Noise and Interference basics (Raymond Schouten) txtpage3
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              INTERFERENCE COUPLING & SOURCES   2



	
	Some power supplies use a switched mode (20-80kHz)
	conversion (using a transformer) giving radiation 
	up to 1MHz. (All PC's, some scope's & lock-in's)
	Even battery powered equipment can use this type of
	circuits, so called dc-dc converters. 


 	Digital equipment (DVM,Lock-in) and interfaces (cables)
	(GPIB, RS232) use clock signals with high frequencies
	and/or fast edges ( 1MHz or: 1Hz 10ns pulses). 

	
	Noncontinuous datatransport (PC-GPIB-DVM) can give 
	bursts coupling in the (DVM) measurementleads disturbing 
	even high impedance dc measurements. 


	All equipment using a monitor screen is emitting
	frequencies of typical 16/32kHz and harmonics.
	(analog scope: less and timebase dependant)

	
 	Measurement equipment using a sampling approach
	(DVM, hand-held multimeter) induces transients into
	the measuring leads. 	


	Measurement wires in a static magnetic fields 
	create a microphone.


	Air flow across contacts creates low frequency "noise"
	in the metal-metal junctions forming thermocouples.