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RFID Modulation and Encoding Techniques

When I started playing for RFID tags/readers I was very confused with how the data was encoded and transmitted from the tags to the reader.

The first thing one must realize is that the tag does not actually transmitter anything. Passive tags (tags that are powered from the reader) gets energized by the 125Khz magnetic wave generated by the RFID reader. All that the tag then does is open/close its coils. This causes interface on the reader coil, which is called back scatter. The reader processes this back scatter and decode the “transmitted” data.  Up to this point the only encoding method is Amplitude modulation, the tag changes the amplitude of the reader’s magnetic field.

It is at this point where things starts to get confusing. The data is transmitted using to techniques. First it gets encoded (Normally Manchester or Bi-Phase), and then the encoded data stream gets modulated on a RF level. The modulation is either ASK, FSK, or PSK.

Based on my experience, EM4100 tags are encoded using either ASK or PSK, and HID encoded using FSK.

For a reader to correctly read a tag the following must be done:

Generate a constant 125KHz modulating frequency to the reader coil


The chip manufacturer Microchip has a very good design document with sample code, designs and explanations,  below is FSK/PSK examples extracted from their guide [1].

Exmple of FSK Modulation [2]
Exmple of FSK Modulation
PSK Modulation [3]
Exmple of PSK Modulation
Data Encoding [4]
Data Encoding