NFC Modulation & RF Signal

- notes and essential details about NFC signal interface and the NFC modulation techniques used.

The RF signal format and modulation for NFC systems has been developed to ensure reliable communications while not consuming too much power.

The NFC modulation format has also been chosen to cater for both active and passive modes of operation.


NFC RF signal parameters

NFC uses the global 13.56 MHz allocation as this is an unlicensed radio frequency ISM band.

Using ASK - amplitude shift keying, as the format for the NFC modulation, most of the RF energy is concentrated in the allowed 14 kHz bandwidth, although the sidebands may extend out as far as ± 1.8 MHz.


NFC RF signal coding

NFC employs two different coding systems on the RF signal to transfer data. In most cases a level of 10% modulation is used, with a Manchester coding format. However for an active device transmitting data at 106 kbps, a modified Miller coding scheme is used with 100% modulation. In all other cases Manchester coding is used with a modulation ratio of 10%.


Data rate
kbps
Active Device Passive Device
106
Modified Miller, 100%, ASK Manchester, 10%, ASK
212
Manchester, 10%, ASK Manchester, 10%, ASK
424
Manchester, 10%, ASK Manchester, 10%, ASK

NFC and Manchester coding

Manchester coding is used for the majority of cases for the NFC communications. The Manchester coding utilises the two different transitions that may occur at the midpoint of a period. A low-to-high transition expresses a 0 bit, whereas a high-to-low transition stands for a 1 bit.

To achieve these conditions it is sometimes necessary to have a transition at the middle of a bit period. Transitions at the beginning of period are disregarded.

Manchester coding
Manchester coding used for NFC data transfer


NFC and Modified Miller coding

The modified Miller code is a little less intuitive, but provides an efficient form of coding. It is characterised by the pauses occurring in the carrier at di?erent positions of a period. Depending on the information to be transmitted, bits are coded as shown below. A high or "1" is always encoded inth e same way, but a low or "0" is encoded differently dependent upon what preceded it.

Modified Miller coding
Modified Miller coding used for NFC data transfer
used for 106 kbps active device transfers

By Ian Poole


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