It is a known fact of science that an electrical current flowing through a wire, or loop of wires, will generate a magnetic field, in itself nothing much. Now, if that current is disconnected, connected, and disconnected, etc, in succession, the magnetic field will collapse and grow - pulse - with the current. Basically, as the circuit switches off, the field collapses and the energy must go somewhere but since the circuit is open, there is no path of escape, so the energy is lost as EMR.
Now imagine the few million circuits, with their switching transistors, inside the expensive bits of your PC. When an AM or FM radio wave is generated, only a narrow band, or specific frequency, of EMR is used so that less energy is required, degradation is reduced, and reception is simpler. However, there are no such frequency limitations for our wire circuit, which generates a wide band of frequencies.
An experiment; try connecting one terminal of battery to the end of a metal file and the other terminal to a short length of wire. Now rub the wire along the file. You have just made a crude broadband radio transmitter.
With the advent of technology, Radio Frequency integrated circuits are incredibly sensitive and are often susceptible to interference from sources - high tension power lines, televisions, motorcycle magnetos, motor vehicle alternators and distributors (the bane of automotive-audiophiles) – including just about anything you need to plug into a household 240VAC outlet.
There is usually very little you can do to limit the interference. Other than maybe, building a Faraday cage around your computer, which also encompasses everything connected to it, running it on a separate power circuit to any of your audio equipment, and grounding the whole lot to a 1 metre length of copper pipe set in moist friable soil outside your window.
If you are using the PC power supply to power your circuit, the better switch mode power supplies are filtered to allow the AC in/DC out, but keep the RF contained within. Cheap power supplies often have RF suppression components left out to cut costs and in EU that's illegal. However if you are a victim of such a power supply, try adding a ferrite choke to power connection (as close to the device as possible). This will stop signals from getting in/out of the device by the power cable route.
EMI Noise Filter
- EMI Noise Filter.JPG (39.75 KiB) Viewed 4163 times