Fig 3: RFID reader internal block diagram
Note that there are two identical channels for driving antenna #1 and antenna #2. The RFID digital
circuit controls both the antenna channels. The EMC filter provides the necessary attenuation to block
higher frequency harmonics from reaching the antenna. The matching network along with the balun
provides the required matching to a 50 Ohm antenna.
The RFID reader uses loop antenna to radiate RF energy. The antennas are tuned to 50 Ohm impedance
with a phase angle of +/- 10 degrees. The following diagram shows a simplified circuitry of the antenna.
Fig 4: RFID loop antenna circuit
The antenna board has two loop antennas tuned to operate at 13.56 MHz. Antenna #1 is around the
glass window and antenna #2 is below the front face of the product. Variable capacitors are used in
place of C1 and C2 so that the antenna could be tuned accurately during production. The following plots
show a typical antenna impedance plots for both antenna #1 and #2.
