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Task
Actual Frequency to Measure
Timebase Frequency
Actual Number of Timebase
Periods
Worst Case Measured Number
of Timebase Periods
Measured Frequency
Error
Error %
© National Instruments Corporation
Choosing a Method for Measuring Frequency
The best method to measure frequency depends on several factors
including the expected frequency of the signal to measures, the desired
accuracy, how many counters are available and how long the measurement
can take.
•
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 1600 cycles of the
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, as Table 5 illustrates.
Table 5. 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 K + 1
measurements are required. 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
50.031 kHz
31 Hz
0.06%
53
NI cDAQ-9172 User Guide and Specifications
Example 2
5 MHz
80 MHz
16
15
5.33 MHz
333 kHz
6.67%
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