Sheath current filter
Encyclopedia
Sheath current filters are electronic components that can prevent noise
signals travelling in the sheath of sheathed cables, which can cause interference. Using sheath current filters, ground loops
causing mains hum
and high frequency common-mode signal
s can be prevented.
Depending on the type, sheath current filters can remove or ameliorate hum in audio equipment, scanning frequencies in AV equipment and unwanted common-mode signals in coaxial cables
s are transformer
s for low frequency
analog and digital audio connections or rarely for high-frequencies in antenna cables between TV outlets and devices (tuner, VCR, TV, etc.). This filter then suppresses low-frequency ground loop currents on the sheath and core of coaxial cables, which can result from multiple grounds at different potentials. They affect the signal because of their upper and lower frequency limits and therefore can not transmit DC
. In addition, analog signals can suffer from nonlinear distortion, especially near the frequency limits of the device.
of the two conductors. Such elements are available as adapters called braid-breakers
or ground breakers and have, in both the signal and ground connection, coupling capacitors (with a capacitance of approximately 1 nF
). They are generally only capable of passing frequencies greater than approximately 50 MHz - ripple current cannot flow. Capacitive coupling adapters have an upper limit frequency of around 1 GHz, so UHF signals can pass through. A passband
of approximately 50 MHz to 1 GHz makes the devices useful for analog and digital television reception, and broadcast FM radio reception. Such ground breakers cannot be used in commercial satellite receivers, since low-frequency control signals and the supply voltage for the low-noise block converter
have to be transferred.
They block to high-frequency common-mode currents above about 50 MHz and affect the signal and the ground connection is not in terms of their Low frequency properties or protective function. Ferrite sheath current filters cannot effectively attenuate ground loop noise.
Cables for connection of computer peripherals often have a ferrite bead. The cable can be used to increase the inductance also repeatedly passed through a ferrite core.
Ferrite sheath current filters can only work effectively if a common-mode signal can flow on a line. This is generally the case when a cable bundle or a coaxial cable has a ground connection at both ends to the grounded equipment.
For a cable bundle between two devices but grounded at only one device, in general a ferrite sheath current filter is not effective. With such an arrangement, a ferrite bead would only be effective to reduce sheath current standing waves. When used to eliminate standing waves, the ferrite sheath current filter must be placed at a current antinode, but not at standing wave nodes.
Ferrite beads are available for different frequency ranges and power capacity.
.
Capacitive coupling filters can be used to prevent hum loops in antennas and radio frequency cables. They also have a galvanic separation.
Ferrite sheath current filters are used for noise suppression, combating noise such as radio frequency interference. They have no electrical isolation and cannot prevent ground loops.
Electronic noise
Electronic noise is a random fluctuation in an electrical signal, a characteristic of all electronic circuits. Noise generated by electronic devices varies greatly, as it can be produced by several different effects...
signals travelling in the sheath of sheathed cables, which can cause interference. Using sheath current filters, ground loops
Ground loop (electricity)
In an electrical system, a ground loop usually refers to a current, almost always unwanted, in a conductor connecting two points that are supposed to be at the same potential, often ground, but are actually at different potentials. Ground loops created by improperly designed or improperly installed...
causing mains hum
Mains hum
Mains hum, electric hum, or power line hum is an audible oscillation of alternating current at the frequency of the mains electricity, which is usually 50 Hz or 60 Hz, depending on the local power line frequency...
and high frequency common-mode signal
Common-mode signal
Common-mode signal is the component of an analog signal which is present with one sign on all considered conclusions. In electronics where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:...
s can be prevented.
Depending on the type, sheath current filters can remove or ameliorate hum in audio equipment, scanning frequencies in AV equipment and unwanted common-mode signals in coaxial cables
Type
There are various types of sheath current filter. Different types have different characteristics and are used to combat different forms of sheath current.Isolation transformer
Isolation transformerIsolation transformer
An isolation transformer is a transformer used to transfer electrical power from a source of alternating current power to some equipment or device while isolating the powered device from the power source, usually for safety...
s are transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
s for low frequency
Low frequency
Low frequency or low freq or LF refers to radio frequencies in the range of 30 kHz–300 kHz. In Europe, and parts of Northern Africa and of Asia, part of the LF spectrum is used for AM broadcasting as the longwave band. In the western hemisphere, its main use is for aircraft beacon,...
analog and digital audio connections or rarely for high-frequencies in antenna cables between TV outlets and devices (tuner, VCR, TV, etc.). This filter then suppresses low-frequency ground loop currents on the sheath and core of coaxial cables, which can result from multiple grounds at different potentials. They affect the signal because of their upper and lower frequency limits and therefore can not transmit DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
. In addition, analog signals can suffer from nonlinear distortion, especially near the frequency limits of the device.
Capacitive coupler
The propagation of (low-frequency) ripple current through antenna cables may be prevented by capacitive couplingCapacitive coupling
In electronics, capacitive coupling is the transfer of energy within an electrical network by means of the capacitance between circuit nodes. This coupling can have an intentional or accidental effect...
of the two conductors. Such elements are available as adapters called braid-breakers
Braid-breaker
A braid-breaker is a filter that prevents television interference . In many cases of TVI, caused by a high field strength of a nearby high frequency transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element...
or ground breakers and have, in both the signal and ground connection, coupling capacitors (with a capacitance of approximately 1 nF
Farad
The farad is the SI unit of capacitance. The unit is named after the English physicist Michael Faraday.- Definition :A farad is the charge in coulombs which a capacitor will accept for the potential across it to change 1 volt. A coulomb is 1 ampere second...
). They are generally only capable of passing frequencies greater than approximately 50 MHz - ripple current cannot flow. Capacitive coupling adapters have an upper limit frequency of around 1 GHz, so UHF signals can pass through. A passband
Passband
A passband is the range of frequencies or wavelengths that can pass through a filter without being attenuated.A bandpass filtered signal , is known as a bandpass signal, as opposed to a baseband signal....
of approximately 50 MHz to 1 GHz makes the devices useful for analog and digital television reception, and broadcast FM radio reception. Such ground breakers cannot be used in commercial satellite receivers, since low-frequency control signals and the supply voltage for the low-noise block converter
Low-noise block converter
A low-noise block downconverter is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception...
have to be transferred.
Ferrite chokes
Ferrite sheath current filters consist of a ferrite sleeve around the line or cable bundle. These are common mode chokes, damping high-frequency common-mode noise on cables.They block to high-frequency common-mode currents above about 50 MHz and affect the signal and the ground connection is not in terms of their Low frequency properties or protective function. Ferrite sheath current filters cannot effectively attenuate ground loop noise.
Cables for connection of computer peripherals often have a ferrite bead. The cable can be used to increase the inductance also repeatedly passed through a ferrite core.
Ferrite sheath current filters can only work effectively if a common-mode signal can flow on a line. This is generally the case when a cable bundle or a coaxial cable has a ground connection at both ends to the grounded equipment.
For a cable bundle between two devices but grounded at only one device, in general a ferrite sheath current filter is not effective. With such an arrangement, a ferrite bead would only be effective to reduce sheath current standing waves. When used to eliminate standing waves, the ferrite sheath current filter must be placed at a current antinode, but not at standing wave nodes.
Ferrite beads are available for different frequency ranges and power capacity.
Application
Transformer sheath current filters are used in low-frequency signal lines, where a ground loop otherwise can not be prevented. They are galvanically isolatedGalvanic isolation
Galvanic isolation is a principle of isolating functional sections of electrical systems, thus preventing the movement of charge-carrying particles from one section to another, i.e. no direct current flows between the sections. Energy or information can still be exchanged between the sections by...
.
Capacitive coupling filters can be used to prevent hum loops in antennas and radio frequency cables. They also have a galvanic separation.
Ferrite sheath current filters are used for noise suppression, combating noise such as radio frequency interference. They have no electrical isolation and cannot prevent ground loops.