Audio Video Isolation Transformer Noise Filters


As electronic equipment gets more and more complicated, the requirements for a clean signal become more and more strict as well. Higher frequencies and lower signal levels are much more susceptible to noise than lower frequencies and higher signal levels. However, each generation of equipment and each new data transfer protocol which is developed includes higher data transfer rates and lower signal voltage levels.

This has made the elimination of spurious noise on transmission lines a critical part of any system design, from the installation of a Hi-definition television, to the installation of a major computer system.

This noise comes from a variety of sources which we find all around us. Some common sources include switch mode power supplies used in a wide variety of electronic equipment, electronic lighting ballasts for fluorescent lights, photocopiers and laser printers. Each of these devices creates spurious electronic “spikes” in electrical line voltage, which can become part of any electronic signal, from audio signals to critical computer data transmission.

There are a number of strategies used to eliminate this electrical noise, including the twisted-pair wires that are common in many types of data-transmission cables. However, tracking down and eliminating electrical noise is no easy task. There is no easy way to measure it and no definite formula for eliminating it

One common method of eliminating high frequency noise is through the use of isolation transformers. These “isolate” the incoming signal from the signal used in the piece of equipment, allowing the noise to be grounded out and eliminated.

How Transformers Work

A transformer is a combination of two electrical coils, wrapped together, usually around a ferrite core. It works under the basic principle that electricity flowing through a wire produces a magnetic field. When a second wire is placed in that magnetic field, the magnetic field creates an equal electrical current in that wire. The wire with the electricity flowing through it is called the primary and the wire which picks up the current from the magnetic field is called the secondary.

The efficiency of this principle can be increased by making the wires into coils, so that the wires take up less space. Voltages can be changed in a proportionate sense by the number of coils that are in the primary and the secondary. If the primary has 100 wraps in the coil and the secondary has only 10, the voltage produced in the secondary is 1/10 of that which is in the primary.

In this basic transformer design, the entire signal is transferred from the primary to the secondary, to include any noise spikes that might have been picked up by the wires leading to the primary. So, a transformer in and of itself has no capability of removing electrical noise.

Even so, one thing that transformers do, quite effectively, is to isolate two circuits from each other. The only connection between the two is a magnetic field; and while that magnetic field may transfer “data” back and forth, it doesn’t transfer any actually electricity between the primary and secondary, enabling it to isolate the secondary circuit, while still passing the signal.

How Isolation Transformers Work

Most transformers are used for voltage stepping, to raise or lower the secondary voltage as compared to the primary voltage. Isolation transformers differ, in that their primary purpose isn’t voltage stepping, but rather isolation. While they may step the voltage up or down, that isn’t their main usage.

The design and construction of isolation transformers differs from that of stepping transformers in one primary way; the inclusion of faraday shields between the primary and secondary. There are two of these shields, a primary one and a secondary one. The primary shield is normally grounded to the ground on the incoming current to the primary, while the secondary shield is normally grounded to the device or system’s ground.

By grounding these faraday shields, the transformer is able to ground out the spurious noise signals, while allowing the signal to travel through the transformer, on to the device’s circuitry. Essentially, the isolation transformer acts as three capacitors, one for the input one for the output and a common one to ground. This “filters” some of the noise to ground.

Transformer Isolation and Ground

Due to the circuit isolation that any transformer provides, it is possible to derive a new ground on the secondary side of the transformer. Typically, electric power companies connect neutral to ground at some point, meaning that ground really isn’t ground. On the secondary side of the transformer, it is possible to connect ground to earth ground, ensuring a good ground for the elimination of the noise “captured” by the faraday shield.

If the transformer is designed to do so, the ground can also be established at some other voltage than zero. This provides the possibility of establishing a ground at a higher voltage, referred to as “lifting the ground,” so that the noise is “swallowed up” into the ground voltage. The difference in ground potential between the primary and secondary in these cases makes it harder for spurious noise to transition from one side of the transformer to the other. This is most effective when the noise is contained in a more electronegitively charged primary circuit, and the secondary is more electropositively charged.

Let’s go back to the power companies tying the neutral to ground for a moment. While this typically reads as zero volts, there is the potential that the neutral leg could have a high voltage potential, even as high as 20 KV. While the potential between the neutral leg and the ground leg would be zero, the potential between the neutral leg or ground and true earth ground could be that full 20 KV.

Isolation Transformers Specifications

Isolation transformers are extremely effective for reducing high frequency common mode noise. However, they have very little effect on low frequency or differential noise. Since isolation transformers are commonly used for noise reduction, it is normal to rate them by their attenuation. This is the amount of noise that they are capable of removing.

Attenuation is always rated in dB (decibels). This can be confusing, because the amount of noise eliminated depends largely upon the amount of noise that exists. When reading this attenuation, keep in mind that each 20dB of attenuation equals a reduction in noise voltage by a factor of 10, making it 1/10 of what it was. Therefore, to get a 1000% reduction in noise requires a 60dB isolation transformer.

Isolation transformers are not as effective in dealing with transients, such as voltage spikes. If the transients are in differential mode (on the neutral leg) they will pass directly through the transformer with no attenuation.

The maximum voltage rating for an isolation transformer is the peak voltage of a spike that the transformer can handle. Voltage spikes or fluctuations higher than this can damage or destroy the isolation transformer, essentially turning it into a fuse. In this case, the transformer will most likely protect any downstream equipment, but at the cost of its own life.

While large isolation transformers can be installed on the power supply for a whole building, there are isolation transformers available for use with many common types of electronic equipment. These are sized and contain the correct connections for use in interconnecting different types of equipment. In no way should these be used for other types of transmission lines by the installation of adapters on those lines. In such cases, the transformer may impede the signal from passing correctly from one device to another; leaving an unusable signal.