Question:
I have a mobile phone, which emits a signal periodically, for the service provider to pinpoint exactly where it is. If I'm near any working audio equipment when this occurs, I hear a series of buzzes through the loudspeakers. If I happen to be making a tape recording at the time, then the buzzes get recorded as well. If the signals can do this when data is being stored onto audio tape, what effects will they have on other data being stored magnetically - videos, bank and identity swipe cards, floppies, even hard disks?
......The radiation from mobile phones is far too weak to affect recorded
magnetic media. The buzzes and clicks that have troubled Mr Somes
must be due to signals from his phone getting into the electronics of
his audio gear and thus getting recorded along with the music!
Theory and practice have shown that magnetic recordings are
essentially unaffected even when tapes etc. are "illuminated" by
mega-watt radars. Magnetic core storage for computers survived for
many years in military applications, after it had been replaced by
semiconductors in commercial kit, because the magnetic cores
maintained their data states when exposed to the electro-magnetic
pulse and radiation experienced close to atomic war-head
explosions. Until recently, the BBC had an audio tape testing and re-furbishment
operation right under the masts of the short wave transmitters at
Daventry! No problems resulted from the high ambient RF
levels which could not be overcome by careful attention to
equipment shielding and earth loops etc.....
The pick-up of radio signals on to nearby bits of wire and other conductors occurs everywhere. It's a problem with hifi because of the small signals and large levels of amplification involved. The signal from a tape head or vinyl record pickup can be tens of microvolts, which is a comparable level to the pickup from a nearby mobile phone. Then it gets amplified by a factor of a million or so, if you like your music loud. So you hear the music, and the interference.
It's not quite the same in a digital system such as a PC. The conductors, PCB track, and wires can all pick up microvolts-worth of interference from all over the place (the biggest interference culprits usually being other boards in the same box, and other ICs on the same board), but to cause a problem, but the interference must be so bad as to cause a voltage that represents a 0 (or a 1) to go out of specification. As a logic 0 might be deemed to be anything between 0V and 0.75 V and a 1 might be deemed to be anything between 4.25V and 5V (dependent on the type of circuitry involved), the interference has to be a very high value to cause a problem. Once it is of a high enough value, it will cause data corruption.
This is why items that are built to CE or FCC standards have to demonstrate a certain level of immunity to to electromagnetic interference (EMI), as it's pretty much expected that there will be interference (both radiated and
conducted) everywhere that the equipment is required to operate.
Typical culprits include phones (mobile and portable), transmitters (TV, phone relay stations, radar sets), TV sets, switching power supplies, sparky old universal motors (as in electric drills, garden tools, vacuum cleaners and so on), light dimmers - the list is endless!
Typical power outputs could be:
noisy motor - some milliwatts
mobile phone - 3 watts
TV transmitter - 1,000,000 watts
I have noticed interference between parts within the PC - once, I found that
having the mouse leads too near to the CD-to-soundcard cable could cause a
buzzing through the speakers, but this is analogue circuitry.
I feel that the problem was worse in the 'old days', when you could buy a CB
radio, then illegally couple it up to a 300W transmitter, install the whole lot in your car and make a bloody nuisance of yourself everywhere you went!
It's a problem that will get worse the more radio emitters we build, for sure, but for many years the achievement of a happy co-existence between otherwise mutually-hostile pieces of equipment has been the everyday bread and butter for those who work in that business.
If you are worried about your magnetic media and disk drives, beware of
powerful permanent magnets as in speakers and lots of other stuff. I don't mix fridge magnets (used to hold memos and notes on the side of my filing cabinet) and floppies any more, for instance! I'd go on for longer, but it's sunny outside!
The other point about computers is that to comply with American FCC regulations they must be well shielded themselves to prevented unnecessary RF radiation. The shielding of course will work both ways preventing ingress as well as egress of the radio waves.
Last updated
February 16th 2004
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