Stanford Research Systems SR865A, Operation Manual

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58 Basics Chapter 2
SR865A DSP Lock-in Amplifier
2) Using a heavy ground bus to reduce the resistance of ground connections.
3) Removing sources of large ground currents from the ground bus used for small
signals.
4) Using differential excitation and signal sensing.
Microphonics
Not all sources of noise are electrical in origin. Mechanical noise can be translated into
electrical noise by microphonic effects. Physical changes in the experiment or cables (due
to vibrations for example) can result in electrical noise over the entire frequency range of
the lock-in.
For example, consider a coaxial cable connecting a detector to a lock-in. The capacitance
of the cable is a function of its geometry. Mechanical vibrations in the cable translate into
a capacitance that varies in time, typically at the vibration frequency. Since the cable is
governed by Q=CV, taking the derivative, we have
i
dt
d
dt
d
dt
d Q C
V
V
C
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 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 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. This second
junction should be held at the same temperature as the first junction.
Noise Measurements
Lock-in amplifiers can be used to measure noise. Noise measurements are generally used
to characterize components and detectors.