DSP TP104-plus Spezifikationen PDF herunterladen (Seite 82)

3-28

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.

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DSP TP104-plus Spezifikationen Seite 82