Most American homes are equipped with microwave ovens. These devices use a magnetron tube to produce several hundred watts of microwave power on a frequency of approximately 2450 MHz. The high voltage applied to the "maggie" is typically pulsating dc [i.e., it is rectified ac, but not rippie-filtered (as are true dc power supplies)). This pulsating dc causes "Tiash" in radio receivers. Although better-quality microwave ovens are equipped with EMÍ filters, many are not. However, most manufacturers or servicers of microwave ovens can install EMI filters inside the oven. Alternatively, one of the EMI filters shown in Fig. 14-3 can be used
The proliferation of personal computers has greatly increased the amount of noise in the radio spectrum. The noise is caused by the internal circuits that operate using digital clock pulses. Older machines, which use internal clock frequencies of 1 to 4.77 MHz (original IBM PC), wipe out large portions of the AM and shortwave bands. If you doubt this, try using an AM radio near the computeri Later computers (XT-turbo and AT-elass machines) used higher clock frequencies (e,grj 8,10, 12,16, 25, or 33 MHz) and these can wipe out the VHF hands as well—including the FM broadcast band IVe noticed that the XT-turbo machine that 1 use for wordprocess-ing my technical articles and books wipes out the AM band in the regular mode and the FM band in the turbo mode—but the lurbo mode produces little interference on AM (a result of the higher clock frequency, I suspect). ^
Most of the noise produced by the computer is radiated from the power line or from the keyboard cable. In the latter case, be sure that a shielded keyboard cable is used. The power line noise is often susceptible to the same kinds of EMI filters.
Another source of noise is the printer (or more commonly, the printer cable). If the cable between the computer and the printer is unshielded then replace it with a
292 What's that mess coming from my we iter?
shielded type. Otherwise, you might want to consider a ferrite clamp-on filter lw. such as the Amidon 2X-43 or equivalent.
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