Nipping Noise in the Bud Jon Titus, Contributing Writer
-- Design News, January 1, 2010
When good shielding techniques cannot keep noise from affecting measurements,
&nbsbsp;The following tips can help:
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Use a spectrum analyzer to help identify noise frequencies and amplitudes. Look for harmonics of noise signals and line-power frequencies.This information can help you identify noise sources.
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Separate digital and analog grounds. Use a single-point ground or cluster ground connections as close together as possible to eliminate ground loops.Do not cut off the earth ground (green wire) on a power plug or use a three-wire to two-wire pe or corroded ground connections cause problems and increase the noise on measurement signals.Do not mix high-power conductors and low-level signal conductors in the same cable or wire bundle. Separate measurement and control signals as much as possible.Check screw-terminal connections to ensure tight contact. Use high-quality cable and connectors.
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Keep connectors clean. 3M's Novec contact cleaner removes grease, silicones and dust, and the 407C audio/video head cleaner from MG Chemicals remove oil and dirt. Neither leaves a residue.
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Switch-mode power supplies, motor controllers, electric welders and power-switching equipment can radiate noise. Discharge-lighting equipment and fluorescent-light ballasts can create radiated and conducted noise.
You might need to replace old ballasts or install an in-line filter such as the Leviton XPF.Use power-line EMI filters to attenuate noise conducted in and out of circuits. Some filters include a choke on the ground line to prevent it from conducting noise, too.I know of a portable-phone battery charger that caused considerable RFI over a wide area due to a faulty component and a poor design. -
Look for coupling between measurement signals and electromagnetic devices s s such as motors, relay coils and SMPS transformers that have time-varying magnetic fields. Reorient signal leads perpendicular to magnetic fields or move wires away from these fields.
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The electrolyte in aluminum capacitors used in power supplies can dry out and allow noise to pass through to sensitive measurement circuits. If you see line-frequency or SMPS switching noise,look for bad capacitors. It might cost les
Do not remove an earth-ground connection or use a "ground" adapter to break a ground loop. Lack of this ground creates a safety hazard.
