This dissertation presents an ultra-low power (ULP) phase-domain RX architecture for the Bluetooth Low Energy (BLE) applications. By 2020, there will be up to 10 to 100 billion wireless sensor devices connected to internet-of-things (IoT)[1]. With the increasing demand on prolonging the battery lifetime, the power consumption of the RF transceivers for such applications has been dramatically reduced in the past few years [2][3]. However, the cost of replacing/recharging the batteries will become a bottleneck for massive deployment of the remote wireless sensors. Therefore, continuous innovation on power and supply reduction of the IoT RF transceivers is needed to extend the battery life time or one step further to achieve complete autonomous operation using energy harvesting. A phase-domain single-channel RX, proposed in [4], transforms the analog-I/Q signal processing into digital-phase processing by combining a phase-rotator based phase tracking loop and the sliding-IF architecture. It demonstrates an approximately 40% power reduction compared to the conventional Cartesian RX architecture. However, it still requires multi-phase generation hardware and suffers from the image rejection issue. The proposed direct frequency demodulator (DIFDEM) RX has a zero-IF architecture and uses minimum possible analog circuitry, which allows it to avoid image issue and achieve low power consumption simultaneously. Instead of phase rotator, the DIFDEM RX uses digitally controlled oscillator (DCO) as a feedback element in the phase tracking loop. Further, to meet tight adjacent channel rejection ratio (ACR) and frequency tolerance specifications of the BLE, DIFDEM employs a 3rd-order elliptic filter and an automatic frequency noise cancellation (AFC) loop. Post layout simulation results indicate that DIFDEM RX can achieve -89 dBm sensitivity and -20/-30 dB ACR at 2/3 MHz, while consuming