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Working principle of Sniffer Nano

We will publish the source code after tidying up the code .To better understand the code, undetstand the working principle of RFID reader is necessary. So we decide to introduce the workflow of our module first.

Sniffer Nano is the device which can transmit the 125KHz RF signal to the tag/card for power supply and receive the signal which reflects from the tag/card. Because the reflect signal is coded, reader receives the RF signal and decodes to valid information.

First, we need the Sniffer Nano to output a 125KHz RF signal, so we use the MCU Timer and PWM pin to export a 125KHz square wave. Of cause this signal can not direct to the coil , it will access the antenna before a power amplifier circuit. The waveform as below:

These 125KHz square waves will drive the antenna which a  air core coil. According to the theory of air core transformer, the 125KHz square wave will be transmitted to the tag/card. To be a load for reader, with the power supply, the tag receives the energy comes from the reader, and is going to be active. Then the tag changes itself from a state to the other state. Seeing from the reader, the load seems to be changed, also the signal pass the antenna will be different – this is the load modulation.Tag information contained in these changes of signals.

The signal with information is analog , so it can not be recognized by MCU. we need to use a detection circuit which include diode and LC filter to extract the valid information ,and change to digital signal.

When the MCU receive these digital signal, it will catch and save into a buffer ,then decode them according to the EM4100 protocol.

The next processing is all by the software burn in the MCU. We will go on to introduce these contents after we release the source code.