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Issue #211 February 2008
RFID Payment Terminal
by Carlos Cossio
Start | System Overview | Energy Transmission | 100% Amplitude Modulation | Load Modulation | MIFARE Card | MIFARE and ISO 14443 | Pay as You Go | Hardware Design | Antenna Design Rules | Antenna Size | Directly Matched Antenna | Firmware Design | Catch the Wave! | Sources & PDF
DIRECTLY MATCHED ANTENNA
I decided to implement a directly matched antenna circuit design in the RFID payment terminal, which enables operating distances of up to 100 mm. The operating distance primarily depends on the size of the antenna and the correct values for the antenna’s matching circuit. The following is a brief description of the necessary components, the EMC filter, the receiving circuit, and the antenna matching itself for the proper functionality of the contactless system (see Photo 3).
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| Photo 3—To test the terminal’s hand-made coil antenna, I used a simple one-turn coil made with a wire attached to an LED. The energy transferred to the test coil is enough to light up an LED to several centimeters. |
The EMC circuit is a filtering and impedance transformation circuit that suppresses higher harmonics and optimizes power transmission to the reader antenna. The contactless circuit is based on an operating frequency of 13.56 MHz. The frequency has to be generated by a quartz oscillator, which also generates higher harmonics. To conform with international EMC regulations, the third, fifth, and higher harmonics of the 13.56 MHz have to be adequately suppressed. In addition to a multilayer layout, it is strongly recommended to implement a low-pass filter circuit for the Tx1 and Tx2 pins of the MF RC531. The low-pass filter is designed with an inductance of 1 mH and a capacitance of 68 pF.
A receiving circuit has to be designed to receive data sent by the card. The internal receiving part of the MF RC531 uses a new receiving concept to enable easy and robust detection of the weak received signal. Because the MIFARE card uses a sub-carrier frequency fSUB to modulate the data sent, instead of using a direct-load modulation of the carrier frequency, it results in the generation of sidebands at ±fSUB around the carrier frequency of 13.56 MHz. Because of the small coupling factor between the reader and the card antenna, the card response is much weaker than the voltage generated by the reader. The detection of that signal requires a well-designed receiving circuit. So, instead of using a direct load modulation, the contactless card uses a sub-carrier frequency fSUB = 847.5 kHz to modulate the data. The sub-carrier load modulation generates two sidebands in the frequency domain: an upper sideband at 14.41 MHz and a lower one at 12.71 MHz around the carrier frequency, which is 13.56 MHz. The sub-carrier load modulation enables easy and robust detection of the received signal. For the design of the receiving circuit, use the internally generated VMID potential as an input potential of the Rx pin. To reduce disturbances, a capacitance to ground of 100 nF has to be connected to VMID. The receiving part of the reader needs a voltage divider connected between the Rx and the VMID pin. This is accomplished with two resistors of values 820 and 560 W, respectively. Additionally, use a serial capacitance of 1 nF between the antenna coil and the voltage divider.
Because the matching of the antenna provides a maximum of power coupled into the antenna depending on its impedance, the voltage at the antenna nodes is slightly different from antenna coil to antenna coil. The Rx input pin of the MF RC531 is high impedance, so a voltage is coupled back into the Rx input of the MF RC531. So, two rules have to be fulfilled. The DC voltage level at the RX input pin has to be kept at VMID (that’s the need of the 820-W resistor and the 100-nF capacitor). In addition, the AC voltage level at the Rx input has to be kept within limits: 1.5 VPP < VRX < 3 VPP. To avoid exceeding the limit of VRX = ±1.5 VPP AC at the Rx input pin, a 3.9-kW resistor is used. A higher input voltage may not destroy the chip, but it results in a receiving failure.
A matching circuit for the antenna coil is necessary in order to achieve the best performance and the antenna coil has to be designed. A connection directly between the antenna and the reader itself is also needed. It is recommended to design the directly matched antenna step by step. First, the antenna coil has to be designed. The antenna itself is a low-ohm device. In order to connect this antenna coil to the MF RC531, a matching circuit is required. Starting with the estimation of the antenna’s equivalent circuit and the calculation of the quality factor, the recommended values for the capacitors of the matching circuit will be reached. Finally, it is worth remembering that the antenna coil inductance and capacitance depend on various parameters, such as antenna construction (type of PCB), the thickness of the conductor, the distance between the turns, the shielding layer, and metal or ferrite in the vicinity. So, it may be necessary to tune the antenna properly in the complete design.
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