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19
1.11
External Noise Sources
In addition to the intrinsic noise sources discussed previously, there are different kinds of external noise sources. Most of
these noise sources are asynchronous and not related to the reference. They do not occur at the reference frequency or its
harmonics. These noise sources affect the measurement mainly by increasing the requirement of dynamic reserve or
lengthening the time constant. However, some external noise sources are related to the reference. If they are picked up in
the signal, noise will add or subtract from the actual signal and cause errors in the measurement. Fortunately, external
noise sources can be reduced through various ways.
Capacitive Coupling
The mutual capacity between wires is often called stray capacity C
stray
. An AC voltage from a nearby piece of apparatus
can couple to a detector via C
stray
. Although C
stray
may be very small, the coupled noise may still be larger than a weak
experimental signal and cause severe instability for the detector. The noise current is given by:
noise
stray
V
C
I
Where
is 2
π
times the noise frequency, V
noise
is the amplitude of noise, and C
stray
is the stray capacitance.
When the noise sources' frequency become larger, the coupling noise will be larger. If the noise source is at the reference
frequency, the noise will be quite large. The lock-in rejects noise at other frequencies, but pick-up at the reference
frequency appears as signal.
Cures for capacitive noise coupling include:
1) Remove or turn off the noise source.
2) Design the experiment to measure voltages with low impedance for most of low frequency noise sources.
3) Install capacitive shielding by placing the experiment and detector in one metal box.
Inductive Coupling
An AC current in a nearby piece of apparatus can couple to the experiment via a magnetic field. A changing AC current
gives rise to a changing magnetic field which induces voltage. The larger the frequency, the larger the electromotive force,
the greater the measurement error.
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.
