Chapter 8 Counters
NI USB-621x User Manual 8-12 ni.com
• Method 2: Measure High Frequency with Two Counters—In this
method, you measure one pulse of a known width using your signal
and derive the frequency of your signal from the result. This method is
good for high frequency signals.
In this method, you route a pulse of known duration (T) to the Gate of
a counter. You can generate the pulse using a second counter. You also
can generate the pulse externally and connect it to a PFI terminal. You
only need to use one counter if you generate the pulse externally.
Route the signal to measure (F1) to the Source of the counter.
Configure the counter for a single pulse-width measurement. Suppose
you measure the width of pulse T to be N periods of F1. Then the
frequency of F1 is N/T.
Figure 8-12 illustrates this method. Another option would be to
measure the widt
h of a known period instead of a known pulse.
Figure 8-12. Method 2
Pulse
F1
Pulse
F1
Gate
Source
1 2 … N
Pulse-Width
Measurement
T =
N
F1
Frequency of F1 =
T
Width of
Pulse
N
Width of Pulse (T)
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