NI cDAQ-9178 - Page 43

© National Instruments Corporation 43 NI cDAQ-9178/9174 User Guide and Specifications
Figure 34. Sample Clocked Buffered Frequency Measurement (Averaging)
When CI.Freq.EnableAveraging is set to false, the frequency measurement returns the frequency of
the pulse just before the sample clock. This single measurement is a single frequency measurement and
is not an average between clocks as shown in Figure 35.
Figure 35. Sample Clocked Buffered Frequency Measurement (Non-Averaging)
With sample clocked frequency measurements, ensure that the frequency to measure is twice as fast as
the sample clock to prevent a measurement overflow.
Choosing a Method for Measuring Frequency
The best method to measure frequency depends on several factors including the expected frequency of
the signal to measure, the desired accuracy, how many counters are available, and how long the
measurement can take. For all frequency measurement methods, assume the following:
fx is the frequency to be measured if no error
fk is the known source or gate frequency
measurement time (T) is the time it takes to measure a single sample
Gate
(fx)
Source
(fk)
Sample
Clock
Counter Armed
S1 S2 S3
T1 T2
1
7
T1 T2
17
210
T1T2
1
7
2
10
1
6
Buffer
121
7106
Gate
Source
Sample
Clock
Counter Armed
Latched
Values
6
646
4
6
4
6
6