National Instruments USB-621 Series - Frequency Measurement

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