A Cryogenic LC VCO Utilizing Cryogenic Models of Active Devices

Upcoming advances in the space research demand a better understanding of the analog and radio frequency circuit behavior at very low temperatures. For this purpose, a LC cross-coupled voltage controlled oscillator in the S-band is designed for space communications as a case study using a commercial 0.18 mu m CMOS Mixed-Mode/RF device technology. At 77 K, performance variations are observed with respect to the room temperature through the model extracted from transistor measurements that have been conducted using liquid nitrogen. One notable observation indicates that by cooling down to the cryogenic temperatures, the oscillator can fall into the "waste of power" region and therefore the phase noise of the circuit does not necessarily improve in the cryogenic operation regime. Even though there is a gap in the literature about the cryogenic behavior of passive devices, it is shown that the variations on the varactor threshold value and the quiescent node voltages of the circuit can notably shift the needed control voltages. Moreover, a voltage controlled oscillator is designed, which ensures practical operation both at 300 K and 77 K, thereby demonstrating how the tuning range and phase noise performance of the oscillator changes at different temperatures.