Bi-stable oscillator circuit for microbalance

摘要

In accordance with the present invention there is provided a method of detecting a load imparted to the surface of a transducer, for example a load caused by the binding of target molecules to the surface of the transducer. The transducer, which may be resonant device such as a bulk acoustic wave (BAW) transducer, a MEMS or NEMS device, or a quartz crystal 202a, forms part of a quartz crystal microbalance (QCM) and is arranged to be exposed to an environment under test. The crystal has a resonant frequency and forms part of a bi-stable oscillator circuit. The oscillator also comprises an impedance network 201, a buffer amplifier 203 and an AGC amplifier 204. When a load imparted to the surface of the resonator is less than a pre-determined threshold value the oscillator circuit oscillates at the resonant frequency of the quartz resonator. When the load imparted to the surface is greater than the threshold value the oscillator circuit oscillates at a free-running frequency set by the impedance network. Bi-stable switching of the circuit oscillation frequency (fig.3) is indicative of the load crossing the predetermined threshold value. The frequency switching can easily be detected without the use of accurate frequency measurements.