National Instruments NI USB-621x User Manual page 94

Chapter 9 Counters
Task
Actual Frequency to Measure
Timebase Frequency
Actual Number of Timebase
Periods
Worst Case Measured Number of
Timebase Periods
Measured Frequency
Error
Error %
NI USB-621x User Manual
80 MHz Timebase. Your measurement may return 1600 ± 1 cycles
depending on the phase of the signal with respect to the timebase. As
your frequency becomes larger, this error of ±1 cycle becomes more
significant; Table 9-2 illustrates this point.
Table 9-2. Frequency Measurement Method 1
Equation
F1
Ft
Ft/F1
(Ft/F1) – 1
Ft F1/(Ft – F1)
[Ft F1/(Ft – F1)] – F1
[Ft/(Ft – F1)] – 1
• Method 1b (measuring K periods of F1) improves the accuracy of the
measurement. A disadvantage of Method 1b is that you have to take
K + 1 measurements. These measurements take more time and
consume some of the available USB bandwidth.
• Method 2 is accurate for high frequency signals. However, the
accuracy decreases as the frequency of the signal to measure
decreases. At very low frequencies, Method 2 may be too inaccurate
for your application. Another disadvantage of Method 2 is that it
requires two counters (if you cannot provide an external signal of
known width). An advantage of Method 2 is that the measurement
completes in a known amount of time.
• Method 3 measures high and low frequency signals accurately.
However, it requires two counters.
Example 1
50 kHz
80 MHz
1600
1599
50.125 kHz
125 kHz
0.06%
9-14
Example 2
5 MHz
80 MHz
16
15
5.33 MHz
333 kHz
6.67%
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