小常識

Nipping Noise in the Bud Jon Titus, Contributing Writer -- Design News, January 1, 2010

When good shielding techniques cannot keep noise from affecting measurements, the following tips can help:

  • 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.

  • 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 power-outlet adapter to break a ground connection (see photo, below). If you cannot eliminate a ground loop, an isolation amplifier on your signal lines can help.

  • Suspect all connectors — even those from high-quality suppliers. Broken, high-resistance 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.

  • 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.

  • 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 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.

  • 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 less to replace a bad supply than its bad capacitors.

  • Look for slots in chassis and enclosures that can radiate RFI signals. If possible, decrease the slot width with conductive tape that connects to the conductive chassis.

  • Use wide-band ferrite cores, or ferrite noise suppressors, on signal cables to attenuate conducted RFI signals.

                                                Nipping Noise in the Bud
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.