Unwanted radiation.

What is it?
Many electronic circuits can generate radio frequency signals. These signals can then be radiated by wires or PCB tracks within the device or leads to other parts of the system.
Examples of electronic devices that can cause interference include computers, power supplies, and televisions.

What can suffer from interference from these signals?
The levels of these signals are usually so low that only receivers, such as radios and televisions, will be affected by them.

Who's fault is it?
This is usually caused by poor design and filtering within the device emitting these signals.

What causes it?
The cause may be poor filtering or just poor design, causing internal signals to be radiated from wires, leads, or PCB tracks. Sometimes, these signals can also be sent back down the mains supply lead and so to many other electrical or electronic items in the area.

Digital signals:
These are used within computers and many control systems. These signals contain harmonics, which can be radiated from wires, leads and PCB tracks.

Timing signals:
These are generated within many electronic systems and are square waves - so contain harmonics.

Scan output:
Used in televisions and computer monitors, these generate the height and width of the picture. The signals are a sawtooth waveform, with many harmonics. The scan coils on the back of the tube, plus the leads to them, can radiate these signals.

What should I do about it?
All electronic equipment should be well designed and filtered to minimise the chances of such interference. However, the problem can still exist, and some equipment has been found to be poorly filtered.
Finding the rogue circuit and adding filtering is not an easy job so, if the few basic ideas here do not work, you may be best getting the help of a qualified engineer.

* Metal cases on computers, etc. should be in place and bolted down with all screws in place.
* Keep all leads as short as possible.
* You may be able to get clip on ferrite filters for leads. These are easy to fit, as you just open them up, put the lead in, then close them.
* Try to locate computer equipment away from any equipment which may be affected.