8.3 A 28GHz Scalable Inter-Core-Shaping Multi-Core Oscillator with DM/CM-Configured Coupling Achieving 193.3dBc/Hz FoM and 205.5dBc/Hz FoMA at 1MHz Offset

Abstract

The high-order modulation schemes of mm-wave wireless communication networks (e.g., 5GNR) have stringent requirements for the LO phase noise. Besides, to minimize the power consumption of oscillators, a high figure-of-merit (FoM) is targeted, especially for the very-low-phase-noise specifications. Various harmonic-shaping techniques have been reported to reduce the noise sensitivity [1]–[3], and thus improve the FoM. By utilizing the mutual coupling of an inductor and a single-ended capacitor, the implicit common-mode technique [1] can set the common-mode (CM) resonance at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$2f_{\mathrm{L}0}$</tex> without an extra CM resonator. Then, the phase noise degradation due to the triode operation is eliminated, while the flicker noise upconversion can be minimized. However, the mutual coupling <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$k$</tex> pushes CM to a higher frequency by cancelling the magnetic flux, which reduces the inductor quality factor (Q) in CM by a factor of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1-k$</tex> as shown in Fig. 8.3.1-top-left. Meanwhile, the Q of the single-ended switched capacitor is inherently lower than the differential type for the same tuning range. At a mm-wave frequency, the switched capacitor Q becomes more critical, and the reduced coil diameter of a small inductor decreases the Q due to the inner-edge destructive coupling. Currently, to overcome the destructive coupling of a small inductor and to increase the maximum power taken by a single-core resonate tank, the mode-rejection multi-core oscillators [4]–[6] are reported, where multiple inductors are arranged in a circular shape with a larger diameter. Then, the inductance of each core can be reduced while keeping a high Q. By coupling N oscillators together, the phase noise can be reduced by <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$10\log(\mathrm{N})$</tex> with the FoM unchanged. Nevertheless, due to the limitation of the layout configuration, the inductance ratio and mutual coupling cannot meet the requirement of harmonic shaping. Then, a 1MHz-offset FoM at mm-wave is about 186dBc/Hz, which is worse than of other single-core oscillators [3]. Moreover, for a many-core configuration (i.e., the number of cores > 4) [6], inner-tank decoupling network is needed to prevent the performance degradation caused by the parasitics in the connections to a multi-core supply.