National Instruments DAQ X Series, Руководство пользователя

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Chapter 7 Counters
© National Instruments 7-21 X Series User Manual
Again the meas u rement time for the one co u nter meas u rement is lowest b u t
the acc u racy is lower. Note that the acc u racy and meas u rement time of the
sample clocked and two co u nter large range are almost the same. The
advantage of the sample clocked method is that even when the freq u ency to
meas u re changes, the meas u rement time does not and error percentage
varies little. For example, if yo u config u red a large range two co u nter
meas u rement to u se a divide down of 50 for a 50 k signal, then yo u wo u ld
get the acc u racy meas u rement time and acc u racy listed in Table 7-3. B u t if
yo u r signal ramped u p to 5 M, then with a divide down of 50, yo u r
meas u rement time is 0.01 ms, b u t yo u r error is now 0.1%. The error with a
sample clocked freq u ency meas u rement is not as dependent on the
meas u red freq u ency so at 50 k and 5 M with a meas u rement time of 1 ms
the error percentage is still close to 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.
• Low freq u ency meas u rements with one co u nter is a good method for
many applications. However, the acc u racy of the meas u rement
decreases as the freq u ency increases.
• High freq u ency meas u rements with two co u nters is acc u rate for high
freq u ency signals. However, the acc u racy decreases as the freq u ency
of the signal to meas u re decreases. At very low freq u encies, this
method may be too inacc u rate for yo u r application. Another
disadvantage of this method is that it req u ires two co u nters (if yo u
cannot provide an external signal of known width). An advantage of
high freq
u ency meas u rements with two co u nters is that the
meas u rement completes in a known amo u nt of time.
• Meas u ring a large range of freq u encies with two co u nters meas u res
high and low freq u ency signals acc u rately. However, it req u ires
two co u nters, and it has a variable sample time and variable error %
dependent on the inp u t signal.
• Again, the meas u rement time for the one co u nter meas u rement is
lowest, b u t the acc u racy is lower. Note that the acc u racy and
meas u rement time of the sample clocked and two co u nter large range
are the same. The advantage of the sample clocked method is that even
when the freq
u ency to meas u re changes, the meas u rement time and
error % does not. For example, if yo u config u red a large range
two co u nter meas u rement to u se a divide down of 50 for a 50 kHz
signal, then yo u wo u ld get the acc u racy meas u rement time and
acc u racy listed in table 7-3. B u t if yo u r signal ramped u p to 5 MHz,
then with a divide down of 50, yo u r meas u rement time wo u ld be
0.01 ms, b u t yo u r error wo u ld now be 0.001%. The error with a sample
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