20.3 A 60GHz 186.5dBc/Hz FoM Quad-Core Fundamental VCO Using Circular Triple-Coupled Transformer with No Mode Ambiguity in 65nm CMOS

The recent development of the 5th-generation (5G) communication sytems has set increasingly strict requirements on the spectral purity of millimeter-wave (mm-wave) local oscillators (LO). Low phase noise is crucial to enable advanced modulation formats for high communication data-rates. Much effort has been made to improve the phase noise performance of the mm-wave LOs. A lower frequency voltage-controlled oscillator (VCO) together with a frequency multiplier can lower the phase noise [1]; however, the high-order harmonic components in VCOs are usually very weak, which requires additional power-consuming mm-wave amplification stages to satisfy the LO swing requirement. For single mm-wave fundamental VCOs, the minimal achievable phase noise is bounded by the smallest realizable inductor that displays a high Q factor. To avoid the “small inductor” problem, N oscillators with relatively large inductors can be coupled together to improve the phase noise by 10log
₁₀
(N) [2-5]. Authors in [2] presented a quad-core bipolar VCO working around 15GHz as shown in Fig. 20.3.1 (Left), where four one-turn inductors are star-connected with the active cores placed in the middle. Resistors (Rc) are added to avoid undesired multi-tone concurrent oscillations. However, the four one-turn inductors still suffer from the Q-factor drop when the inductance decreases, thus limiting the highest achievable oscillation frequency. Besides, VDD at the inductor central taps and VSS at the tail current source are far from each other, making the V
_DD
-V
_SS
current return path long. This path has to be carefully modeled in simulations, especially in the mm-wave frequency range, where the return path inductance is comparable to the tank inductance. Instead of the star-connected topology, authors in [3] presented a circular-connected quad-core VCO working close to 30GHz, where the inductors are arranged in a circular topology as shown in Fig. 20.3.1 (Middle). The destructive coupling between the inner edges inside a small inductor is eliminated. Therefore, the minimal realizable inductance is further reduced while keeping a high Q factor. The central taps are connected by narrow metal traces to avoid latching and mode ambiguity. The VCO adopts a CMOS configuration, which limits the highest operating frequency. It would be difficult for this topology to be adopted in NMOS-only VCOs because the central taps have to be resistively isolated to suppress unwanted modes; therefore, they cannot be connected to the AC-ground power supply simultaneously as required by the NMOS-only configuration. Due to the lack of harmonic impedance control in the circular inductors, extra tail filtering transformers are added to improve the phase noise.