At first, we present an analysis and a discussion of the design options and tradeoffs for a passive microwave transponder. We derive a set of criteria for the optimization of the voltage multiplier, the power matching network and the backscatter modulator in order to optimize the operating range. In order to match the strictly power requirements, the communication protocol between transponder and reader has been chosen in a convenient way, in order to make the architecture of the passive transponder very simple and then ultra-low-power. From the circuital point of view, the digital section has been implemented in subthreshold CMOS logic with very low supply voltage and clock frequency. We present different solutions to supply power to the transponder, in order to keep the power consumption in the deep sub-µW regime and to drastically reduce the huge sensitivity of the subthreshold logic to temperature and process variations. Moreover, a low-voltage and low-power EEPROM in a standard CMOS process has been implemented. Finally, we have presented the implementation of the entire passive transponder, operating in the UHF or microwave frequency range
