Spectral Shifting—A Signal to Noise Enhancer
Dan DeBra and Brian Lantz, Stanford University

Reversal allows the separation of biases from measurements. But disturbances and or measurement noise is seldom a constant. In fact they seldom make contributions which are uniform over all frequencies. Many times there are regions of frequency for disturbances or measurements which are quieter and the signal to noise ratio can be improved if the signal can be shifted to that region of the spectrum. Examples will include rotating instruments (or whole satellites), lock-in amplifiers, frequency beating in laser interferometers, chopping in photomultiplier tubes, ac excitation of thermisters and others. We will discuss some common noise characteristics e.g. 1/f-noise. In the process of shifting and sampling one may limit the available information so there become limits on the highest and lowest frequencies. Windowing as a means of correcting for some of the distorting effects of shifting will also be discussed.