Stanford Research Systems SR785, Operating Manual And Programming Reference

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2-74 External Noise Sources
SR785 Dynamic Signal Analyzer
1) Eliminate mechanical vibrations near the signal path.
2) Tie down cables carrying sensitive signals so they do not move.
3) Use a low noise cable that is 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 generally changes slowly. This effect is large on the scale of many
signals (10’s of
µ
V) 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 signal source 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. This
second junction should be held at the same temperature as the first junction.