National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 50)

NI cDAQ-9172 User Guide and Specifications 50 ni.com
Method 1b—Measure Low Frequency With One Counter
(Averaged)
This method is good for low to medium frequency signals. Use this method
to measure several periods of your signal using a known timebase.
Figure 25 illustrates this method.
Figure 25. Frequency Measurement—Method 1b
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.
Average the remaining K period measurements to determine the average
period of F1. The frequency of F1 is the inverse of the average period.
F1
Ft
F1
Intervals Measured
Gate
Source
Buffered Period
Measurement
Average Period of F1 =
N
1
+ N
2
+ …N
K
K
Frequency of F1 =
N
1
+ N
2
+ …N
K
K × Ft
Ft
T
1
T
2
… T
K
1
Ft
×
1 2 ...N
1
1... ...N
2
… 1... ...N
K