National Instruments PCIe-6323 User Manual page 153

Chapter 7 Counters
Which Method Is Best?
This depends on the frequency to be measured, the rate at which you want to monitor the
frequency and the accuracy you desire. Take for example, measuring a 50 kHz signal. Assuming
that the measurement times for the sample clocked (with averaging) and two counter frequency
measurements are configured the same, Table 7-3 summarizes the results.
Table 7-3. 50 kHz Frequency Measurement Methods
Variable
fx
fk
Measurement time (ms)
N
Max. frequency error
(Hz)
Max. error %
From these results, you can see that while the measurement time for one counter is shorter, the
accuracy is best in the sample clocked and two counter large range measurements. For another
example, Table 7-4 shows the results for 5 MHz.
Table 7-4. 5 MHz Frequency Measurement Methods
Variable
fx
fk
Measurement time (ms)
N
Max. Frequency error
(Hz)
Max. Error %
7-18 | ni.com
Sample One
Clocked Counter
50,000 50,000
100 M 100 M
1 .02
— —
.512 25
.00102 .05
Sample One
Clocked Counter
5 M 5 M
100 M 100 M
1 .0002
— —
50.01 263 k
.001 5.26
Two Counter
High
Frequency Large Range
50,000
1,000
1
—
1,000
2
Two Counter
High
Frequency Large Range
5 M
1,000
1
—
1,000
.02
50,000
100 M
1
50
.5
.001
5 M
100 M
1
5,000
50
.001