Stanford Research Systems SR865A Operation Manual page 74

56 Basics
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 detector via a stray
capacitance. Although C
a weak experimental signal. This is especially troublesome if the coupled noise is
synchronous (at the reference frequency).
We can estimate the noise current caused by a stray capacitance by:
i C
where ω is 2π times the noise frequency, V
stray capacitance.
For example, if the noise source is a power circuit, then f = 60 Hz and V
C can be estimated using a parallel plate equivalent capacitor. If the capacitance is
stray
roughly an area of 1 cm
current will be 400 pA (at 60 Hz). This small noise current can be thousands of times
larger than the signal current. If the noise source is at a higher frequency, the coupled
noise will be even greater.
If the noise source is at the reference frequency, then the problem is much worse. 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) Removing or turning off the noise source.
2) Keeping the noise source far from the experiment (reducing C
3) Designing the experiment to measure voltages with low impedance (noise current
4) Installing capacitive shielding by placing both the experiment and detector in a
SR865A DSP Lock-in Amplifier
stray
d V
Z
C V
stray stray noise
dt
2
separated by 10 cm, then C
the signal cables close to the noise source.
generates very little voltage).
grounded metal box.
may be very small, the coupled noise may still be larger than
is the noise amplitude, and C
noise
noise
is 0.009 pF. The resulting noise
stray
). Do not bring
stray
Chapter 2
is the
stray
= 120 V.