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Cures for inductively coupled noise include:
1) Remove or turn off the interfering noise source.
2) Reduce the area of the pick-up loop by using twisted pairs or coaxial cables.
3) Use magnetic shielding to prevent magnetic field from crossing the area of measurement.
Resistive Coupling or Ground Loops
The ground loop is an interference source which can generate noise voltage between the grounds. If the noise voltage is
large enough, it will cause measurement errors. Ground loop is a physic loop, which generates from many ground methods.
These grounding methods can act as a big loop wire. They pick up noises from the environment and generate voltages in
the grounding system. The 50 Hz magnetic field of the AC power is a normal noise source that the ground loop always
pickup. For distributed grounding systems, the ground voltage can cause the ground current flow in the ground loop. Since
the ground is with low impedance, noise current is always very high.
Cures for ground loop problems include:
1) Connect all grounds to the same physical point.
2) Use a heavy ground bus to reduce the resistance of ground connections.
3) Remove sources of large ground currents from the ground bus.
Microphonics
Not all noise source are electrical in origin. According to microphonic effects, mechanical noise can be transformed into
electrical noise. Physical changes in the device or cables (due to vibrations for example) will cause electrical noise over
the whole bandwidth of the lock-in.
Solutions to minimize microphonic signals:
1) Eliminate mechanical vibrations near the experiment.
2) Tie down cables carrying sensitive signals.
3) Use a low noise cable that is designed to reduce microphonic effects.
Thermocouple Effects
When two dissimilar metals contact, there will be potential difference between them. The reason for potential difference
includes: (1) different electronic work function of two metals. (2) different electron concentration of two metals.
Suppose that metal A and metal B have work functions Va and Vb respectively. The electromotive force (emf) between A
and B is:
)
ln(
b
a
b
a
ab
N
N
q
kT
V
V
V
Where K=1.38
×
10
-23
J/K is Boltzmann's constant. T is the temperature in
°
Kelvin;q is the elementary charge of 1.60
×
10
-19
