Telephone line protection against power line interference esp. at switchboards - has series inductive coil followed by parallel inductive coil on each incoming telephone line, with series capacitor leading to earth

摘要

The two inductors (12,13) in series with the telephone line (2,3) are preferably identical, and wound on a common ferrite core as a transformer (14). The parallel inductors (17,18) are similarly wound as a transformer pair (19), and have the same inductance as the series pair. Their associated capacitors (20,21) have the same capacitance. The attenuation is sensitive to interference at 50 Hz which interferes with speech, ringing, or teletext signals. Inductances of 5 H with 1 microfarad ceramic capacitors yield an attenuation of about 47 dB for a 800 Hz 4 V signal with 4 mV noise. USE/ADVANTAGE - Protection near industrial installations esp. railway overhead power lines. Simple, small size, easy installation.