Earth loop
A ground loop or ground loop , with the participation of a grounding also ground loop called, is in the electrical engineering a closed into a loop ground connection of an electrical wiring or wiring due to the at low frequency noise currents impedance of the loop an unwanted voltage drop generated in the signal path. As a result, an interference signal is added to the useful signal. The interference current can be transmitted into the useful circuit via a common impedance with an interference circuit or inductively via magnetic field coupling.
Occurrence
The interference signal can z. B. express in audio systems as an unwanted, annoying hum. In measuring devices and sensors, the low-frequency interference signal falsifies oscillograms or measurement signals. Depending on the cause, the interference signal contains the mains frequency (50 Hz or 60 Hz) and a more or less high proportion of its harmonics or harmonics. Odd-numbered harmonics (in Europe 150 Hz, 250 Hz, etc.) and (e.g. when mains rectifiers are involved ) also even-numbered harmonics (in Europe 100 Hz, 300 Hz, etc.) occur. Often the network base frequency can hardly or not at all even in audio systems, because it is at the lower limit of the LF transmission range.
( DC ) overhead tram lines and their feed lines and rails can cause interference magnetic fields at 300 Hz, which arise in the substation when rectifying from the three-phase network ( ripple current ).
causes
The transmission of electrical signals, whether analogue or digital, requires the transmission of a reference or reference potential . In the case of symmetrical signals, this is the other wire (analog telephone: a - against b - wire) or in the case of asymmetrical signals, ground. If the reference potentials of the devices to be connected are not the same, an interference signal is generated in the amount of the reference potential difference.
If you connect z. B. the ground of two devices via a cable shield and the device grounds are connected via the protective contacts of the sockets ( protection class I ) at the same time , this creates a closed circuit (highlighted in yellow in the picture), which is applied to all magnetic alternating sources ( transformers of power supplies , electric motors , Chokes , etc.) of the environment acts like a short-circuited secondary winding of a transformer.
The current flowing through the earth connection generates an interference voltage via the resistance of the earth connection (contact resistance + cable resistance) :
The reference potential of the receiver thus differs by this voltage from the reference potential of the transmitter. is applied to the received signal as an interference signal, which sets the voltage at the receiver:
Ground loops are created even without a connection to the protective conductor e.g. B. in metallic control cabinets. There, too, they can cause malfunctions if there are multiple ground connections in different ways or if grounded antenna and telephone cables are connected to one another multiple times. Large equalizing currents can particularly flow between different points of a protective conductor connection (different electrical circuits in the house installation) or between those and an earthed antenna or TV cable system. In addition, direct and alternating currents in the milliampere range flow in the protective conductor between buildings.
This can be remedied by differential signal transmission with two wires that are not connected to the grounding of the devices or are only connected to a single location (principle with analog telephones ).
Examples
Video and sound technology
Individual audio devices ( amplifiers , mixing consoles , but also computers) are often protective earthed and have a connection between the protective conductor and the signal ground. A further ground connection via the signal cable then leads to a closed ground loop and interference. Equalizing currents arise between different earth connections (for example the antenna cable and the protective conductor ) due to slightly different earth potentials. They cause voltage drops on the ground connections of the signal ground. These voltage drops add directly to the useful signal (NF) or cause in ferrite filter coils an amplitude modulation in (amplitude modulated) television signal. In the second case, horizontal bars running through the image appear, which may also have a lack of line synchronization if the sync pulses are lost due to saturation of the ferrites.
Audio devices (internal)
Ground loops within audio devices are caused by improper design: if the ground points of the input sockets and other points of the internal circuit are connected to one another or even to the protective conductor via several paths, this is the case. After rectification, the supply current often also flows via a common section of the ground connection. This leads to humming noises - depending on the cause with the mains frequency or twice the mains frequency and harmonics thereof.
Computer networks
Older, asymmetrical standards for electronic data transmission ( RS-232 , parallel port , 10BASE2 ) in particular have problems with ground loops. The EDP devices, which are usually provided with protective earth ( protection class I ), together with the outer conductors of the shielded cables, caused earth loops between the devices that could interfere with data transmission. Today's network connections for long distances contain isolating transformers ( Ethernet - "Magnetics"). In USB cable differential signal transmission is used to reduce potential interference caused by the operating voltage and ground connection and the shielding ground loops.
Sensors
In machines and systems, ground loops often occur with sensors connected via coaxial cables if these are connected at the same time to different points on the protective conductor or the chassis of the machine. This can be remedied by using differential, floating inputs (see pseudo-differential signal transmission ) or only placing the screen at one end of the cable connection.
Circuit boards, switching power supplies
With circuit boards and device wiring, there are two mutually contradicting requirements:
- the lowest possible induction of the ground connections ( ground plane )
- the avoidance of earth loops by means of star point earthing : all earth connections meet at just one point.
These mutually exclusive requirements are handled differently depending on the assembly or device:
- With motherboards and often also with switched-mode power supplies , a closed ground plane is used as a separate layer.
- If analog and digital signals appear mixed, separate ground planes are used, which are only connected at one point
- In the case of low-frequency devices (amplifiers), neutral grounding can be used to a large extent and, if necessary, partial areas can be covered with a ground plane which, however, must not be traversed by large currents (rectifier, outputs).
Telephone and communications technology
Attempts in the 19th century to transmit telephone calls asymmetrically (i.e. with only one line and with earth as the opposite pole) were limited to a few kilometers - the interference became too great. In communications engineering, for example, it was first recognized that the transmission of signals over long distances is only possible if not only a reference signal is carried as a second line, but this mass also remains unaffected and unused by other signals. The principle of differential signal transmission was born. See also symmetrical signal transmission .
Countermeasures
Avoiding a loop
So-called protective insulation can provide a remedy for protective conductor ground loops . This is why many audio devices are fully insulated and - even with metal housings - have no connection to the protective conductor. A ground connection between the devices only exists in the form of the shield of the signal lines. However, as soon as devices are connected by several signal ground paths, ground loops can also arise here.
Therefore, among other things, the symmetrical signal transmission is used in professional audio equipment . Any equalizing currents that may occur are kept away from the useful signal and differences in the level of the signal grounds between different devices do not cause any problems. The mass then only has a shielding effect; Voltage drops on it do not affect the signal. Often in audio systems also are transformer inserted. This breaks the connection of the signal grounds.
Sound equipment occasionally has a so-called ground lift switch. This can be used to break the connection between the protective conductor connection of the device and the signal ground in the device. In this way, a possibly existing hum loop can be broken, although this can result in disadvantages for the insensitivity to radio interference and other capacitive interference. A good solution can be a capacitor (approx. 0.1 µF) between the signal ground of a device and its protective conductor / housing: the capacitor forms a high resistance to the ripple current, but still ensures the shielding effect of the housing against higher-frequency interference.
Requirement for these two measures (ground lift switch or capacitor) is a so-called safe separation between voltage-carrying parts on the one hand and audio signals as low-voltage-transmitting parts on the other hand: The devices must meet the requirements of a protective low voltage ( PELV - Protective Extra Low Voltage meet).
The ground connections must therefore not be separated by interrupting the protective contacts of the mains connection of protection class I devices: This would cancel the protective function ( protective earthing ), which in the event of a fault can lead to life-threatening voltages on the device housing.
Reduction of equalizing currents
Devices are often used whose signal ground is connected to the protective conductor (antenna or cable systems, computers) so that earth loops are unavoidable. Sheath current filters , isolating transmitters or optical data transmission methods often help here . For high-frequency useful signals and low-frequency interference signals, e.g. B. on antenna lines, it may be sufficient to galvanically interrupt the outer and inner conductors or just the outer conductor of the antenna line using a capacitor. High-frequency useful signals are then transmitted capacitively. The hum loop is interrupted for low-frequency interference signals. Such adapter plugs are sometimes referred to as sheath current filters, but they only reduce low-frequency sheath currents.
Reduction of interference
Usually, the area enclosed by the signal lines is kept small by using twisted pair lines or coaxial lines. The interference in ground loops can occur by reducing their enclosed area (parallel or even twisted laying of signal cables, separated from power cables). In special cases, hum compensation can be achieved by laying them in the shape of a figure eight.
The source of the interference fields can often not be removed, but the effects can, in the case of transformers or series reactors, e.g. B. can be reduced by changing their position. Magnetic shields around the source can also help, but are expensive for low-frequency magnetic fields and are therefore usually only found in tape recorders or turntables.
Evenly wound toroidal transformers have the lowest magnetic stray fields of all transformer designs.
Reducing the resistance of part of the loop
The interference current circulating in the hum loop influences the useful signal because it flows at least partly in the same cabling as the useful signal. As a rule, it flows in the connection between the signal grounds of two devices. Since the signal ground represents the reference point for the transmission of the useful signal from one device to another, any difference in the reference point between the devices will appear as an interference signal . If you make sure that these reference points are as similar as possible, circulating interference currents will have less of an impact on the useful signal.
It is therefore advantageous if the ground connection between the devices has the lowest possible resistance . This can be achieved with cables and plug connections with low resistance (high cross-section of the shields, low contact transition resistance) or with additional ground connections with a high cross-section. Some devices have additional ground screws for this purpose. Then the equalizing current is maintained (or possibly increases), but the voltage drop shifts to areas of the loop that do not have signal ground.
Separation of the signal path from the loop
Better results can be achieved if the interference currents do not flow over the signal ground at all. Interference currents may then flow through the cable shield, but the signal ground is routed separately within this shield. The prerequisite for this is the galvanic separation of the signal ground from the shield ground, as is the case with the measuring device connector in communications technology. Both are connected at a single point at most.
Equivalent to this is the symmetrical signal transmission , here no signal ground connection is required - the signal is the differential voltage between two signals in antiphase. In addition, this solution also eliminates capacitive coupling - they have the same effect on both signal lines in opposite phase and have no influence on the differential voltage.
Digital and optical connections
Digital connections, in which the audio signals are transmitted as binary coded packets, eliminate the problem of hum loops, since the digital signals have to be decoded on the receiving end and the (analog) hum component is not taken into account. The S / PDIF standard used for digital transmission provides for different cable types. With coax or jack cable connections, there is still the possibility of influencing the analog parts of the signal processing if the decoupling is poor on the receiver side. Optical TOSLINK cables provide a complete remedy . These optical fibers are electrically non-conductive and the optical signal cannot be influenced by magnetic or electric fields. Their complex optical signal converters are disadvantageous.
literature
- Joachim Franz: EMC. Fail-safe construction of electronic circuits . Teubner, Stuttgart et al. 2002, ISBN 3-519-00397-X .
- Adolf J. Schwab , Wolfgang Kürner: Electromagnetic Compatibility . 5th, updated and supplemented edition. Springer, Berlin et al. 2007, ISBN 978-3-540-42004-0 .
