Frequency Measurement; Method 1-Measure Low Frequency With One Counter - National Instruments NI USB-621x User Manual

Chapter 9 Counters

Frequency Measurement

NI USB-621x User Manual
This condition ensures that correct values are returned by the counter. If this
condition is not met, the counter returns a zero. Refer to the
Count Prevention section for more information.
For information about connecting counter signals, refer to the
Counter/Timer Pinouts
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 9-11 illustrates this
method.
F1
Gate
Ft
Source
Single Period
Measurement
section.
Interval Measured
F1
1 2 3
Ft
Period of F1 =
Frequency of F1 =
Figure 9-11. Method 1
9-10
Duplicate
Default
... ... N
N
Ft
Ft
N
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