National Instruments Data Acquisition Device NI USB-621x, User Manual

Page: 107 / 185

Chapter 8 Counters
NI USB-621x User Manual 8-14 ni.com
80 MHz Timebase. Yo u r meas u rement may ret u rn 1600 ± 1 cycles
depending on the phase of the signal with respect to the timebase. As
yo
u r freq u ency becomes larger, this error of ±1 cycle becomes more
significant; Table 8-1 ill u strates this point.
• Method 1b (meas u ring K periods of F1) improves the acc u racy of the
meas u rement. A disadvantage of Method 1b is that yo u have to take
K + 1 meas u rements. These meas u rements take more time and
cons u me some of the available USB bandwidth.
• Method 2 is acc u rate for high freq u ency signals. However, the
acc u racy decreases as the freq u ency of the signal to meas u re
decreases. At very low freq u encies, Method 2 may be too inacc u rate
for yo u r application. Another disadvantage of Method 2 is that it
req u ires two co u nters (if yo u cannot provide an external signal of
known width). An advantage of Method 2 is that the meas u rement
completes in a known amo u nt of time.
• Method 3 meas u res high and low freq u ency signals acc u rately.
However, it req u ires two co u nters.
Table 8-1. Frequency Measurement Method 1
Task Equation Example 1 Example 2
Act u al Freq u ency to Meas u re F1 50 kHz 5 MHz
Timebase Freq u ency Ft 80 MHz 80 MHz
Act u al N u mber of Timebase
Periods
Ft/F1 1600 16
Worst Case Meas u red N u mber of
Timebase Periods
(Ft/F1) – 1 1599 15
Meas u red Freq u ency 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%