National Instruments Data Acquisition Device NI USB-621x User Manual page 103

Chapter 8 Counters
NI USB-621x User Manual
You can configure the counter to measure one period of the gate signal.
The frequency of F1 is the inverse of the period. Figure 8-10 illustrates
method 1.
F1
Gate
Ft
Source
Single Period
Measurement
• Method 1b: Measure Low Frequency with One Counter
(Averaged)—In this method, you measure several periods of your
signal using a known timebase. This method is good for low to medium
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 make K + 1 buffered period
measurements. Recall that the first period measurement in the buffer
should be discarded.
Interval Measured
F1
1 2 3
Ft
Period of F1 =
Frequency of F1 =
Figure 8-10. Method 1
8-10
... ... N
N
Ft
Ft
N
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