National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 49)

© National Instruments Corporation 49 NI cDAQ-9172 User Guide and Specifications
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
Duplicate Count Prevention section.
For information on connecting counter signals, refer to the Default
Counter/Timer Routing section.
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.
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.
F1
Ft
F1
Ft
Gate
Source
1 2 3 … N
Single Period
Measurement
…
Period of F1 =
N
Ft
Frequency of F1 =
N
Ft
Interval Measured