SRS Labs SR850 manual Noise

SR850 Basics

Which method you use depends upon the require- ments of the experiment.

R noise

The quantity Rn can be somewhat hard to under- stand. For example, suppose X and Y are equally noisy and centered about zero. The values of R are always positive (magnitude) and thus average to a nonzero value. In this case, X and Y noise result in an average R which can be interpreted as the minimum detectable value of R. Increasing the time constant reduces the X and Y output noise and reduces this average value of R. The calcula- tion of R noise by either method will typically yield a value smaller than either Xn or Yn. This is because X and Y have both positive and negative values with a zero center yielding large deviations while R is always positive with a non zero mean and has smaller deviations. In this case, R noise is mathematically defined but not indicative of the Gaussian noise typically measured.

If there is a nonzero steady state value of R such that the noise excursions of R are small compared with the mean R, then R noise is meaningful. This is the case when measuring noise in the presence of real detectable signal. In this case, the value Rn approaches Xn and Yn.

3-28