© National Instruments Corporation 45 NI cDAQ-9178/9174 User Guide and Specifications
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 shows
the results for 5 MHz.
Again the measurement time for the one counter measurement is lowest but the accuracy is lower. Note
that the accuracy and measurement time of the sample clocked and two counter large range are almost
the same. The advantage of the sample clocked method is that even when the frequency to measure
changes, the measurement time does not and error percentage varies little. For example, if you
configured a large range two counter measurement to use a divide down of 50 for a 50 k signal, then you
would get the accuracy measurement time and accuracy listed in Table 6. But if your signal ramped up
to 5 M, then with a divide down of 50, your measurement time is 0.01 ms, but your error is now 0.125%.
The error with a sample clocked frequency measurement is not as dependent on the measured frequency
so at 50 k and 5 M with a measurement time of 1 ms the error percentage is still close to 0.00125%.
One of the disadvantages of a sample clocked frequency measurement is that the frequency to be
Table 6. 50 kHz Frequency Measurement Methods
Var iable Sample Clocked One Counter
Two Counter
High Frequency Large Range
fx 50,000 50,000 50,000 50,000
fk 80 M 80 M 1,000 80 M
Measurement time
(mS)
1 .02 1 1
N — — — 50
Max. frequency
error (Hz)
.638 31.27 1,000 .625
Max. error % .00128 .0625 2 .00125
Table 7. 5 MHz Frequency Measurement Methods
Var iable Sample Clocked One Counter
Two Counter
High Frequency Large Range
fx 5 M 5 M 5 M 5 M
fk 80 M 80 M 1,000 80 M
Measurement time
(mS)
1 .0002 1 1
N — — — 5,000
Max. Frequency
error (Hz)
62.51 333 k 1,000 62.50
Max. Error % .00125 6.67 .02 .00125
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