A Portable Multichannel Frequency Acquisition System for Resonant Gas Sensor Arrays

Resonant gas sensors are widely adopted for complex gas identification tasks due to their high sensitivity and fast response. The acquisition of their resonance frequency, however, often relies on frequency counters or FPGAs, which compromises the portability and cost of the system. This paper reports a low-cost and highly integrated multichannel frequency acquisition system for resonant gas sensor arrays. A single-channel frequency meter based on an STM32F103C8T6 microcontroller and a Pierce oscillator is first developed to track the resonance of a 10 MHz quartz crystal microbalance (QCM). Various filtering techniques are evaluated for enhancing the stability of the frequency signal, such that a low limit of detection can be achieved by the sensor. The recursive median average filtering method is found suitable for the sensing system. It exhibits a maximum frequency variation of 5 Hz and a standard deviation of 1.46 Hz, which satisfies most gas sensing tasks. Lastly, an analog multiplexer was employed to continuously switch among an array of oscillators while the microcontroller keeps monitoring the resonances. The multichannel frequency acquisition system shows great potential to contribute to a portable electronic nose system based on resonant sensor arrays such as capacitive micromachined ultrasonic transducer and QCM arrays.