Frequency Measurement; Low Frequency With One Counter - National Instruments NI cRIO-905x Series User Manual

one point of data and returned in units of seconds or ticks. In a pulse measurement, each pair
of high and low times is considered one point of data and returned as a paired sample in units
of frequency and duty cycle, high and low time or high and low ticks. When reading data, 10
points in a semi-period measurement will get an array of five high times and five low times.
When you read 10 points in a pulse measurement, you get an array of 10 pairs of high and low
times.
Also, pulse measurements support sample clock timing while semi-period measurements do
not.

Frequency Measurement

You can use the counters to measure frequency in several different ways. Refer to the
following sections for information about cRIO controller frequency measurement options:
•

Low Frequency with One Counter

• High Frequency with Two Counters
• Large Range of Frequencies with Two Counters
• Sample Clocked Buffered Frequency Measurement
For more information about choosing the best frequency measurement option, refer to the
Choosing a Method for Measuring Frequency
Low Frequency with One Counter
For low frequency measurements with one counter, you measure one period of your signal
using a known timebase.
You can route the signal to measure (fx) to the Gate of a counter. You can route a known
timebase (fk) to the Source of the counter. The known timebase can be an onboard timebase,
such as 80 MHz Timebase, 20 MHz Timebase, or 100 kHz Timebase, or any other signal with
a known rate.
You can configure the counter to measure one period of the gate signal. The frequency of fx is
the inverse of the period. The following figure illustrates this method.
fx
Gate
fk
Source
Single Period
Measurement
Figure 61. Low Frequency with One Counter
fx
1
fk
and Which Method is Best?
Interval Measured
2 3 ...
N
Period of fx =
fk
fk
Frequency of fx =
N
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