Robust Subthreshold CMOS Voltage-Controlled Frequency Synthesizer for Wireless Dosimeter Application

Detailed study of MOSFET behavior in the subthreshold region of operation is conducted with the 0.13 µm CMOS IBM technology. High sensitivity of subthreshold CMOS circuits to process variations is addressed. Two body-biasing circuits for stabilizing the subthreshold current of n-channel (NMOS) and p-channel (PMOS) devices are introduced. These stabilization schemes are applied to a voltage-controlled frequency synthesizer (VCFS) operating in the subthreshold region to design a robust, high resolution and ultra-low power (ULP) signal processing unit (SPU). The intended application is a wireless dosimeter, which converts radiation doses in the range of -900 mV to 0 to pulses with varying frequencies. Simulation results show a linear frequency vs. sensitivity response, generating pulses with 50% duty cycle for all inputs. The SPU yields a maximum error of 210 mV or 24% (6 Gy), minimum resolution of 15 KHz/Gy, minimum bandwidth of 805 KHz and maximum power consumption of 1.72 µW.