SYNCHRONOUS REJECTOR FILTER.

Electromagnetic measurements in the low-frequency part of the spectrum are complicated by noise at mains frequency and its harmonics. An effective way of countering this form of interference is by connecting rejector filters in the circuit carrying the signal. The traditional scheme based on passive filters with RC tuned circuits does not provide a high enough Q band and suppression at the rejection frequency. Furthermore, the rejection frequency is fixed. Should the mains frequency change by as little as the plus or minus 2% set by existing standards, the traditional filters become ineffective. Better results can be achieved by the use of synchronous rejector filters that track mains frequency. Two filter schemes have been studied experimentally.