We present the design of the RF section of a long range passive RFID transponder. It consists of a voltage multiplier and a voltage regulator, that convert the RF signal into a regulated DC supply voltage, a PWM demodulator and a PSK backscatter modulator. The entire circuit has been designed with AMS 0.35 /spl mu/m BiCMOS technology. Post-layout simulations show the correct operation of the whole section. The supply voltage generator, which provides a supply voltage of 0.6 V - appropriately regulated for the subthreshold CMOS logic to be used for the digital section - exhibits a power efficiency of 20% in the RF/DC conversion. Proper modulation and demodulation is obtained.