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Capacitive Coupling. A voltage on a nearby
piece of apparatus (or operator) can couple to a
detector via a stray capacitance. Although
C
stray
may be very small, the coupled in noise
may still be larger than a weak experimental
signal.
Capacitive Noise Coupling
To estimate the noise current through C
stray
into
the detector we have
i=C
stray
dV = jwC
stray
V
noise
dt
where a reasonable approximation to C
stray
can
be made by treating it as parallel plate capacitor.
Here, w is the radian frequency of the noise
source (perhaps 2 * π * 60Hz ), V
noise
is the
noise voltage source amplitude (perhaps 120
VAC). For an area of A = (.01 m)2 and a
distance of d = 0.1m, the 'capacitor' will have a
value of 0.009 pF and the resulting noise current
will be 400pA. This meager current is about
4000 times larger than the most sensitive
current scale that is available on the SR510
lock-in.
Cures for capacitive coupling of noise signals
include:
1) removing or turning off the interfering noise
source,
2) measuring voltages with low impedance
sources and measuring currents with high
impedance sources to reduce the effect of i
stray
,
3) installing capacitive shielding by placing both
the experiment and the detector in a metal box.
Inductive Coupling. Here noise couples to the
experiment via a magnetic field:
Inductive Noise Coupling
A changing current in a nearby circuit gives rise to a
changing magnetic field which induces an emf in the
loop connecting the detector to the experiment.
(emf = dØ
B
/dt.) This is like a transformer, with the
experiment-detector loop as the secondary winding.)
Cures for inductively coupled noise include:
1) removing or turning off the interfering noise
source (difficult to do if the noise is a broadcast
station),
2) reduce the area of the pick-up loop by using
twisted pairs or coaxial cables, or even twisting the 2
coaxial cables used in differential hook-ups,
3) using magnetic shielding to prevent the magnetic
field from inducing an emf (at high frequencies a
simple metal enclosure is adequate),
4) measuring currents, not voltages, from high
impedance experiments.