Abstract: Corruption of digital data on a telephone line to which a telephone test set is to be coupled is obviated by inserting a low pass filter circuit in the tip-ring connection path to the test set. The low pass filter provides a low impedance path for only voice band signals, while blocking signals that fall within the high frequency range of digital data that may be present on the telephone line. The low pass filter circuit includes inductors coupled with respective tip and ring leads and a controllably switched, shunt-capacitor circuit coupled across the tip and ring leads. Also, resistors that compensate for impedance variations are coupled in series with the leads and are controllably by-passed, when said telephone test set is to go off-hook.

Abstract: An electromagnetic interference by-pass filtering mechanism suppresses RF noise currents conducted over the tip and ring leads of a telephone line-powered telephone instrument. Such RF noise currents may result from amplitude modulated radio broadcast signals emanating in the vicinity of the telephone instrument. The filtering mechanism comprises a conductive material coated on the interior surface of the housing of the telephone instrument, so as to surround the printed circuit board containing the telephone circuitry of the instrument. The conductive material is coupled to earth. First and second capacitors are coupled between the tip and ring leads and the conductive material, and first and second inductors are coupled in series with the tip and ring conductors and connections of the tip and ring conductors to the printed circuit board containing the telephone circuitry.