National Instruments Eight-slot USB Chassis NI cDAQ-9172 - Manual (Page 53)

© National Instruments Corporation 53 NI cDAQ-9172 User Guide and Specifications
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.
• 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.
Table 5. Frequency Measurement Method 1
Task Equation Example 1 Example 2
Actual Frequency to Measure F1 50 kHz 5 MHz
Timebase Frequency Ft 80 MHz 80 MHz
Actual Number of Timebase
Periods Ft/F1 1600 16
Worst Case Measured Number
of Timebase Periods (Ft/F1) – 1 1599 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%