Period Measurement - National Instruments NI cDAQ-9181 User Manual

Chapter 5 Counters
• Measuring a large range of frequencies with two counters measures high and low frequency
signals accurately. However, it requires two counters, and it has a variable sample time and
variable error % dependent on the input signal.
Table 5-5 summarizes some of the differences in methods of measuring frequency.
Table 5-5. 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
section.

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.
You can calculate the period of the Gate input by multiplying the period of the Source signal by
the number of edges returned by the counter.
Period measurements return the inverse results of frequency measurements. Refer to the
Frequency Measurement
5-18 | ni.com
Number
of Number of
Counters Measurements
Used Returned
1
1 or 2
2
1
section for more information.
Measures
High
Frequency
Signals
Accurately
1 Poor
1 Good
1 Good
1 Good
Default Counter/Timer Routing
Measures Low
Frequency
Signals
Accurately
Good
Poor
Good
Good