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An ac technique is described for measuring low-frequency resistance fluctuation spectra with improved sensitivity over dc methods achieved by avoiding preamplifier 1/f noise. The technique, easily implemented with decade resistors and a lock-in amplifier, allows the current noise of low-resistance (r<10 kΩ) specimens to be measured to frequencies below 1 mHz. Use of a center-tapped, four-probe specimen geometry allows discrimination between specimen and contact noise and eliminates noise due to bath temperature variations. The technique is demonstrated in use to determine the dependence of the 1/f noise of Cr films on film area. Measurements with simultaneous direct and alternating currents provide means to study the noise of nonlinear devices and frequency-dependent conductors.

The basic principles behind the operation of a lock-in amplifier are described. Particular emphasis is placed on looking at the frequency components of the signal present at the various stages of the lock-in during a typical measurement. The description presented here has been used successfully at Oberlin College to explain lock-in operation to students.

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