25.3 A VCO with implicit common-mode resonance

CMOS VCO performance metrics have not improved significantly over the last decade. Indeed, the best VCO Figure of Merit (FOM) currently reported was published by Hegazi back in 2001 [1]. That topology, shown in Fig. 25.3.1(a), employs a second resonant tank at the source terminals of the differential pair that is tuned to twice the LO frequency (F_LO). The additional tank provides a high common-mode impedance at 2×F_LO, which prevents the differential pair transistors from conducting in triode and thus prevents the degradation of the oscillator´s quality factor (Q). As a consequence, the topology can achieve an oscillator noise factor (F)-defined as the ratio of the total oscillator noise to the noise contributed by the tank- of just below 2, which is equal to the fundamental limit of a cross-coupled LC CMOS oscillator [2]. There are, however, a few drawbacks of Hegazi´s VCO: (1) the additional area required for the tail inductor, (2) the routing complexity demanded of the tail inductor, which can degrade its Q and limit its effectiveness, and (3) for oscillators with wide tuning ranges, the need to independently tune the second inductor, which again can degrade its Q. Moreover, it can be shown that the common-mode impedance of the main tank at 2×F_LO also has a significant effect on the oscillator´s performance, which if not properly modeled can lead to disagreement between simulation and measurement, particularly in terms of the flicker noise corner. To mitigate these issues, this work introduces a new oscillator topology that resonates the common-mode of the circuit at 2×F_LO, but does not require an additional inductor.