Figure 61. High Frequency with Two Counters
Pulse
fx
Pulse
fx
Gate
Source
1 2 … N
Pulse-Width
Measurement
T =
N
fx
Frequency of fx =
T
Width of
Pulse
N
Width of Pulse (T)
Large Range of Frequencies with Two Counters
By using two counters, you can accurately measure a signal that might be high or low
frequency. This technique is called reciprocal frequency measurement. When measuring a
large range of frequencies with two counters, you generate a long pulse using the signal to
measure. You then measure the long pulse with a known timebase. The cRIO controller can
measure this long pulse more accurately than the faster input signal.
Note Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Counter 3.
You can route the signal to measure to the Source input of Counter 0, as shown in the
following figure. Assume this signal to measure has frequency fx. NI-DAQmx automatically
configures Counter 0 to generate a single pulse that is the width of N periods of the source
input signal.
86 | ni.com | cRIO-904x User Manual
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