National Instruments Data Acquisition Device NI USB-621x - Manual (Page 103)

Chapter 8 Counters
NI USB-621x User Manual 8-10 ni.com
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.
Figure 8-10. Method 1
• 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.
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