A popular vision in the field of wireless communications has been to achieve ubiquitous communications, in which nearly everything we interact with is connected wirelessly to a network. When combined with sensors, such a system could have applications ranging from biomedical ones, such as health monitoring sensors, to smart houses that can track their occupants' presence and automatically adjust such things as lighting, temperature, and music or video to suit the individual's preferences. In both instances, the radios that connect the sensors to the network must consume very little energy to allow long battery life or operation with energy scavenged from the environment, must be very small in size to permit them to disappear into the environment, and must be able to be created at a minimum cost to enable a large number of devices to be used.

This thesis addresses some of the design concerns facing the receivers for such sensing systems, with a particular focus on improving the sensitivity, reliability, and robustness of ultra-low power receivers. Two prototype radios have been implemented exploring different applications. The first focuses on a general purpose low power receiver for an Active RFID tag, which provides a highly integrated wireless sensing platform including a full transceiver along with power conditioning and a DC-DC converter to interface with an energy harvester. This receiver consumes 48uW of analog power, along with 61uW of digital power for demodulation and synchronization and achieves a sensitivity of -66dBm for 100kbps of data while being powered by the on-chip power supply. The second implements a wakeup radio, which operates along with a more powerful, full-featured radio such as a WiFi or Bluetooth and listens to the channel to determine if signals are present. This design focuses on minimizing the active power consumption while improving the sensitivity, and achieves a -90dBm sensitivity for a 10kbps OOK wakeup signal while consuming only 37.5uW of power.