RFID 13.56MHz RECEIVER CIRCUIT
The following circuit is a receiver which detects the presence of the ISM frequency 13.56MHz which most
RFID systems use. The central part is an NE615 or SA615 integrated circuit. This chip can be found on the
PHILIPS web site. It is comprised of a mixer/oscillator, two limiting intermediate detectors, muting,and a logarithmic received signal strength indicator (RSSI). The input frequency is picked up by the loop antenna
which can be made using a 75 x 75mm area, and tuned with C1 and C8.
The 13.56MHz frequency is mixed with the oscillator 8MHz resulting in an IF signal of 13.56-8.0 = 5.56MHz
which is then taken through two 5.5MHz ceramic filters FL1 and FL2. This signal is measured by the RSSI
function on pin 7. The RSSI output supplies a current between 0 and 80uA depending on the signal strength. The sensitivity can be adjusted with R5 and will turn on LED D1 when enough signal is present.