Noise and Interference basics (Raymond Schouten) txtpage2
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              INTERFERENCE COUPLING & SOURCES   1


	Most equipment uses a mains power supply,
	this can give cause to several problems:


	1-Ground loops (50,100,150Hz etc)
          Displacement currents between several (unequal potential)
	  line-power grounds. Becomes a problem when these currents
	  run via signal lines (induces voltage).
          Use differential inputs, avoid coax cables
          (use cables with seperate signal,zero,shield)
	  or route the coax in a copper braiding (two sides connected)
	  to lower the induced voltage.
  
	2-Magnetic loop coupling (50,150Hz,250Hz etc)
	  Magnetic flux inducing a voltage in a signal wire-loop.
          Power transformers in equipment produce a lf stray magnetic
	  field, not blocked by a standard metal housing (only blocks
	  electric field).
	  Minimize (or rotate) loop area, move power-transformers

	3-Electric stray field (50Hz) 
	  Most powerlines are unshielded and capacitive couple e-field
 	  to the environment.
	  This also couples to you, so when you touch a high impedance 
	  device you will induce volts, EVEN WHEN "GROUNDED" VIA
	  A COMMERCIAL "GROUND-STRAP" on your arm because this has
          a build-in 1Mohm series-resistor (it is only good for 
	  ESD, ElectroStatic Discharge)
	  Shield wires, equipment, sample, 
	  reroute mains cables

	4-Coupling of mains interference (>1kHz) by
	  coupling capacitance of supply transformers
	  (and internal mains wiring,switches,lamps)
	  in (commercial) equipment.

	5-Injection of mains interference (>1kHz) in
	  ground wiring by implementation of mains 
	  interference filters in (commercial) equipment.

	6-Switched-mode line-power supplies (PC's, commercial 
	  measurement equipment) couple and radiate 
	  20..80kHz spikes.(see next page)