National Instruments NI 6221, User Manual

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Chapter 7 Counters
© National Instruments Corporation 7-9 M Series User Manual
Note that 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 about connecting counter signals, refer to the Default
Counter/Timer Pinouts 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—Measure Low Frequency with
One Counter
In this method, you measure one period of your signal using a known
timebase. This method is good for low frequency signals.
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. Figure 7-10 illustrates this
method.
Figure 7-10. Method 1
F1
Ft
F1
Ft
G a te
S o u rce
1 2 3 … N
S ingle Period
Me asu rement
…
Period of F1 =
N
Ft
Fre qu ency of F1 =
N
Ft
Interv a l Me asu red