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Note
If you are using an external signal as the Source, at least one Source pulse should
occur between each active edge of the Gate signal. This condition ensures that correct
values are returned by the counter. If this condition is not met, consider using duplicate
count prevention, described in the
F1
Gate
Ft
Source
Single Period
Measurement
© National Instruments Corporation
Duplicate Count Prevention
For information on connecting counter signals, refer to the
Counter/Timer Routing
Frequency Measurement
You can use the counters to measure frequency in several different ways.
You can choose one of the following methods depending on your
application: Method 1, Method 1b, Method 2, and Method 3.
Method 1—Measure Low Frequency With One Counter
This method is good for low frequency signals. Use this method to measure
one period of your signal using a known timebase. Figure 24 illustrates this
method.
F1
1
Ft
Figure 24. Frequency Measurement—Method 1
You can route the signal to measure (F1) to the Gate of a counter. You can
route a known timebase (Ft) to the Source of the counter. The known
timebase can be 80MHzTimebase. For signals that might be slower than
0.02 Hz, use a slower known timebase.
You can configure the counter to measure one period of the gate signal. The
frequency of F1 is the inverse of the period.
section.
Interval Measured
2
3
...
N
Period of F1 =
Ft
Frequency of F1 =
49
NI cDAQ-9172 User Guide and Specifications
section.
Default
...
N
Ft
N
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