The isolated star ground
The most common system for technical grounding is the isolated start ground system This approach to grounding has a minimum of technical compromises, meets the requirements of equipment grounding and provides a system that is relatively practical to install, troubleshoot and maintain. There are other approaches, such as using a ground plane, which will be discussed later; however, the isolated star ground is the most common.
Figure 3 illustrates the basic geometry for a star ground system. We can see from the illustration that the ground system consists of a central point that stars out to local points, which star out to equipment within a given area. Within that equipment, these grounds can further star out to the electronics, the shields and other systems and subsystems that require a ground reference.
The entire technical ground system of conductors and ground buses is insulated and isolate from all other systems, with the exception of a single point at the center of the ground system, which must be, by electrical code requirements, connected to the other grounding systems within a facility.
Several key points can be used to explain why this approach works:
-All of the electronic equipment within a given area has individual conductors providing it with a reference.
-Each piece of equipment within a given area has a ground reference to the same level.
-Every piece of equipment has only one possible path to earth.
-Each piece of equipment has a similar resistance to ground.
The first item means the common impedance coupling is eliminated by pieces of equipment within a given area. Because this equipment will normally have many signal interconnections, it is subject to this type of EMI generation.
The second item means all pieces of equipment in a given area will have a similar reference because they are connected to the same point. Again, this will reduce common impedance coupling and the effects of common-mode noise on differential lines. Also, any ground loops between equipment in a given area will have a minimum loop area.
Item three means there will not be any ground loops because there is only a single path to earth. Note that, although ground loops are not created by the grounding system, a ground loop can be created through interconnecting cable. This point will be discussed later.
The fourth item—all equipment having a similar resistance to ground—means that, because the system is picking up a certain amount of EMI and sinking these noises to ground, all branches in the system will have a similar ground-reference voltage (potential).
Levels of ground
The star grounding system results in various levels of ground. First is circuit ground. All electronics require a ground routed through the circuit board. Each circuit board is housed within a chassis that has an equipment ground. At one point within the chassis, the circuit ground is grounded to the equipment ground point for that chassis. This point is the star ground point for the unit.
The second level is shield ground. Each piece of electronics will have interconnecting cables for its signal inputs and outputs. The shields of these cables are normally grounded (often at one end only). The most common place to ground the shield is at the electronics unit where it terminates. Therefore, it is necessary to take the input-output connector ground pin to a ground within the chassis. How this is done is critical and will be discussed later in the section on equipment wiring.
Of course, shield grounding can also occur at interim jackfields. In this case, it is necessary to take a technical ground wire to the jackfields for this purpose.
The equipment ground refers to the ground reference to each individual piece of electronic equipment within an audio of video system. This ground is part of the safety ground of the electrical system and enters the equipment through the ac power cord via the third prong, a requirement of the electrical code. The code’s intention is that, when the piece of equipment is plugged I, it is also grounded with that same connection point. In this way, it is not possible to have a piece of equipment powered up and not grounded. Tampering with this equipment ground is illegal.
The next level, the local or area technical ground-reference bus, is connected to the master ground reference by a single heavy conductor. One or more of these are typically in a facility located near equipment centers such as control rooms, machine rooms, remote amplifier rooms or mobile truck locations.
The last level, the master technical ground-reference bus, is the central hub for all technical ground conductors. Only one of these buses is within any given facility, and it is this point that connects the technical ground system to the ground electrode system of the building and to the electrical grounding system for the building. This point also grounds to the neutral conductor for the power distribution.
Common impedance coupling
Common impedance coupling can occur between any two pieces of electronics whenever they have a shared conductor that has impedance between them. A common example of this is a shared grounding conductor used by more than one piece of equipment. This is one of the main reasons why the star ground system is employed. However, the trade-off between loop area and common impedance coupling has to be considered. Figure 4 shows two examples of common impedance coupling. In Figure 4A, the example shows common impedance coupling via the neutral conductor. Figure 4B shows common impedance coupling via a daisy-chain ground. In this figure, we see that any noise currents created by the two left amplifiers will create a voltage across R3 that will modulate the ground reference of the third amplifier. The common impedance R3 results in common impedance coupling.
A major cause of failure of technical ground systems is ground loops. They result in electromagnetic interference. A ground loop, as the name implies, is created when a conductive loop is formed by the technical ground conductors and some additional conductor. A ground loop can be formed when a short to ground occurs in a technical ground system, as shown in Figure 5. It is also possible to have a ground loop in a technical ground system when two points of the technical ground system are connected together, usually through a piece of interconnecting signal cable, as shown in Figure 6. In other words, ground loops can occur when a piece of technical equ8pment9ecomes grounded to building steel or some other conductive member. Ground loops can also occur when a piece of signal cable has the shield terminated to the technical equipment (and hence ground) at both ends.
Ground loops are detrimental to audio, video and related equipment because they cause stray currents to flow within the technical ground conductors. These ground conductors are used as the ground reference to the electronic equipment of the system. The stray currents consequently induce noise in the ground reference, and this noise can be induced into the signal lines of the system.
Loop area and impedance
The idea of loop area is important to technical ground systems. We discussed earlier that when a conductive loop is formed a magnetic field will induce a current into that loop. The current will be a function of the size of the loop or the strength of the magnetic field. (A larger loop will contain more magnetic field lines.) If the field increases or the loop area increases, the current will increase. The EMI problem associated with this then increases with the loop area or the magnetic field strength.
Figure 7 shows different scenarios where the loop area varies because of how the wiring was done and illustrates the varying effect of these situations on the magnitude of the EMI. In one case, the equipment is grounded locally, and the loop area of the ground loop is small. In the second case, the equipment is grounded not locally but some distance away, and now the loop area is much larger. We wold anticipate that the EMI problem, when a ground loop is present, is more in the latter case. This example illustrates an important issue of technical grounding: You must consider the geometry of the ground and wires. Although two different grounding systems may look equivalent, the fact that the one has a larger loop area (a geometry problem, not a circuit problem) means they might behave quite differently in practice. Figure 8 presents another look at loop area and EMI.
Impedance is defined as the ac resistance to electrical current flow. Impedance describes the resistance of a piece of wire in the presence of an alternating current. Interestingly, impedance will vary with the frequency of the current in the wire. As the frequency increases, so does the impedance. Two major effects cause the impedance of a piece of wire to increase with frequency. Because many forms of EMI are high in frequency and in some cases very high in frequency (in MHz), the impedance of the wire becomes a significant factor in determining its ability to drain away stray electrical noise.
The two main effects that increase impedance are the skin effect and the inductance of the individual conductor. One explanation of skin effect is that the internal inductance of a wire increases toward the center, making current flow easier toward the outside of the wire. Therefore, at very high frequencies, most of the current flows around the outside of the conductor, and consequently the conductor has effectively a smaller area, so its resistance increases. The incremental self inductance of a single piece of wire is a function of its length and the radius of the conductor. A piece of wire with a bend in it will have a greater inductance than a single straight piece. Therefore, it is important to route ground conductors with a minimum number of bends and turns.
Because of the skin-effect problem, it is common to use braided conductors or flat pieces of copper plate or ribbon, which have a greater surface area and hence less skin effect. However, under normal EMI conditions, the use of braid and straps is not required.
As the frequency of alternating signals increases, the manner in which the signals propagate through a wire is governed by transmission line theory. Although this topic is beyond the scope of this article, an intuitive understanding of this topic can be gained by considering waves traveling down a river or channel. If any obstacles are in the channel, the waves become broken up and may be reflected back up the channel. The extreme case of this is a wall at the end of the channel, where the waves traveling down the channel and the waves being reflected back up the channel interact with each other to create waves that do not move longitudinally in the channel but simply move up and down.
One of the effects of standing waves is that, at certain frequencies of excitation, a wire behaves as an open circuit. No electrical energy is transmitted through the wire. In this case, where a wire is being used as a grounding conductor, no grounding is taking place. The frequency at which the standing waves take place is a function of the length of the wire. So for a given frequency there is always some wire length at which, if there is a termination discontinuity at the end of the line, some energy will be reflected back down the line, resulting in an impedance characteristic as shown in Figure 9. One solution to this problem is to provide multiple grounding paths of varying lengths, which unfortunately can create ground loops. This phenomenon is one of the reasons why good high-frequency grounding (in the megahertz region) is so difficult to achieve.
Ground planes—the final solution?
Given that at certain frequencies a conductor of a given length acts as an open circuit or at least its impedance increases, multiple ground routes are desirable. A ground plane consists of a large conductive surface. Obviously, any two points between the surface can be connected with a large number of paths through the ground plane. At any frequency, the impedance between any two points on the ground plane will always be low. For this reason, ground planes are a common technique used in circuit boards where they are easily implemented.
The concept of ground planes can be applied to audio and video systems grounding; however, it’s clear that implementing a ground plane is considerably more difficult than implementing discreet insulated grounding conductors.
Consider, for example, a recording studio with a large conductive mat placed below a carpeted floor. Every piece of equipment the studio could be grounded to this mat at a point immediately below the equipment. Such a system would provide extremely low impedance among all pieces of equipment in the studio. However, it would be difficult to ensure that this ground plane did not become inadvertently shorted to building steel. These systems are somewhat unusual, therefore, and are found in only the most challenging of electromagnetic interference environments. One example of how grounding planes might be used in a facility is shown in Figure 10.