Demodulation of phase-modulated signals with small modulation index without digital noise limitation

Abstract

In many communication and sensor applications, receivers digitize phase-modulated carrier signals directly by analog-to-digital converters. The obtained phase-modulated digital carrier signals are numerically demodulated in order to extract the relevant information. However, the limited dynamic range of the analog-to-digital converter may reduce the carrier-to-noise ratio of carrier signals after digitization. Correspondingly, the resolution of the demodulated digital signal is degraded. We demonstrate a sampling and demodulation scheme for phase-modulated signals with a small modulation index. Our new scheme overcomes the limitation due to digital noise defined by the analog-to-digital converter. Through simulations and experiments, we provide evidence that our method can improve the resolution of the demodulated digital signal, when the carrier-to-noise ratio of phase-modulated signals is limited by digital noise. Our proposed method is applicable, for example, to solve the problem of a possible degradation of measurement resolution after digital demodulation in heterodyne interferometers measuring small vibration amplitudes.