Sheath current filter
A sheath current filter is a component that prevents either a hum loop via a double earth or ground connection or the propagation of high-frequency , unwanted common-mode signals on coaxial cables .
Depending on the type, sheath current filters prevent hum in audio systems, horizontal bars running through the image in AV devices or the propagation of high-frequency interference signals on cables. Often sheath current filters are wrongly equated with sheath wave filters .
How different sheath flow filters work
Depending on the type of signal and the interference situation, a distinction is made between the following types:
Isolating transformer
Isolation transformers are transformers for low frequency for analog and digital ( S / PDIF ) audio connections or, more rarely, for high frequency in antenna cables between antenna socket and end device ( tuner , video recorder , television , etc.). These sheath current filters suppress the low-frequency current on the shielding (the sheath) and the core (inner conductor) of coaxial cables , which can arise through multiple earthing at different potentials, through galvanic separation between input and output . They influence the signal due to their upper and lower limit frequency and therefore cannot transmit any direct current. Furthermore, non-linear distortions occur, which z. B. can lead directly to quality losses with analog signals.
Capacitive coupler
The spread of (low-frequency) hum currents on antenna cables can be prevented by capacitive coupling of the two conductors. Such as an adapter plug available elements as a ground breaker (ger .: Ground Loop Isolator ) or mass separator (Engl. Groundbreaker ) called and have the signal path and the ground connection coupling capacitors that the basis of their capacitance value (about 1 nF) only Are able to pass frequencies from about 5 MHz - hum currents can no longer flow.
Usual capacitive isolating adapter plugs have an upper limit frequency of 1 GHz so that UHF signals can also pass through them. They can therefore be used for VHF broadcasting as well as analog and digital television reception including cable television .
Such mass separators cannot be used with commercially available satellite receivers , since low-frequency control signals and the supply voltage for the LNB must be transmitted simultaneously.
application
Transmitter sheathed current filters are used in low-frequency signal lines when a hum loop cannot be prevented in any other way. They have galvanic isolation.
Capacitive sheath current filters can be used in antenna and radio frequency lines to prevent hum loops. They have no galvanic isolation.