Research Activity: Low power RC oscillators with high frequency accuracy across temperature*

Crystal oscillators are often preferred frequency stability across temperature. The achievable temperature stability is around 50ppm over the [-40,125C] industrial temperature range. However crystal oscillators come at the expense of extra power consumption as well as real estate. Recently there have been many works aiming to replace crystal oscillators using calibrated RC oscillators especially for use in IoT applications. These applications require 1000ppm variation over the temperature range.

In this work, we have developed an RC oscillator based FLL loop that aims to achieve the target frequency by equalizing the resistance of a switched capacitor resistor to an integrated resistor. Therefore the resistor variation across temperature will be determining the frequency variation across temperature. Moreover the resistor can be modulated to simulate a larger (and different) resistor value.  Using two resistors with temperature coefficients of different signs in parallel, the equivalent resistor can achieve a near zro temperature coefficient.

The design is realized in a 0.18u CMOS process using low voltage devices.. The resulting oscillator operates at 1MHz and draw a total current of 30uA.

*Melih Bilmez, Doç. Dr. Tufan Coşkun Karalar


RC oscillator system block diagram