User's Manual

Chapter 9 Counters
NI USB-621x User Manual 9-14 ni.com
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.
• 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.
Table 9-2. 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 1600 16
Worst Case Measured Number of
Timebase Periods
(Ft/F1) – 1 1599 15
Measured Frequency Ft F1/(Ft – F1) 50.125 kHz 5.33 MHz
Error [Ft F1/(Ft – F1)] – F1 125 kHz 333 kHz
Error % [Ft/(Ft – F1)] – 1 0.06% 6.67%