Question: Name three things you have NOT experienced in your A/V system installations. Yeah, right (don't forget, my office is located not far from Fantasyland). All three are common terms for disturbances seen or heard in A/V systems everywhere. They are the visual or aural result of 'common-mode noise.' It could be described as common-mode because it's a common occurrence when we connect together the components of a system. When common-mode noise presents itself we usually refer to it as a 'ground loop' problem regardless of the real cause.

Where Tomato Worms Come From

Like the insidious green worm that mysteriously shows up on that luscious tomato, ground loops always seem to crop up in a system…moreover, metaphorically or otherwise, crawling slowly up that new vision, the fruit of our efforts, imparting that same feeling of dread. No one wants to deal with ground loops; or worms. But, are we really dealing with a 'ground loop?' Interestingly, many noise problems in systems are not literally ground loops at all, although this term has become part of the vernacular. There are three common methods of noise pickup: conduction, common impedance, and electric/magnetic field coupling.

Conduction is the most obvious and most commonly overlooked. In this situation a wire may be running through a noisy environment wherein noise is induced and conducted to another circuit. Either the wire must be removed from the source of the noise or appropriate decoupling methods must be used to arrest the noise.

Common impedance coupling is probably the most often encountered system of interference coupling. This occurs when electrical currents from two different circuits flow through a common impedance. Each circuit creates a voltage drop across the common impedance thereby creating a noise source which influences the other. See Figure 1. This may be the most common situation in A/V systems where equipment is connected to AC power mains through common, daisy-chained neutrals and safety ground returns to the main service panel. The first step in eliminating this source of noise is the use of 'home run' style wiring, which establishes the single point ground shown in Figure 2. Do not allow systems to share neutrals and grounds. All electrical systems have a common impedance. The idea is to connect electronic systems at a single point in the system where the common impedance is the lowest possible. This point is usually at the main service entrance panel.

Radiated electric and magnetic fields provide another source of interference. If the noise source is close by, we need to consider the electric field and the magnetic field separately. If the source is far away (e.g., a radio station), the interference is considered to be lumped into electromagnetic radiation.

Altogether, the coupling of interfering noise involves a source, a coupling medium, and a receiver. Elimination or reduction of interfering noise is possible only by evaluating the problem and taking appropriate action on one or more of these three components. Although cabling quality and installation is very important to noise elimination, cables are not always the culprit behind induced system noise. The interfering noise we see or hear in the system will be converted to differential mode (mixed with the desired signal) once it has entered the system component following its creation. And, in some cases, it is this piece of equipment that is responsible for coupling the noise. At this juncture, the noise is virtually impossible to remove.

Figure 1. Equipment connected on a series AC circuit to a single point ground, but sharing ground current.

Figure 2: Preferred single-point ground method that eliminates shared current. Also called "home run" wiring

Becoming Well Grounded

While grounding is the best way to minimize or eliminate interfering noise, it is of little value if the grounding system is not properly considered in the system design. Knowing where and when to ground components and cables is equally important. Safety ground and signal ground are the two categories of grounding. The connection of circuits to the earth as a ground point implies a safety ground since our AC power distribution system is earth ground referenced. To electronics engineers, the term ground is simply a reference point of zero potential difference between two points within a circuit or system. In an ideal ground system, all points on the ground have zero potential difference. However, in reality, anytime there is a current flowing through the ground return of a circuit, a potential difference is created. This difference is the noise voltage generator across the ground. Whether it will infect your system depends on whether this interfering current travels through the system of interest.

Figure 3. US standard AC wiring showing safety ground and lowest common impedance point

The U.S. standard power distribution system is a three-wire system: hot wire (115 VAC), neutral wire, and safety ground wire. See Figure 3. The circuit current is intended to flow from the hot wire, through the load, and through the neutral, not the safety ground. The safety ground is connected only to the appliance's enclosure to protect the user from electrocution in the event of a ground fault. However, the neutral and safety ground are common back at the main entrance panel. This should be the point of lowest common impedance, or the theoretical single point ground. This point equates to the reference ground for the power transformer in the mains power system. Appliances wired for 230 VAC may not use a neutral at all, but the enclosure is typically connected to the safety ground in case of dielectric failure. The reality is that, due to capacitive coupling, some currents flow in the safety ground. This is often true of switch-mode power supplies having line filters that couple noise energy into the safety ground. Although the noise is typically very small, the potentially interfering voltage is there and may infect low voltage systems like audio and video equipment.

Figure 4: Best connection method for audio on shielded twisted pair when outer shield must be grounded at both ends.

Keep in mind that any equipment system referenced to another earth ground WILL inevitably create ground currents in your system. Connecting signal equipment to a separate ground rod than that used at the electrical service entrance will set up differences in potential between those two ground points. Therefore, two grounds are not necessarily better than one.

Having the third wire safety ground is the beginning of the so-called ground loop problem. Initially, we may not know if the interfering noise is a loop phenomenon or just noise induced by virtue of a common impedance. We must analyze the actual return current path in the circuit, not the path we think it should be flowing within. In some cases, the wiring of a system dictates (usually economically) that ground returns be a combination of series and parallel single point grounds. Certainly this is a compromise so the designer must take care in grouping ground returns selectively such that equipment or circuits of widely varying power levels do not share the same ground return circuit. Some equipment is more susceptible to noise problems by virtue of the internal grounding topology taken by the manufacturer. The grounding premises discussed here are just as meaningful within the design of equipment as they are at the system interconnect level.

Whether considering audio or video, the grounding of cable shields is not always clear. There are two scenarios to consider—one for low frequency noise and another for high frequency noise. The hum and buzz in the audio or creeping bar in the video typically falls within the realm of low frequency noise interference. For balanced audio or frequencies below 1 MHz, grounding the cable shield at only one point (the source end) is recommended. Grounding both ends sets up a loop for current to flow along the ground shield. This approach is effective against magnetic pickup, but may not help with low frequency ground loops caused by difference in potential between the grounded shields at each end. If a shield surrounding a twisted pair must be grounded at both ends by virtue of the system connection design, then the scenario in Figure 4 should be followed.

Figure 5: Best connection method for unbalanced coaxial lines

Unbalanced coaxial lines such as typically used for video and consumer grade audio connections should follow the ground connection method shown in Figure 5 in order to realize the best noise rejection possible. Figure 6 shows the typical wiring scenario for a ground loop condition. When additional noise rejection is required, consider breaking the signal ground utilizing a coupling transformer, an optical coupler, a differential amplifier, or a common-mode choke. All four of these possible solutions may work well for audio situations because of the limited signal bandwidth. Figure 7 shows how an isolation transformer breaks the ground loop for an audio installation.

Figure 6: Equivalent circuit for a ground loop between two components not on equal potential grounds.

Figure 7: An isolation transformer is a quick, cost-effective method for removing a ground loop in an audio system.

For video, the transformer and optical coupler are difficult to implement because of their impact on system high frequency performance. This leaves the differential amplifier or the common-mode choke as viable solutions. Eliminating the effects of the circulating current created by the differential voltage between two equipment grounds is our goal. Let's take the transformer concept, modify it somewhat, and implement as a common-mode choke as in Figure 8. A common-mode choke passes DC and differential-mode signals, but blocks common-mode signals. A common-mode signal manifests when the voltage amplitude and direction are the same on both conductors within the connection system. The desired signal's current flows in opposite directions within the common-mode choke and is passed on through by its low impedance. In the case of common-mode current, flow is in the same direction and the choke presents high impedance. The interfering noise magnetic field is absorbed, or dissipated within the common-mode choke. Electrically, it is as if the ground line were opened. When the interfering frequency is several megahertz, the common-mode choke's effectiveness may be limited by internal shunt capacitance between windings on the choke core that allow the interfering signal to pass through. This limitation is highly dependent on the quality of the common-mode choke design.

Figure 8: Common-mode choke arrangement is very effective in blocking common-noise but passes DC bias and a wide band of frequencies.

The differential amplifier style isolation for a ground loop condition is still an effective method for isolating the interfering ground current. Like the balanced input on a professional audio amplifier, the differential style isolator cancels the common-mode voltages on the signal line. This type isolation amplifier is very effective at high frequencies. It can be limited at low frequencies depending on the quality of the internal balance of the amplifier's inputs and its dynamic voltage range.

In contrast, common-mode chokes can typically handle higher common-mode voltage levels and are better balanced at low frequencies. Therefore, the selection between common-mode choke or differential amplifier noise elimination solutions is based more on the frequency band of the interfering signal. For the typical low frequency hum, buzz, and rolling bars, the common-mode choke of today will likely provide the best performance. Recent developments in extremely high permeable materials make the common-mode choke concept a better performer for video applications compared to a few years ago. In addition, its simplicity is enhanced in that no power supply is required for its operation. The correct point of application is in series with the signal line(s) at the input of the noise-receiving device in your system. For video applications, this will be at the display's input. Care must be taken that the shields of the input side cables on the common-mode choke do not contact the chassis or any other metallic connection that is connected to the display's ground; otherwise its positive effects will be defeated.

Illustration B shows how a shielded cable can decrease the magnetic area of the loop when both ends are properly grounded.

Think In Small Loops

Maximization of radio and TV signal reception is best done by installing the largest antenna you can afford. Conversely, the best way to protect against magnetic noise pickup is to reduce the area of the receiver pickup loop, the antenna, as much as possible. The term 'receiver' in this context is any device acting as the receptor to undesirable signals. The loop area to which I refer is the total area enclosed by the circulating current in the receiving circuit. This is where determination of the interfering current's actual return path is very important.

A shield around a wire affords some protection against magnetic induction when the shield reduces the area of pickup. Figure 9 shows the effect of the shield on the loop area. Quite often signal currents return to the source by means other than that which was intended. Only when we know the actual path of the return current can we predict the shielding effectiveness of a system. To obtain the maximum protection from interfering low frequency noise the shield should not be one of the signal carrying conductors; and, if possible, one end of the circuit system should be isolated from ground. In most cases, due to our three wire electrical system, this is difficult to attain without considering use of a transformer (audio) or a common-mode choke (video).

Coax: Why It Works, Why It Doesn't

Wrapping a shield around a conductor and grounding at one end creates a shunt for the electric field around the conductor. However, the magnetic field around the conductor created by current flow in the conductor is unimpeded, meaning that the conductor can radiate a magnetic field. By grounding the coax shield at both ends, a current is induced in the shield by the magnetic field. This shield current flows in the opposite direction to the field created around the center conductor. Therefore, magnetic shielding is afforded by cancellation of the magnetic field currents. At frequencies around 1 MHz and above, the skin effect comes into play. Magnetic fields created by the signal inside the coaxial cable are cancelled internally wherein the field current travels on the interior of the shield only. External interfering fields create currents traveling on the outside of the coax shield only. So, at high frequencies, coaxial cable performs more like a double-shielded cable.

In the close-coupled presence of low frequencies (i.e. 60 Hz power line) the effectiveness of a shield around a conductor (as in a coaxial cable) is nil. This is why all low voltage signal cables should not be run parallel near AC power lines. More noise coupling rejection is attained by physical separation of the low voltage signal cables from the power lines since the induced energy falls off at a rate proportional to the square of the distance of separation.

Relative to capacitive pickup, a double-shielded coaxial cable (triax) having insulation between the two shields ensures that the signal-carrying shield does not have noise potentials induced within it. The outer shield carries ground currents and shunts them away. While noise rejection is enhanced with triaxial cable, it is more expensive and difficult to use, or terminate.

Twisted Pair: Is It Any Better?

Shielded twisted pair, STP, wire has characteristics very similar to triaxial cable for capacitive pickup and magnetic pickup rejection. It is certainly easier to work with as witnessed by its prolific use in professional audio cabling systems. This type cable provides excellent resistance to magnetic and electric field coupling at low frequencies. The desired signal currents flow in the two twisted conductors and any noise currents flow within the shield. Therefore, the common impedance type coupling is eliminated in this system [Note: It is possible to experience noise pickup on balance inputs using STP if the downstream component's differential input has improper internal grounding with respect to the cable shield connection. This is referred to by some as the 'pin 1 problem.'] Unshielded twisted pair, UTP, wire has very little capacitive pickup rejection capability unless its termination is very well balanced. Its rejection of magnetic pickup is excellent, however, because magnetic field currents travel within it in opposite directions and cancel.

But, realize that twisted pair cables perform best at frequencies below about 100 KHz with satisfactory performance possibly extending to several megahertz. In the megahertz range, high frequency losses in the cable increase rapidly, aided by non-uniformity characteristics of the cable construction. This explains, in part, the proliferation of coaxial cable from low megahertz frequencies on up through VHF.

Are You Magnetic Or Electric?

Troubleshooting system noise problems is challenging. Determining whether undesired signals result from capacitive pickup (electric field) or magnetic pickup can aid the troubleshooter toward a fast resolution of the problem. For identifying the point of noise ingress, reference 3 at the end of this article provides an excellent step-by-step approach utilizing simple fixtures and techniques. In addition, here is something you can do using a digital multi-meter or oscilloscope for low frequency interference.

Measure the noise voltage across the terminating impedance at one end of the cable while decreasing the impedance at the other end of the cable (i.e., loading it down with a parallel resistance). If the measured noise decreases, the pickup is from an electric field. This is because, for an electric field, a noise current is induced between the receiving conductor and ground. Lowering the impedance at the opposite end loads down this interfering current. But, if the measured noise increases, the pickup is due to a magnetic field. For magnetic field coupling, a noise voltage is produced in series with the receiver's conductor. Loading down the opposite end of the conductor lowers the impedance of the series loop and the magnetic induction is enhanced.

Strive For The Uncommon Mode

Hum, buzz, and ground loops should not engulf the bulk of your system integration time. First, consider the equivalent circuit of your system by identifying all possible routes for interfering ground currents. Remember, defeating the AC power third-wire ground connection in any system may solve a ground loop problem, but not only is this illegal, it could result in the electrocution of your customer. While there is no substitute for good design practice where grounding systems are concerned, both differential amplifier and common-mode choke solutions are worthwhile. In light of new common-mode choke technology you will want to consider what this tool can accomplish for you in troublesome noise situations with its simplicity of implementation. These new chokes can easily pick off that green worm, known as common-mode noise.

References:

1. Noise Reduction Techniques in Electronic Systems, Second Edition, Henry W. Ott, 1988, John Wiley & Sons, ISBN 0-471-85068-3.
2. Grounding of Industrial and Commercial Power Systems, IEEE Green Book, 1996, The Institute of Electrical and Electronics Engineers, ISBN 1-55937-141-2.
3. Audio Hardware Applications, Ted Uzzle, Larry Garter, Gene Patronis, March 2001, National Systems Contractors Association.