app1995| Application Note

Application Note 1995
Using Spread-Spectrum Oscillators to Reduce Radiated Emissions in Consumer Products www.maxim-ic.com

OVERVIEW
In 1975 the Federal Communications Commission (FCC), the government agency that regulates radio frequency (RF) emissions in the United States, enacted new regulations called FCC Part 15. These were not directed at controlling equipment such as radio and TV transmitters, or aircraft-navigation and emergency beacons that deliberately radiate high-power RF energy. Instead, these regulations sought to control equipment that did not deliberately radiate RF energy such as televisions, automobiles, and low-power, unregulated RF radiators such as walkie-talkies and electronic remote controls. A good example of the motivation behind these regulations occurred years ago at Dallas/Fort Worth Airport. Pilots, during takeoffs and landings, reported loss-of-control situations with recently developed electronic flight-control systems. The FCC traced the cause to interference from remote-controlled garage-door openers in homes in the surrounding suburban areas. The Part 15 regulations addressed this problem by requiring tha
t all electronic equipment sold in the U.S. be tested and certified to ensure against radiated RF energy that could cause malfunctions in other electronic devices. During the 1980s and 1990s, electronic devices from microwave ovens to cell phones proliferated. Cross interference between these devices became a problem. Microwave ovens interfered with pacemakers, while cable modems interfered with cordless phones. Similarly, computer monitors radiated enough RF energy that interfered with most other electronic equipment in their vicinity. The FCC and other regulatory bodies such as the Electro Magnetic Compatability (EMC) agency in Europe, responded with tighter regulations regarding these emissions in all electronics. In the U.S., FCC Part 68 regulated industrial and commercial electronic devices. Part 68, Class A involved equipment used in an industrial environment while Part 68, Class B addressed consumer products. This article will concentrate on Class B electronics only.

Reducing Radiated Emissions
Radiated emissions occur in any piece of electronics that has a changing electrical signal radiates emissions. In the case of the PC motherboard with a clock running at a certain frequency, it will radiate RF energy tightly centered on the frequency of the master oscillator. Definitions: Radiated susceptibility is the sensitivity of a piece of electronic equipment to interference from outside RF energy sources. Radiated emissions are RF energies unintentionally radiated by a piece of electrical equipment, such as a PC or monitor, which might have some impact on another piece of electrical equipment. If the radiated RF energy from a PC motherboard is measured, radiation spikes at the oscillator's center frequency are present (Figure 1). FCC requirements for consumer products (FCC Part 68, Class B) require that radiated emissions must be below a specified maximum value at any given frequency.

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