The method includes generating a binary signal (S4A to S4D) which is time modulated. To do this, an alternating signal (S1) is compared with a known cyclical signal (S5) and a threshold signal. There is measured the durations between the transitions of the modulated signal by means of a high speed clock...http://www.google.com.tw/patents/US4645881?utm_source=gb-gplus-share專利 US4645881 - DC level trip detector
