Ectron 441A, Instructions Manual, Page: 39 / 74

interest. During setup, the lowest and highest signal fre-
quencies should be tested for proper operation.
Output Filter
This two-pole active filter is used to reduce noise of
the output analog signal. Corner frequencies are
1
Hz,
10
Hz,
100
Hz, and
WB
(approximately 1.5 kHz).
Since the lower the filter frequency the slower the re-
sponse, a compromise must be made between noise and
response. (Response of the digital filter adds to the re-
sponse of the analog output filter.) Table 4-2 indicates
the response to a step-frequency change for the four
filter frequencies assuming the digital filter is set to a
PULSES/REV
olution of
001
. In general, the user
should select the lowest filter setting consistent with the
response requirements of the application.
Table 4-2
Response Time Versus Output Filter Setting
Output Filter in Hz
Approximate Response Time
in Seconds
WB (1500) 0.005*
100 0.055*
10 0.5*
1 5.0*
* add
1
⁄
f
in
where
f
in
is the input frequency
Digital Filter
For operational frequencies up to 1 kHz, this unique
filter uses digital averaging of the converted input signal
to eliminate the unwanted effects of a signal whose fre-
quency varies in a periodic manner. For example, the
output frequency of a paddle-wheel flow sensor usually
changes as the wheel rotates because the paddles are
not evenly spaced. If the sensor has eight paddles, its
output will consist of a repeating sequence of eight
pulses. If this signal is fed to a frequency-to-voltage
converter without this filtering, the output analog signal
will have a cyclic component with a period equal to
the period of rotation of the device. However, by using
the digital filter with an eight-pulse running average,
the period variation is averaged out and a much more
stable analog output is obtained.
Figure 4-6 illustrates the function of the digital filter
for a signal from an eight-paddle sensor. At first, the
unfiltered (stair-step) analog output varies about 5 V;
then the speed of rotation is increased so the signal
varies about 5.4 V. Keying in eight pulses per revolution
Model 441A Applications
4-9