Group3 DTM-133, Руководство пользователя

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4.5 DIGITAL FILTERING
The teslameter software includes a digital filtering algorithm which may be enabled by turning on internal DIP switch
1. See section 3.5. Filtering is useful for smoothing out small fluctuations in the field reading. When filtering is enabled,
the FILTER indicator is
In order to speed up the response to large field changes when filtering is on, a window is set to define a band about the
current displayed field value. Filtering will only occur while the unfiltered field value remains within the window. If the
field value changes rapidly enough, the filtered field reading will not be able to follow fast enough to keep the
unfiltered value within the window, and filtering is temporarily disabled. This allows the field reading to follow large
rapid field changes, while providing good filtering of constant or slowly varying fields.
The default window width is 20 resolution increments on each side of the displayed field reading, for a total window
width of 40 increments.
The digital filter performs the following computation:
F(new) = F(old) + F - F(old)
where F(old) is the previous field reading display F(new) is the updated field reading display F is the most recent unfiltered
field reading J is the filter factor.
The effective time constant of the filter is dependent upon both the rate at which field measurements are made and
the value of J, according to the formula:
T = P/In[J/(J-1)]
where T is the filter time constant
P is the period between field measurements.
Field measurements are made at a fixed rate of 30 per second, so P = 0.033. With the default value of J - 8, the filter
time constant is 0.25 seconds. This gives a cut-off frequency in the displayed field reading of 0.64Hz.
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DTM-133 User's Manual
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