User's Manual

Resistive Coupling (or 'Ground Loops').

Currents through common connections can give

rise to noise voltages.

Resistive Coupling

Here, the detector is measuring the voltage

across the experiment, plus the voltage due to

the noise current passing through the finite

resistance of the ground bus. This problem

arises because we have used two different

grounding points which are not at exactly the

same potential. Some cures for ground loop

problems include:

1) grounding everything to the same physical

point,

2) using a heavier ground bus to reduce the

potential drop along the ground bus,

3) removing sources of large currents from

ground wires used for small signals.

Microphonics provides a path for mechanical noise

to appear as electrical noise in a circuit or

experiment. Consider the simple circuit below:

The capacitance of a coaxial cable is a function of its

geometry so mechanical vibrations will cause the

cable capacitance to vary with time. Since C=Q/V,

we have

C dV + V dC

= dQ = i

dt dt dt

so mechanical vibrations will cause a dC/dt which in

turn gives rise to a current i, which will affect the

detector. Ways to eliminate microphonic signals

include:

1) eliminate mechanical vibrations,

2) tie down experimental cables so they will not

sway to and fro,

3) use a low noise cable that is designed to reduce

microphonic effects.

Thermocouple Effect. The emf created by dissimilar

metal junctions can give rise to many microvolts of

dc potential, and can be a source of ac noise if the

temperature of the junction is not held constant.

This effect is large on the scale of many low level

measurements.