Figure 62. Large Range of Frequencies with Two Counters
Source Out
Counter 0
Source
Gate
Out
Counter 1
Signal to
Measure (fx)
Signal of Known
Frequency (fk)
CTR_0_SOURCE
(Signal to Measure)
CTR_0_OUT
(CTR_1_GATE)
CTR_1_SOURCE
Interval
to Measure
0 1 2 3 … N
Next, route the Counter 0 Internal Output signal to the Gate input of Counter 1. You can route
a signal of known frequency (fk) to the Counter 1 Source input. Configure Counter 1 to
perform a single pulse-width measurement. Suppose the result is that the pulse width is J
periods of the fk clock.
From Counter 0, the length of the pulse is N/fx. From Counter 1, the length of the same pulse
is J/fk. Therefore, the frequency of fx is given by fx = fk * (N/J).
Sample Clocked Buffered Frequency Measurement
Sample clocked buffered point frequency measurements can either be a single frequency
measurement or an average between sample clocks. Use CI.Freq.EnableAveraging to set the
behavior. For buffered frequency, the default is True.
A sample clocked buffered frequency measurement with CI.Freq.EnableAveraging set to
True uses the embedded counter and a sample clock to perform a frequency measurement. For
each sample clock period, the embedded counter counts the signal to measure (fx) and the
primary counter counts the internal time-base of a known frequency (fk). Suppose T1 is the
number of ticks of the unknown signal counted between sample clocks and T2 is the number
of ticks counted of the known timebase as shown in the following figure. The frequency
measured is fx = fk * (T1/T2).
cRIO-904x User Manual | © National Instruments | 87
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