Fault tolerant carrier frequency tracking for closed loop measurement systems

Carrier frequency based capacitance measurement is commonly used for measuring extremely small values of capacitive angle and distance sensors. If operated in a closed-loop system with a synchronous demodulator it offers significant advantages over conventional capacitance measurement methods. However, it is vulnerable to in-band disturbance signals. This drawback can only be overcome by selecting a different (undisturbed) carrier frequency. Fully integrated capacitive sensors without any means of external frequency reference are compulsory requirements for many industrial applications. These systems purely rely on an internal oscillator, which has to be digitally tunable to cope with temperature and process effects. We present a combined method for tuning the operating frequency to an external reference circuit (LC-filter) and at the same time detecting in-band noise signals.