National Instruments DAQ X Series, Руководство пользователя

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Chapter 7 Counters
X Series User Manual 7-22 ni.com
clocked freq u ency meas u rement is not dependent on the meas u red
freq u ency so at 50 kHz and 5 MHz with a meas u rement time of 1 ms
the error % will still be 0.001%. One of the disadvantages of a sample
clocked freq
u ency meas u rement is that the freq u ency to be meas u red
m u st be at least twice the sample clock rate to ens u re that a f u ll period
of the freq u ency to be meas u red occ u rs between sample clocks.
Table 7-5 s u mmarizes some of the differences in methods of meas u ring
freq u ency.
For information abo u t connecting co u nter signals, refer to the Default
Counter/Timer Pinouts section.
Period Measurement
In period meas u rements, the co u nter meas u res a period on its Gate inp u t
signal after the co u nter is armed. Yo u can config u re the co u nter to meas u re
the period between two rising edges or two falling edges of the Gate inp u t
signal.
Yo u can ro u te an internal or external periodic clock signal (with a known
period) to the So u rce inp u t of the co u nter. The co u nter co u nts the n u mber
of rising (or falling) edges occ u rring on the So u rce inp u t between the
two active edges of the Gate signal.
Table 7-5. Frequency Measurement Method Comparison
Method
Number of
Counters
Used
Number of
Measurements
Returned
Measures High
Frequency
Signals
Accurately
Measures Low
Frequency
Signals
Accurately
Low freq u ency with
one co u nter
1 1 Poor Good
High freq u ency with
two co u nters
1 or 2 1 Good Poor
Large range of
freq
u encies with
two co u nters
2 1 Good Good
Sample clocked
(averaged) 1 1 Good Good
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