Analysis and Design of an Ultra-Low-Power Bluetooth Low-Energy Transmitter With Ring Oscillator-Based ADPLL and 4x Frequency Edge Combiner

In this paper, we present an all-digital ring oscillator (RO)-based Bluetooth low-energy (BLE) transmitter (TX) for ultra-low-power radios in short range Internet-of-Things (IoT) applications. The power consumption of state-of-the-art BLE TXs has been limited by the relatively power-hungry local oscillator (LO) due to the use of LC oscillators for superior phase noise (PN) performance. This paper addresses this issue by analyzing the PN limit of a BLE TX and proposes an RO-based solution for power and cost savings. The proposed transmitter features: 1) a wideband all-digital phase-locked loop (ADPLL) featuring an fRF/4 RO, with an embedded 5-bit TDC; 2) a 4x frequency edge combiner to generate the 2.4-GHz signal; and 3) a switch-capacitor digital PA optimized for high efficiency at low transmit power levels. These not only help reduce the power consumption and improve PN performance but also enhance the TX efficiency for short range applications. The TX is prototyped in 40-nm CMOS, occupies an active area of 0.0166 mm(2), and consumes 486 mu W in its low-power mode, while configured as a non-connectable advertiser. The TX has been validated by wirelessly communicating beacon messages to a mobile phone.