HP 34970A Shielding Techniques, Separation of High-Leveland Low-LevelSignals

Chapter 8 Tutorial

System Cabling and Connections

Shielding Techniques

Shielding against noise must address both capacitive (electrical) and inductive (magnetic) coupling. The addition of a grounded shield around the conductor is highly effective against capacitive coupling. In switching networks, this shielding often takes the form of coaxial cables and connectors. For frequencies above 100 MHz, double-shielded coaxial cable is recommended to maximize shielding effectiveness.

Reducing loop area is the most effective method to shield against magnetic coupling. Below a few hundred kilohertz, twisted pairs may be used against magnetic coupling. Use shielded twisted pair for immunity from magnetic and capacitive pickup. For maximum protection below

1 MHz, make sure that the shield is not one of the signal conductors.

	Recommended Low-Frequency Cable:			Recommended High-Frequency Cable:
	Shielded twisted pair			Double-shielded coaxial cable
LO			HI
			LO
HI
			Center Conductor
	Twisted Pair	Shield		Shield Foil Shield Braid PVC Jacket

Separation of High-Level and Low-Level Signals

Signals whose levels exceed a 20-to-1 ratio should be physically separated as much as possible. The entire signal path should be examined including cabling and adjacent connections. All unused lines should be grounded (or tied to LO) and placed between sensitive signal paths. When making your wiring connections to the screw terminals on the module, be sure to wire like functions on adjacent channels.

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