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Pulse
Gate
F1
Source
Pulse-Width
Measurement
Method 3—Measure Large Range of Frequencies
Using 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. In this method, you generate a long pulse using the signal to
measure. You then measure the long pulse with a known timebase. The
M Series device can measure this long pulse more accurately than the faster
input signal.
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 F1.
Configure Counter 0 to generate a single pulse that is the width of N periods
of the source input signal.
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Width of Pulse (T)
Pulse
1
2
...
F1
Width of
T =
Pulse
Frequency of F1 =
Figure 7-13. Method 2
7-13
Chapter 7
Counters
N
N
F1
N
T
NI 6238/6239 User Manual
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