In this brief I introduce a new synthesizable VCO using (n+1) mux2i cells from the SkyWater130 PDK to form a (2n+1) hybrid ring oscillator.
Although is possible to make current-starved inverters using nand2 cells, for example, it's difficult to balance the output because most standard cell logic realizations are asymmetric combinations of series and paralled transistors. Voltage control range is limited with nand cells compared to balanced current-starved inverters.
An alternative to using the mux2i cell with swapped bias inputs is to make one of the cells using the mux2 cell which adds an inverter
to the same layout as a mux2i cell.
For the 7th Skywater130 High-Voltage library without a mux2i the same technique may be adapted using the positive and negative enable tristate buffers, einvp and einvn.
Voltage control can be observed in the following simulation output:
The inverting voltage follower bias generator may have higher current consumption than an optimized bias generator. However, since this new design can be synthesized so easily and implemented in multiple libraries on Skywater130, it may serve as a useful benchmark of those libraries.
It would be useful to design a benchmark comparison by following the basic layout in .
A CMOS Voltage Controlled Ring Oscillator with Improved Frequency Stability, Goran Jovanovic, Mile Stojcev, Zoran Stamenkovic, Scientific Publications of the State University of Novi Pazar, Ser. A: Appl. Math. Inform. and Mech, Vol. 2, pp. 1-9, 2010.
CMOS ring oscillator with combined delay stages, Ramazani, A., Biabani, S., & Hadidi, G. (2014), AEU-International Journal of Electronics and Communications, 68(6), 515-519.
 Improved accuracy equation for propagation delay of a CMOS inverter in a single ended ring oscillator, Zafarkhah, E., Maymandi-Nejad, M., & Zare, M., 2017, AEU - International Journal of Electronics and Communications, 71, 110-117.
 A 46.8 microW/1.12 GHz 7th Stage New Ring Voltage Controlled Oscillator, Madhusudan Maiti, Suraj Kumar Saw, Vijay Nath, Swarnendu Kumar Chakraborty and Alak Majumder, V. Nath and J. K. Mandal (eds.), Nanoelectronics, Circuits and Communication Systems, Lecture Notes in Electrical Engineering 642,