High speed high precision frequency counter for QCM sensor data acquisition

The Quartz Crystal Microbalance (QCM) sensor is a mass-sensitive sensor. The resonance frequency of the QCM sensor changes due to the deposited mass on the sensor surface. A QCM sensor with a 10MHz resonance frequency has a sensitivity of 4.4ng/Hz. It is required to have high precision and a stable frequency counter to achieve sensitivity. A resolution of 1 Hz with a second sampling time can be realized using discrete components, microcontroller, CPLD, or FPGA based on a conventional or reciprocal frequency counter. However, in some applications, the sensor's response to mass change is quick enough so that a significant frequency change happens in less than one second. A frequency counter with a faster sampling time with high resolution is required. This work shows a frequency counter design using FPGA, which can measure the frequency with a resolution of less than 1Hz with a sampling rate of 100ms. The frequency counter is implemented based on a reciprocal counter and is used TCXO reference clock source to achieve high stability. The counter has been tested using a rubidium clock oscillator and shows long-term stability. The developed frequency counter performs better than the available frequency counter reported in some publications.