National Instruments X Series User Manual page 150

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 time-base. The frequency measured will be fx = fk * (T1/T2).
Figure 7-15. Sample Clocked Buffered Frequency Measurement (Averaging)
Counter Armed
S1
Gate
( fx )
Source
( fk )
Sample
Clock
Buffer
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.
Figure 7-16. Sample Clocked Buffered Frequency Measurement (Non-Averaging)
Counter Armed
Gate
Source
Sample
Clock
Latched
Values
With sample clocked frequency measurements, ensure that the frequency to measure is twice as
fast as the sample clock to prevent a measurement overflow.
1
6
T1 T2
1 6
6
6
S2
2
10
T1 T2
1 7
2 10
4
6
4
X Series User Manual
S3
1
6
6
6
4
6
© National Instruments | 7-15
T1T2
1 7
2 10
1 6