Thermocouple Effects; Baluns - Stanford Research Systems SR570 User Manual

Mechanical vibrations in the cable which cause a
dC/dt will give rise to a current in the cable.
This current affects the detector and the
measured signal.
Some ways to minimize microphonic signals
are:
1) Eliminate mechanical vibrations near the
experiment.
2) Tie down cables carrying sensitive
signals so they cannot move.
3) Use a low noise cable designed to
reduce microphonic effects.

Thermocouple effects

The emf created by junctions between dissimilar
metals can give rise to many microvolts of
slowly varying potentials. This source of noise is
typically at very low frequency since the
temperature of the detector and experiment
generally changes slowly. This effect is large on
the scale of many detector outputs and can be a
problem for low frequency measurements,
especially in the mHz range. Some ways to
minimize thermocouple effects are:
1) Hold the temperature of the experiment
or detector constant.
2) Use a compensation junction, i.e. a
second junction in reverse polarity
which generates an emf to cancel the
thermal potential of the first junction
(both held at the same temperature).
A few words about Baluns
To reduce the effects of ground loops, the SR570
has a balun (BALanced/UNbalanced common-mode
choke) connected to the output stage. This may be
thought of as two wires wrapped about a magnetic
core, forming a pair of inductors. One wire carries
the output signal and the other the return, forming
essentially a differential signal which can pass
through the balun with little to no attentuation over
the SR570's bandwidth.
Non-differential ("common-mode") signals, like
ground-loop pickup, however, are effectively
blocked by the balun. The degree of rejection varies
with frequency and is determined by the choke
inductance and the resistance of the windings.
The presence of the balun effectively breaks the
ground loop which would have occurred if the
SR570 and the device connected to the output were
both connected to ground. However, this also means
that the output is "differential" and cannot be used as
a ground reference when attempting to trace noise in
the SR570 circuitry. Measurements of this type
should be referenced to the input ground or the
SR570 chassis.
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