Best Practices for Phase Noise Simulations of Free Running Ring Oscillators for Crystal-Free Narrowband FSK Communication Systems

Low-power, crystal-free wireless systems use free-running oscillators for both RF communications and sleep mode timers. However, these oscillators suffer from poor frequency stability, leading to reciprocal mixing and carrier mismatches for RF local oscillators, and additional guard time for sleep mode timers. Meeting the frequency stability requirements of standards like IEEE 802.15.4 and BLE is a significant challenge in crystal-free IoT mote design as the non-linear nature of oscillators demands special care in early simulations. Designers can use quick AC simulations, such as PNoise and HBNoise, or transient simulations, which are broadly applicable but take several weeks to gather sufficient data. Simulating the phase noise of free-running oscillators is especially tricky, and minor errors can lead to large miscalculations, which can be problematic for designers aiming to meet specifications on the first silicon iteration. This paper aims to cover the key methods used for oscillator phase noise simulations along with some conversion methods for interpreting the results in time domain for better understanding. As a design example, a ring oscillator circuit in Intel’s 16nm FinFET process is simulated and the resulting Allan Deviation is also calculated. Discrepancies between the simulated results are reported and recommendations are made for the most suitable method for free-running ring oscillators.