Period Measurement - National Instruments DAQ X Series User Manual

Chapter 7 Counters
Table 7-5 summarizes some of the differences in methods of measuring
frequency.
Table 7-5. Frequency Measurement Method Comparison
Method
Low frequency with
one counter
High frequency with
two counters
Large range of
frequencies with
two counters
Sample clocked
(averaged)
For information about connecting counter signals, refer to the
Counter/Timer Pinouts

Period Measurement

In period measurements, the counter measures a period on its Gate input
signal after the counter is armed. You can configure the counter to measure
the period between two rising edges or two falling edges of the Gate input
signal.
You can route an internal or external periodic clock signal (with a known
period) to the Source input of the counter. The counter counts the number
of rising (or falling) edges occurring on the Source input between the
two active edges of the Gate signal.
X Series User Manual
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clocked frequency measurement is not dependent on the measured
frequency so at 50 kHz and 5 MHz with a measurement time of 1 ms
the error % will still be 0.001%. One of the disadvantages of a sample
clocked frequency measurement is that the frequency to be measured
must be at least twice the sample clock rate to ensure that a full period
of the frequency to be measured occurs between sample clocks.
Number of Number of
Counters Measurements
Used Returned
1 1
1 or 2 1
2 1
1 1
section.
7-22
Measures High Measures Low
Frequency Frequency
Signals Signals
Accurately Accurately
Poor Good
Good Poor
Good Good
Good Good
Default
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