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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
LOAD MODULATION
Data transmission from the card back to the reader is completed using the principle of load modulation (see Figure 3). The card is designed as a resonance circuit and consumes energy generated by the reader. The energy consumption has the same reactive effect as a voltage drop on the reader side. This effect can be used to transfer data from the card back to the reader by changing a load or a resistance in the card IC.
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| Figure 3—The principle of load modulation enables data transmission from the card back to the reader by changing a resistance in the card IC. |
The reader antenna should be tuned to a resonance frequency fR of 13.56 MHz. In fact, the resonance circuit generates voltages in the reader antenna several times higher than the supply voltage. Due to the small coupling factor between the reader and the card antenna, the card response is about 60 dB weaker than the voltage generated by the reader. The detection of such a signal requires a well-designed receiving circuit. Instead of using a direct-load modulation, the contactless card uses a sub-carrier frequency fSUB to modulate the data. The result of the sub-carrier modulation is the generation of sidebands at ±fSUB around the carrier frequency of 13.56 MHz. The sub-carrier load modulation enables easy and robust detection of the received signal.
The contactless RF interface uses Manchester coding for the data in the baseband before the sub-carrier modulation is done. Figure 4 shows the typical data coding and the sub-carrier load modulation in the time domain.
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| Figure 4—Manchester data coding is used in the RF interface to transmit data back to the reader. |
First, data are internally coded to the Manchester coding. The data rate of the contactless card for the communication from the card to the reader is 106 kHz. The same goes for the communication between the reader and the card. So, the length of a bit frame is 9.44 ms. The Manchester code uses rising and falling edges to code the data.
A logical “1” is expressed with a falling edge in the middle of the bit frame. A logical “0” is expressed with a rising edge in the middle of the bit frame.
The contactless card generates the sub-carrier frequency fSUB = fR/16 = 847.5 kHz. The time T0 expresses the pulse length of the operating frequency, T0 = 1/fR = 74 ns. The Manchester coded data is modulated at the sub-carrier frequency. Finally, the sub-carrier load modulation is completed. Thus, 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.
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