© National Instruments | 5-13
NI cDAQ-917x User Manual
Sample Clocked Buffered Frequency Measurement
Sample clocked buffered point frequency measurements can either be a single frequency
measurement or an average between sample clocks. Use CI.Freq.EnableAveraging to set the
behavior. For buffered frequency, the default is True.
A sample clocked buffered frequency measurement with CI.Freq.EnableAveraging set to True
uses the embedded counter and a sample clock to perform a frequency measurement. For each
sample clock period, the embedded counter counts the signal to measure (fx) and the primary
counter counts the internal time-base of a known frequency (fk). Suppose T1 is the number of
ticks of the unknown signal counted between sample clocks and T2 is the number of ticks
counted of the known timebase as shown in Figure 5-15. The frequency measured is:
fx = fk * (T1/T2)
Figure 5-15. 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 5-16.
Figure 5-16. Sample Clocked Buffered Frequency Measurement (Non-Averaging)
Gate
(fx)
Source
(fk)
Sample
Clock
Counter Armed
S1 S2 S3
T1 T2
1
6
T1 T2
17
210
T1 T2
1
7
2
10
1
6
Buffer
121
6106
Gate
Source
Sample
Clock
Counter Armed
Latched
Values
6
646
4
6
4
6
6
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