National Instruments 622 Series User Manual page 109

Table 7-2. 50 kHz Frequency Measurement Methods (Continued)
Variable
Maximum
frequency error
(Hz)
Maximum 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 7-3 shows the results for 5 MHz.
Table 7-3. 5 MHz Frequency Measurement Methods
Variable
fx
fk
Measurement
time (mS)
N
Maximum
frequency error
(Hz)
Maximum error
(%)
• Using one counter for low frequency measurements is a good method for many
applications. However, the accuracy of the measurement decreases as the frequency
increases.
• Measuring with one counter (averaged) measures high and low frequency signals
accurately. The advantage of this method is that it requires only one counter. Disadvantages
include the possibility of FIFO overflow at high frequencies and high N for this method.
These measurements take more time and consume some of the available PCI or PXI
One Counter
— Averaged
25
0.05
One Counter
— Averaged
5 M
100 M 100 M
0.0002 1.0002
—
263 k
5.26
High
Frequency
0.5 1,000
0.001
High
Frequency
5 M
1,000
5,000
50 1,000
0.001
M Series User Manual
Two Counters
Large Range
2 0.001
Two Counters
Large Range
5 M 5 M
100 M
1
— 5,000
0.02 0.001
© National Instruments | 7-13
0.5
1
50