Chapter 7 Counters
© National Instruments 7-15 X Series User Manual
Figure 7-13 illustrates this method. Another option is to measure the width
of a known period instead of a known pulse.
Figure 7-13. High Frequency with Two Counters
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 X Series device
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 Figure 7-14. 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.
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 )
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