EMI/RFI Line Filter
A filter made up of inductors and capacitors connected in series in a device's supply line. Its job is to limit the conducted noise that fast-switching devices such as inverters pump into the mains, and to protect sensitive devices sharing the same line from that noise.
Noise spreads by two routes: radiated (through the air) and conducted (over the cables). A line filter targets the second. The common mode choke and capacitor arrangement inside it redirects the high-frequency components to earth and back to their source, preventing them from reaching the mains.
- Where: on the input side of the inverter, right next to the device and with a short cable. A long cable between the filter and the device gives back part of what the filter gained.
- Mounting: the filter body must sit on the panel metal with a wide contact surface; screwing it onto a painted surface raises the earthing impedance.
- Separation: the filter's input and output cables must not be run parallel to each other, otherwise the noise bypasses the filter.
For the radio receiver the practical order of priority is this: first move the receiver and its antenna away from the noise source, then correct the shielding and earthing and try a ferrite; the line filter comes into play as a permanent solution at system level.
The context in which this term is used: 433 MHz or 2.4 GHz?