Stanford Research Systems SR844, User Manual

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SR844 Basics 2-27
SR844 RF Lock-In Amplifier
Cures for capacitive coupling include:
•
Removing or turning off the noise source.
•
Keep the noise source far from the experiment (reducing C
STRAY
). Do not bring signal
cables close to the noise source or reference cables.
•
Designing the experiment with low-impedance detectors (so that noise current generates
small voltages).
•
Installing capacitive shielding by placing both the experiment and detector in a metal
box.
Inductive coupling
An AC current in a nearby piece of apparatus can couple to the experiment 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 detector to the experiment. This is
like a transformer with the experiment–detector loop as the secondary winding.
Cures for inductively coupled noise include:
•
Removing or turning off the interfering noise source.
•
Reducing the area of the pick-up loop by using twisted pairs or coaxial cables.
•
Using magnetic shielding to prevent the magnetic field from crossing the area of the
experiment.
Resistive coupling or Ground Loops
Currents flowing through ground connections can give rise to noise voltages. This is
especially a problem with reference frequency ground currents.
Signal
Source
Noise
Source
Detector
Signal
Source Noise
Source
Detector
B(t)