Sheath current filters are electronic components that can prevent

noise Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

signals travelling in the sheath of sheathed cables, which can cause interference. Using sheath current filters, ground loops causing

mains hum Mains hum, electric hum, cycle hum, or power line hum is a sound associated with alternating current which is twice the Utility frequency, frequency of the mains electricity. The fundamental frequency of this sound is usually double that of the l ...

and high frequency

common-mode signal In electrical engineering, a common-mode signal is the identical component of voltage present at both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal volta ...

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 cable Coaxial cable, or coax (pronounced ), is a type of electrical cable consisting of an inner Electrical conductor, conductor surrounded by a concentric conducting Electromagnetic shielding, shield, with the two separated by a dielectric (Insulat ...

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 transformer An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons or to ...

s are

transformer In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...

s for

low frequency Low frequency (LF) is the ITU designation for radio frequencies (RF) in the range of 30–300 kHz. Since its wavelengths range from 10–1 km, respectively, it is also known as the kilometre band or kilometre waves. LF radio waves ...

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.

Capacitive coupler

The propagation of (low-frequency)

ripple current Ripple (specifically ripple voltage) in electronics is the residual periodic variation of the DC voltage within a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alt ...

through antenna cables may be prevented by

capacitive coupling Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...

of the two conductors. Such elements are available as adapters called

braid-breaker A braid-breaker is a filter that prevents television interference (TVI). In many cases, TVI is caused by a high field strength of a nearby high frequency (HF) transmitter, the aerial down lead plugged into the back of the TV acts as a longwire an ...

s 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 A passband is the range of frequency, frequencies or wavelengths that can pass through a Filter (signal processing), filter. For example, a radio receiver contains a bandpass filter to select the frequency of the desired radio signal out of all t ...

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 A low-noise block downconverter (LNB) is the receiving device mounted on satellite dishes used for satellite TV reception, which collects the radio waves from the dish and converts them to a signal which is sent through a Coaxial cable, cable t ...

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 wave In physics, a standing wave, also known as a stationary wave, is a wave that oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with respect t ...

s. When used to eliminate standing waves, the ferrite sheath current filter must be placed at a current

antinode A node is a point along a standing wave where the wave has minimum amplitude. For instance, in a vibrating guitar string, the ends of the string are nodes. By changing the position of the end node through frets, the guitarist changes the effec ...

, but not at standing wave nodes. Ferrite beads are available for different frequency ranges and power capacity.

Application

{{See also, Electromagnetic compatibility, Ground loop (electricity) Transformer sheath current filters are used in low-frequency signal lines, where a ground loop otherwise can not be prevented. They are galvanically isolated. 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.

External links

How to build a homemade sheath current filter. (German)
Filters