Campbell Scientific CR9000X Measurement and Control System - Manual (Page 285)

Section 7. Measurement Instructions
7-61
The minimum
i
is:
( )
⎟⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎝
⎛
−
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
2
1
2 log
log
ceiling
10
10
ref
SR
bin
f N
f
S
where
( )
x ceiling is the smallest integer that is not less than x . The
maximum
i
is:
( )
⎟⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎝
⎛
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
2
1
2 log
2
log
floor
10
10
ref
SR
bin
f
f
S
where
( )
x floor is the largest integer that is not greater than x .
Users can select whether the CR9052DC returns the entire spectrum or only
part of the spectrum by setting ILow and IHigh. To return the entire spectrum,
set ILow to its minimum value, and set IHigh to its maximum value. As an
alternative to computing the minimum ILow and maximum IHigh from the
equations given above, let the CR9000X perform the calculations: Set ILow a
very negative value (like -1000) and set IHigh to a very positive value (like
1000). When the program is downloaded, the CR9000X compiler will issue an
error that gives the minimum ILow and maximum IHigh for the current
FFTFilt programming. These values can then be entered into the program and
used to calculate the size required for the destination array.
To limit the lower end of the final spectrum by frequency, select a minimum
frequency of interest,
low
f , and then calculate ILow:
ILow =
( )
⎟⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
2 log
log
round
10
10
ref
low
bin
f
f
S ,
where
( )
x round is x rounded to the nearest integer.
To limit the upper end of the final spectrum, select a maximum frequency of
interest,
high
f , and then calculate IHigh:
IHigh =
( )
⎟⎟
⎟
⎟
⎟
⎟
⎠
⎞
⎜⎜
⎜
⎜
⎜
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
2 log
log
round
10
10
ref
high
bin
f
f
S .
The total number of spectral components returned by FFTFilt is
IHigh - ILow + 1.