SRS Labs SR530, Lock-In Amplifier manual Capacitive Noise Coupling

Capacitive Coupling. A voltage on a nearby piece of apparatus (or operator) can couple to a detector via a stray capacitance. Although Cstray 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 Cstray into the detector we have

i= Cstray dV = jwCstrayVnoise dt

where a reasonable approximation to Cstray can be made by treating it as parallel plate capacitor. Here, w is the radian frequency of the noise source (perhaps 2 * π * 60Hz ), Vnoise 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 istray,

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.

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