Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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2-72 External Noise Sources
SR785 Dynamic Signal Analyzer
current. If the noise source is at a higher frequency, the coupled noise will be even
greater.
If the noise source is at the signal frequency, then the problem is much worse. Vector
averaging rejects noise at other frequencies, but pick-up at the signal 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 signal source (reducing C
stray
). Do not
bring the signal cables close to the noise source.
3) Designing the experiment to measure voltages with low impedance (noise
current generates very little voltage).
4) Installing capacitive shielding by placing the signal source in a metal box.
Inductive Coupling
An AC current in a nearby piece of apparatus can couple to the signal path via a
magnetic field. A changing current in a nearby circuit gives rise to a changing magnetic
field which induces an emf (d
Φ
B
/dt) in the loop connecting the signal source to the
analyzer. This is like a transformer with the analyzer-source loop as the secondary
winding.
Figure Chapter 2 -4 Inductive Coupling
Cures for inductively coupled noise include:
1) Removing or turning off the interfering noise source.
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 connections.
3) Using magnetic shielding to prevent the magnetic field from crossing the area
of the signal path.
4) Measuring currents, not voltages, from high impedance sources.
Signal
Source Noise
Source
Analyzer
B(t)