National Instruments NI cDAQ-9172 - page 44

NI cDAQ-9172 User Guide and Specifications 44 ni.com
You can route the signal to measure to the Source input of Counter0, as
shown in Figure27. Assume this signal to measure has frequency F1.
Configure Counter0 to generate a si ngle pulse that is the width of N periods
of the source input signal.
Figure 27. Frequency Measurement—Method 3
Then route the Counter 0 Internal Output signal to the Gate input of
Counter 1. You can route a signal of known frequency (F2) to the Counter1
Source input. F2 can be 80MHzTimebase. For signals that might be slower
than 0.02Hz, use a slower known timebase. Configure Counter 1 to
perform a single pulse-width measurement. Suppose the result is that the
pulse width is J periods of the F2 clock.
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).
SOURCE OUT
COUNTER 0
SOURCE
GATE
OUT
COUNTER 1
Signal to
Measure (F1)
Signal of Known
Frequency (F2)
CTR_0_SOURCE
(Signal to Measure)
CTR_0_OUT
(CTR_1_GATE)
CTR_1_SOURCE
Interval to
Measure
0123…
N