Development of PVT immune VCO based clock generator for wide frequency range

Clock generation block is an important component in any digital circuit. For example,high resolution data converters require high precision clock that has very little variationover process, voltage and temperature (PVT) variations. The most common clock referencesare typically based on quartz crystal oscillators which provide excellent stabilitywith variations in PVT. However, the incompatibility of on-chip integration due to largesize makes the investigation of alternative ways to generate on-chip clock necessary. Thiswork presents a Voltage Controlled Oscillator (VCO) based clock generator that is capableof producing PVT immune frequencies from tens of kHz to tens of MHz in 180nm SOI technology. The VCO design is based on differential ring architecture employingdelay variation by partial positive feedback to obtain wide frequency tuning range.The designed VCO has a tuning range from 54.37 MHz to 113.2 MHz across process andtemperature variations. The centre frequency, 87 MHz, is compensated for a temperaturerange of the automotive application by biasing the VCO using a complimentary toabsolute temperature (CTAT) reference current. The designed CTAT reference currentis capable of producing variable temperature coefficient across nominal, worst speed andworst power process corners by means of resistor trimming. Worst case deviation of thecentre frequency has been reduced from 30.3% to 5.5%. The static power consumptionof the VCO is 198 mW and at centre frequency has a phase noise of -95.17 dBc/Hz @ 1MHz offset frequency. The VCO has a Figure of Merit of -140 dBc/Hz. Frequency dividerwith a division ratio of 2 is designed to obtain a frequency division of the order 3 andPVT immune clock frequencies from 85 kHz to 87 MHz are achieved. The desired clockfrequency is selected by the means of a 16:1 Multiplexer (MUX). All mentioned resultswere obtained from schematic simulations using Cadence Virtuoso