in coil 125 having an inductance L2. Then the coupling coefficient k is given by:
The criteria for a “zero” coupling antenna in accordance with the invention is that k≦10%.
In the active card mode operation of a transceiver device, such as a Near Field Communication (NFC) device, planar loop antenna 110 is connected to the integrated circuit chip comprising the driver circuit (e.g. an NFC chip) such that common ground 150 is connected to connection point 130 between coils 115 and 125. The driver output of the integrated circuit is connected to common ground 150 and end pad 135 of coil 115 and is used to drive the active load modulation signal. The receiver input of the integrated circuit is connected to common ground 150 and end pad 145 of coil 125 and is used to sense the 13.56 MHz carrier phase of the contactless reader.
To make planar loop antenna 110 insensitive to the influence of metallic objects nearby and thereby reduce unwanted harmonic emissions a layered structure (see
Coil antennas 115 and 125 may be etched antennas, wire antennas, galvano-antennas or printed antennas. For example, for etched antennas, substrate 320 made of PVC having a copper layer (typical thickness of about 1.8 μm) on both sides of substrate 320 may be used. Photoresist material is placed over the copper layers on each side of substrate 320. A photographic process then projects the antenna coil layout onto the photoresist residing on top of the copper layers on each side of substrate 320. Using a chemical process, the exposed photoresist is removed, leaving the layout for coils 115 and 125 in the copper layers. A chemical etch then removes the exposed copper leaving only the copper layouts covered by the photoresist material. The photoresist is then chemically removed to yield planar coils 115 and 125. Coil antennas 115 and 125 may be electrically connected by drilling a hole and filling the hole with conductive paste to create connection 150.
Finally,
The layer structure of planar loop antenna 110 in accordance with the invention also provides directionality as the H-field emission occurs preferentially in the direction away from metal shield layer 370 as shown in
While the invention has been described in conjunction with specific embodiments, it is evident to those skilled in the art that many alternatives, modifications, and variations will be apparent in light of the foregoing description. Accordingly, the invention is intended to embrace all other such alternatives, modifications, and variations that fall within the spirit and scope of the appended claims.
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