An active tracking filter reduces low frequency noise (such as a 20 Hz ringing signal) riding on a DC voltage (e.g., on the order of (40 VDC) applied to a load such as a subscriber line interface circuit (SLIC). A controllable impedance device, such as a FET operating in its linear range, is coupled...http://www.google.ca/patents/US6489755?utm_source=gb-gplus-sharePatent US6489755 - Active ripple and noise filter for telecommunication equipment powering
