A device used to suppress noise in a transmission line or cable, caused by electromagnetic interference (EMI). EMI is produced by nearby power lines, motors, generators, and other sources. EMI can introduce noise into a transmission line or cable that can degrade the quality of a signal or even make communication impossible. By inserting a line filter at the appropriate point, you can suppress the noise and potentially improve transmission speeds.
Line filters might be needed in homes or small businesses that use modems to connect to the Internet through a dial-up connection over the local loop. High-speed V.90 modems sometimes have difficulty attaining their top data transfer speeds because of ambient line noise caused by nearby sources of EMI. By placing a line filter at the customer premises between the modem and the Plain Old Telephone Service (POTS) connection, you can filter out noise, which could improve modem speeds.
Graphic L-8. Line filter.
Before installing a line filter, you should use a radio frequency (RF) spectral analyzer to determine the general frequency of the source of EMI so that you can choose an appropriate line filter. Line filters typically filter out one of the following frequency ranges: low frequency (LF), high frequency (HF), very high frequency (VHF), or ultra high frequency (UHF) signals.