Frequency range s of digital noise s

I'll second Kijanki's response, and add some experimental evidence:

I have a portable battery powered transistor radio which can tune AM signals from 150 KHz (long wave) to 18 MHz (short wave), plus the FM band from 88 to 108 MHz. If I tune it in between stations, so that its AGC goes to max gain, and bring it within about two feet of an i7-950 based computer I built, at moderate settings of the volume control loud noise is induced in the radio throughout that entire 150 KHz to 18 MHz range. There is perceivable but lesser noise on FM, although some or most of that, rather than being at 88 to 108 MHz, MIGHT be lower frequencies that are picked up in the radio at circuit points at or downstream of where the signal is downconverted to FM's 10.7 MHz intermediate frequency.

I should add that the two foot distance applies to the unshielded left side of the computer, nearly all of which is an acrylic window. But the noise can also be picked up on the shielded sides, although I have to bring the radio significantly closer on those sides.

Regards,
-- Al

Thanks Kijanki. Just how high is high frequency? Sound Application is claiming usefulness bandwidth to over 2 gigahertz (about 2.5) and I would like to relate that to noise generated by digital products, power supplies and transformers.

Pss, personal computer clocks might be in gigahertz range but computers also contain switching power supplies that operate in hundreds of kilohertz range and buses operating in hundreds of megahertz. In addition when signal is not continuous, but changes pattern (like data on computer bus) additional frequencies, called sidebands, are produced (since it is modulation). All these frequencies interact with each other creating even more frequencies - ending up in one big noise that Al, picked on his radio. I'm not sure what is Sound Application, but as I said before high frequency digital noise can affect audio band many different ways.

Thanks Kijanki. Just how high is high frequency? Sound Application is claiming usefulness bandwidth to over 2 gigahertz (about 2.5) and I would like to relate that to noise generated by digital products, power supplies and transformers.

Emitted is the key word here. Emissions are the same as radiated emissions. Most products are limited by Class B to less than 40bDuV/m, at least those that pass Class B testing. This is low enough so you will not have interference between components. Interference of clocks and SMPS supplies inside a component is certainly possible, but usually affecting only jitter in digital systems, not causing errors. Jitter is affected because signal noise levels are increased. Generally in good designs, the problem is more power supply electrical noise, magnetic fields and ground return paths that are more problematic, not emissions.

The thing to understand is that usually the only components that are even sensitive to radiated emissions are tuners, and these can pick up uV of EM radiation.

In typical audio systems, the component-component problem is ground-loops picking up EMI. Eliminate the ground loops and the problem is gone. If the ground-loop is retuned by making it larger diameter or smaller and maybe by twisting the power cords together to minimize the loop area, this can be a fix. Its usually when the ground loop is a size that acts like an antenna that serious problems occur. The area tunes a specific frequency and its harmonics.

Steve N.
Empirical Audio