Chapter 7 Counters
X Series User Manual 7-20 ni.com
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.
From this, 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 7-3. 50 kHz Frequency Measurement Methods
Varia ble
Sample
Clocked
One Counter
Two Counter
High Frequency Large Range
fx 50,000 50,000 50,000 50,000
fk 100 M 100 M 1,000 100 M
Measurement time (mS) 1 .02 1 1
N — — — 50
Max. frequency error
(Hz)
.512 25 1,000 .5
Max. error % .00102 .05 2 .001
Table 7-4. 5 MHz Frequency Measurement Methods
Variable
Sample
Clocked
One Counter
Two Counter
High Frequency Large Range
fx 5 M 5 M 5 M 5 M
fk 100 M 100 M 1,000 100 M
Measurement time (mS) 1 .0002 1 1
N — — — 5,000
Max. Frequency error
(Hz)
50.01 263 k 1,000 50
Max. Error % .001 5.26 .02 .001
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