Choosing A Method For Measuring Frequency - National Instruments NI cRIO-905x Series User Manual

Figure 64. 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 as shown in the following
figure.
Figure 65. 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.

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
fk
Measurement Time (T) is the time it takes to measure a single sample
78 | ni.com | NI cRIO-905x User Manual
1
6
T1 T2
1 6
6
6
is the frequency to be measured if no error
is the known source or gate frequency
S2
2
10
T1 T2
1 7
2 10
4
6
4
S3
1
6
T1 T2
1 7
2 10
1 6
6
6
4
6