Effect of Oscillator Phase Noise on Synchronous Demodulation Measurement Systems for Sensing Applications

Synchronous demodulation is a well-established technique for precise definition and reduction of noise bandwidth in various applications. Typically the noise added by the nonlinear elements such as reference oscillator and demodulator is assumed to be negligible. However, with the scaling of the dimensions of micro-sensors, the signals from these sensors generally tend to become weaker. Therefore, there is a need to study the so far neglected noise contributions from the components of the synchronous demodulator. In this paper, we focus on the significance of the phase noise of the reference oscillator on the system performance. A detailed analytical model is developed to investigate the nonlinear interaction. It is shown that the phase noise of the reference oscillator can significantly contribute to the output noise depending on the phase difference between the reference and measured signals. Close-to-resonance phase noise components produce low- frequency components at the output. However, our experimentally tested analysis indicates that the effect of phase noise on the output signal can be eliminated by complete compensation of the phase shift between reference and measured signals. This study applies to the design of low cost, high-performance measurement systems for high precision sensor applications.