A Multi-PLL Approach to Low-Spur, Low-Jitter Frequency Synthesis for Millimeter-Wave Bands

F. Herzel, N. Maletic, C. Carta, G. Fischer (IHP Frankfurt (Oder), Germany)

We present a novel local oscillator architecture for mmWave frequencies consisting of a pair of low-frequency PLLs driving a larger number of integer-N PLLs to be used in phased arrays or MIMO transceivers. It employs two fractional-N PLLs in the lower GHz range, each driven by its own crystal oscillator, with their RF outputs connected to a 2:1 multiplexer (MUX). The MUX output is connected to the input of a mmWave PLL array. By using a small loop bandwidth for the fractional-N and a large loop bandwidth for the integer-N mmWave PLLs, a low level of phase noise and spurious tones (spurs) is predicted. In this topology, the phase noise of the mmWave PLLs is reduced by noise averaging, whereas phase noise and spurs from the fractional-N PLLs are reduced by signal processing after downconversion of the signal to the baseband. The very low level of fractional spurs is achieved by an intelligent control of the MUX, resulting in spur positions much larger than the loop bandwidth.