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MX4428 MXP Engineering / Technical Manual Document: LT0273
MXP Loop Filter Board
Page 9-2 24 March 2006 Issue 1.5
9.1 EXPONENTIAL FILTER
An “exponential filter” is used to smooth values received from all detector types and remove
“noise”.
Value
Time
IN
OUT
This is a kind of moving average of recent samples, with the most recent samples given the
most weighting. The output of the filter will exponentially approach the input (like an
electronic RC filter).
OUT
n
= OUT
n-1
+ (IN - OUT
n-1
) / FD.
FD is the filter divisor. The bigger FD, the slower the filter.
This filter usually requires calculation using units much smaller than the units being filtered.
For instance it could use floating point arithmetic or integer arithmetic using units of 1/256 of
the “detector units”. Conversion from the calculation units back to detector units should use
rounding rather than truncation.
9.2 STEP LIMITING FILTER
“Step Limiting” is used after the exponential filter on all detector types, to reduce sensitivity
to large short term changes in input values which may represent false alarms.
Value
Time
IN
OUT