A Spurious and Oscillator Pulling Free CMOS Quadrature LO-Generator for Cellular NB-IoT

Abstract

A reconfigurable LO-generator (LOG) using an integer divider and a mixed-mode fractional divide-by-2.5 (Div2.5) is presented to generate a spurious and oscillator pulling free output spectrum without additional digital calibration for cellular narrowband Internet of Things (NB-IoT). The proposed LOG includes a fractional divider to mitigate the pulling effect between the oscillator and the power amplifier. The designed fractional Div2.5 consists of a digital Div2.5 core, a duty cycle corrector (DCC), and an I/Q generator (I/Q-GEN). An adaptive bias circuit (ADB) is applied to compensate for process, voltage, and temperature (PVT) variations on delay circuits of the DCC and the I/Q-GEN by adjusting the supply voltage. The designed LOG is fabricated in the 28-nm CMOS process while consuming 40 mW, including ADPLL, digitally controlled oscillator (DCO), local oscillator (LO) distribution buffers, and dividers at 2 GHz. The measured phase noise of the Div2.5 LOG generating 2-GHz carrier frequency is −146.3 dBc/Hz at a 10-MHz offset. The measured fractional spurious tones are below −90 dBc without oscillator pulling effects for the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.6\sim 2.1$ </tex-math></inline-formula> -GHz output frequency.