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Electrical noise can be annoying when it causes audible or visual interference in radios and televisions. If it interferes with a depth sounder, or a back-up camera on a motor coach, then human life may be at risk.
Noise categorization is complex, but falls into two major types, radiated and conducted. Radiated noise is propagated through the air, while conducted noise is present on power and signal lines.
The best rule is to prevent noise from escaping its source, but this isn't always easy to do, and in some cases, proper filtering would cost more than a manufacturer is willing to pay. In other cases, noise exists because no one has considered the problem. In such circumstances, noise might be prevented with easy and inexpensive methods.
High frequency noise is easier to filter than low frequency noise. Intermittent noise, such as that generated by motors, can sometimes be minimized by clamping power lines with diodes or tranient absorbers.
When you're sleuthing noise, it pays to know where noise can be generated. Here's a list of devices that can cause noise in some form.
To find a noise source, turn everything off, and then turn on one device at a time until the noise appears. Don't be surprised to find that the noise requires two or more devices to be powered up.
Finding the source of noise is hardest when the noise is intermittent. Is there a motor that periodically operates and generates the noise?
Once a noise source is identified, treatment can begin. As mentioned, clamping diodes may be a simple solution, but in complex situations, the noise will have to be measured on an oscillscope or spectrum analyzer before a targeted solution can be applied.
While filtering power to each sensitive device may work, it's always better to find the source of noise and prevent it from leaving. A filter on a noise source is always more effective than trying to filter downstream devices.