National Instruments Data Acquisition Device NI USB-621x, Руководство пользователя

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Chapter 8 Counters
© National Instruments Corporation 8-13 NI USB-621x User Manual
Figure 8-13. Method 3
Then ro u te the Co u nter 0 Internal O u tp u t signal to the Gate inp u t of
Co u nter 1. Yo u can ro u te a signal of known freq u ency (F2) to the
Co u nter 1 So u rce inp u t. F2 can be 80MHzTimebase. For signals that
might be slower than 0.02 Hz, u se a slower known timebase. Config u re
Co u nter 1 to perform a single p u lse-width meas u rement. S u ppose the
res u lt is that the p u lse width is J periods of the F2 clock.
From Co u nter 0, the length of the p u lse is N /F1. From Co u nter 1, the
length of the same p u lse is J/F2. Therefore, the freq u ency of F1 is
given by F1 = F2 * ( N /J).
Choosing a Method for Measuring Frequency
The best method to meas u re freq u ency depends on several factors
incl u ding the expected freq u ency of the signal to meas u re, the desired
acc u racy, how many co u nters are available, and the meas u rement d u ration.
• Method 1 u ses only one co u nter. It is a good method for many
applications. However, the acc u racy of the meas u rement decreases as
the freq u ency increases.
Consider a freq u ency meas u rement on a 50 kHz signal u sing an
80 MHz Timebase. This freq u ency corresponds to 1600 cycles of the
S OURCE OUT
COUNTER 0
S OURCE
GATE
OUT
COUNTER 1
S ign a l to
Me asu re (F1)
S ign a l of Known
Freq u ency (F2)
CTR_0_ S OURCE
( S ign a l to Me asu re)
CTR_0_OUT
(CTR_1_GATE)
CTR_1_ S OURCE
Interv a l
to Me asu re
0 1 2 3 … N