National Instruments DAQ X Series, Руководство пользователя

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Chapter 7 Counters
X Series User Manual 7-20 ni.com
Which Method Is Best?
This depends on the freq u ency to be meas u red, the rate at which yo u want
to monitor the freq u ency and the acc u racy yo u desire. Take for example,
meas u ring a 50 kHz signal. Ass u ming that the meas u rement times for the
sample clocked (with averaging) and two co u nter freq u ency meas u rements
are config u red the same, Table 7-3 s u mmarizes the res u lts.
From this, yo u can see that while the meas u rement time for one co u nter is
shorter, the acc u racy is best in the sample clocked and two co u nter large
range meas u rements. For another example, Table 7-4 shows the res u lts for
5 MHz.
Table 7-3. 50 kHz Frequency Measurement Methods
Variable
Sample
Clocked One Counter
Two Counter
High Frequency Large Range
fx 50,000 50,000 50,000 50,000
fk 100 M 100 M 1,000 100 M
Measurement time ( mS ) 1 .02 1 1
N — — — 50
Max. freq u ency error
(Hz)
.512 25 1,000 .5
Max. error % .00102 .05 2 .001
Table 7-4. 5 MHz Frequency Measurement Methods
Variable
Sample
Clocked One Counter
Two Counter
High Frequency Large Range
fx 5 M 5 M 5 M 5 M
fk 100 M 100 M 1,000 100 M
Measurement time ( mS ) 1 .0002 1 1
N — — — 5,000
Max. Freq u ency error
(Hz)
50.01 263 k 1,000 50
Max. Error % .001 5.26 .02 .001
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