Large Range Of Frequencies With Two Counters - National Instruments cRIO-904 Series User Manual

Pulse
fx
Measurement

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
Figure 61. High Frequency with Two Counters
Pulse
Gate
Source
fx
Pulse-Width
Width of Pulse (T)
1 2 ...
N
Width of
T =
Pulse
fx
N
Frequency of fx =
T
N