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High Frequency with Two Counters
For high frequency measurements with two counters, you measure one pulse of a known width
using your signal and derive the frequency of your signal from the result.
Note
Counter 0 is always paired with Counter 1. Counter 2 is always paired with
Counter 3.
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 (fx) to the Source of the counter. Configure the counter for a single
pulse-width measurement. If you measure the width of pulse T to be N periods of fx, the
frequency of fx is N/T.
The image below illustrates this method. Another option is to measure the width of a known
period instead of a known pulse.
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.
76 | ni.com | NI cRIO-905x User Manual
Figure 62. 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
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