DLL‐based signal conditioning system for SAW sensor with digital output

Abstract

The use of a delay-locked loop (DLL) for conditioning signals from a surface acoustic wave (SAW) sensor is proposed. The system consists of a reference oscillator and a DLL. The signal generated in the reference oscillator is propagated through a sensitive SAW delay line (SAW-DL). The SAW propagating through this structure has a phase velocity dependent on environmental parameters such as temperature, humidity, mechanical deformation, or analyte concentration. Therefore, the system measures the physical quantity of interest by tracking the delay caused by the sensitive SAW-DL via a DLL. Fully digital DLL is made use of for the delay-tracking, which provides the conversion of the environmental parameter being sensed directly to the digital domain. The system is demonstrated through results from behavioural simulation, using a model of an SAW-DL sensitive to humidity.Introduction: Electromechanical properties of surface acoustic wave delay lines (SAW-DLs) covered with sensitive thin films are known to be significantly sensitive to environmental parameters such as temperature, mechanical deformation, or analyte concentration. For instance, in [1] Caliendo et al. investigate the response of several chemical interactive material membranes to relative humidity (RH) on the exposure to water, ethanol, and acetone vapours. They report a considerable variation in the phase velocity and in the acoustic attenuation of signals propagating along the surface of the SAW-DL as function of the gas concentration. Recently, graphene oxide (GO) has been employed for enhancing the sensitivity of SAW-DL to humidity [2, 3].The conventional conditioning circuit for SAW sensors consists of an RF oscillator including the SAW-DL with sensitive coat in the feedback loop [4]. The oscillation frequency is determined by the propagation velocity of the SAW-DL, which changes with the sensing parameter. This signal is then mixed with a signal from a reference oscillator (usually the same oscillator, but without the sensitive film), and the frequency difference is measured by a frequency counter. In a second approach [5], a voltage-controlled phase shifter (VCPS) is employed to phase-tune two feedback paths in order to establish a positive-feedback loop and allow oscillations to start. The voltage that controls the VCPS is then related to the RH.In this Letter, we present a new conditioning system in which the sensitive SAW-DL operates in open-loop configuration. Therefore, the information about the environmental parameter under measure is conveyed in the delay of the signal propagating through the SAW structure, rather than in the frequency of an SAW oscillator. To track this delay, we employ a delay-locked loop (DLL) [6]. The DLL's control signal (filtered-error signal) follows the delay of the sensitive SAW-DL, and therefore allows a direct measure of the physical parameter being sensed. This proposed conditioning solution has the advantage of directly converting the physical quantity into a digital number, which can be easily post-processed in ...