Chapter 8 Counters
NI USB-621x User Manual 8-14 ni.com
From Counter 0, the length of the pulse is N/F1. From Counter 1, the
length of the same pulse is J/F2. Therefore, the frequency of F1 is
given by F1 = F2 * (N/J).
Choosing a Method for Measuring Frequency
The best method to measure frequency depends on several factors
including the expected frequency of the signal to measure, the desired
accuracy, how many counters are available, and the measurement duration.
• Method 1 uses only one counter. It is a good method for many
applications. However, the accuracy of the measurement decreases as
the frequency increases.
Consider a frequency measurement on a 50 kHz signal using an
80 MHz Timebase. This frequency corresponds to 1,600 cycles of the
80 MHz Timebase. Your measurement may return 1,600 ± 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 8-1 illustrates this point.
• 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.
Table 8-1. Frequency Measurement Method 1
Task Equation Example 1 Example 2
Actual Frequency to Measure F1 50 kHz 5MHz
Timebase Frequency Ft 80 MHz 80 MHz
Actual Number of Timebase
Periods
Ft/F1 1,600 16
Worst Case Measured Number of
Timebase Periods
(Ft/F1) – 1 1,599 15
Measured Frequency Ft F1/(Ft – F1) 50.031 kHz 5.33 MHz
Error [Ft F1/(Ft – F1)] – F1 31 Hz 333 kHz
Error % [Ft/(Ft – F1)] – 1 0.06% 6.67%
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