Choosing A Method For Measuring Frequency - National Instruments USB-621 Series User Manual

Chapter 8 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
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.
Table 8-1. Frequency Measurement Method 1
Equation
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.
Example 1
F1 50 kHz
Ft 80 MHz
50.031 kHz
8-14
Example 2
5 MHz
80 MHz
1,600 16
1,599 15
5.33 MHz
31 Hz 333 kHz
0.06% 6.67%
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