SRS Labs SR560 manual Resistive Coupling or ‘Ground Loops’, Microphonics, Thermocouple Effect

APPENDIX B

Resistive Coupling (or ‘Ground Loops’)

Currents through common connections can give rise to noise voltages.

Resistive Coupling

Here, the detector is measuring the voltage across the experiment, plus the voltage due to the noise current passing through the finite resistance of the ground bus. This problem arises because we have used two different grounding points, which are not at exactly the same potential. Some cures for ground loop problems include:

1)Ground everything to the same physical point.

2)Use a heavier ground bus to reduce the potential drop along the ground bus.

3)Remove sources of large currents from ground wires used for small signals.

Microphonics

Microphonics provides a path for mechanical noise to appear as electrical noise in a circuit or experiment. Consider the simple circuit below:

The capacitance of a coaxial cable is a function of its geometry so mechanical vibrations will cause the cable capacitance to vary with time.

Since C = Q/V, we have:

C dV + V dC = dQ = i

dt dt dt

So mechanical vibrations will cause a dC/dt which in turn gives rise to a current i, which will affect the detector. Ways to eliminate microphonic signals include:

1)Eliminate mechanical vibrations.

2)Tie down experimental cables so they will not sway to and fro.

3)Use a low noise cable that is designed to reduce microphonic effects.

Thermocouple Effect

The emf created by dissimilar metal junctions can give rise to many microvolts of DC potential, and can be a source of AC noise if the temperature of the junction is not held constant. This effect is large on the scale of many low level measurements.

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