A DDS-based multi-harmonic frequency meter for QCM sensor applications

In this paper we present a prototype of a frequency meter based on an ARM processor and a DDS module. The instrument was developed to be used with QCM-sensor based oscillator circuits for in-liquid applications. For each resonance frequency of the QCM sensor, up to 50 MHz, the instrument can generate a reference signal exploiting the DDS module, which is used to perform a heterodyne demodulation of the oscillator signal. The ARM processor is used to on-line change the DDS settings (to track up to three resonance frequencies components with interleaved sampling of the QCM signals with a sampling time of about 1s) and to perform the frequency measurements. The contemporary use of the information from different QCM resonance frequencies will be exploited to enhance the instrument sensitivity while preserving a resolution of about 1 Hz for short term measurements with 10 MHz QCMs. At present, preliminary tests were performed.