Spider DSA User’s Manual
263
t = 1/f
s
= 1 / k F
span
time between adjacent time points (S)
T
span
= Nt
duration of each time capture or memory load period (S)
F = 1/T
span
difference between adjacent frequency points (Hz)
F
span
= NF / k frequency range presented (Hz)
In order-normalized (order-tracked) analysis, both the frequency range and
sample rate must vary in proportion to the machine speed. This is accomplished
by measuring the shaft speed with a tachometer and deriving a sample rate equal
to k O
span
times the instantaneous shaft speed. O
span
is the maximum number of
shaft-speed orders (multiples) to be measured in a spectrum. The effective anti-
aliasing filter must constantly adjust to limit the incoming signal bandwidth to
O
span
times the shaft-turning frequency. This results in the following spans and
resolutions:
R = 1/f
s
= 1 / k O
span
shaft-angle between adjacent signal samples (Revolution)
R
span
= NR
number of turns in each memory capture (Revolution)
O = 1/R
span
difference between adjacent order points (Order)
O
span
= NO / k order span presented (Order)
Typical analyzers require between 2.56 and 4 samples per maximum order
spanned. This is the same k multiple relating the analyzer’s sample-rate to the
frequency band studied in normal fixed-bandwidth analysis. The exact numeric
value is determined by the analyzer’s design specifics.
Processing Concept
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