Experiments with a commercial multivibrator as a detector for high impedance patterns

Certain industrial detectors call for robust, low-cost varying oscillators in the MHz range. In our application, high impedance patterns in the frequency domain are detected by sensing a small capacitance change related to the pattern passing a detection plane. Various commercial building blocks are readily available e.g. in CMOS IC chip form. We have studied the applicability of some typical products such as TC74HC4060, however, in most cases the obtained dynamic range in frequency domain has been inadequate for our purposes. A compact solution is the HEF4047B type multivibrator from the LOCMOS HE4000B family. A sensitive astable multivibrator circuit with a base frequency of 2.4 MHz has been constructed. By utilizing a simple counter, sensitivity in the range of 500 kHz/pF has been achieved. The response time far exceeds the requirements of this application. The sensor function is based on variable capacitance and therefore all stray sources must be carefully controlled. This has been accomplished through a dedicated printed circuit board layout. A buffer circuit is mandatory in the oscillator output not only to reduce the load pulling effects but also to provide convenient output levels.