National Instruments 6368, Руководство пользователя

Страница: 153 / 278

7-18
|
ni.com
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.
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 7-3.
50 kHz Frequency Measurement Methods
Variable
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