SRS Labs SR510 - Manual (Page 42)

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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.