Stanford Research Systems SR785, Operating Manual And Programming Reference

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External Noise Sources 2-71
SR785 Dynamic Signal Analyzer
External Noise Sources
In addition to the intrinsic noise sources discussed previously, there are a variety of
external noise sources within the laboratory.
Most of these noise sources are asynchronous, i.e. they are not related to the signal and
do not occur at the signal frequency or its harmonics. Examples include lighting fixtures,
motors, cooling units, radios, computer screens, etc. If these noise sources are large, they
can determine the input range and hence the noise floor. They can however be removed
from the FFT spectrum by using triggering and vector averaging. Since the noise signals
are not phase coherent with the trigger and signals of interest, they vector average to
zero.
Some noise sources, however, are related to the signal source and, if picked up in the
signal, will add or subtract from the actual signal and cause errors in the measurement. A
typical source of synchronous noise is a ground loop between the signal source and the
analyzer.
Many of these noise sources can be minimized with good laboratory practice and
experiment design. There are several ways in which noise sources are coupled into the
signal path.
Capacitive Coupling
An AC voltage from a nearby piece of apparatus can couple to a signal path via a stray
capacitance. Although C
stray
may be very small, the coupled noise may still be larger than
a weak signal. This is especially damaging if the coupled noise is synchronous with the
signal.
Figure Chapter 2 -3 Capacitive Coupling
We can estimate the noise current caused by a stray capacitance by,
i = C
stray
dV/dt =
ω
C
stray
V
noise
where
ω
is 2
π
times the noise frequency, V
noise
is the noise amplitude, and C
stray
is the
stray capacitance.
For example, if the noise source is a power circuit, then f = 60 Hz and V
noise
= 120 V.
C
stray
can be estimated using a parallel plate equivalent capacitor. If the capacitance is
roughly an area of 1 cm
2
separated by 10 cm, then C
stray
is 0.009 pF. The resulting noise
current will be 400 pA (at 60 Hz). This small noise current can be larger than the signal
Signal
Source
Noise
Source
Stray Capacitance
Analyzer