SRS Labs SR560 - Manual (Page 31)

APPENDIX B
B-3
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) Ground everything to the same physical
point.
2) Use a heavier ground bus to reduce the
potential drop along the ground bus.
3) Remove sources of large currents from
ground wires used for small signals.
Microphonics
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.