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Large Range of Frequencies with Two Counters

    Last Modified: February 26, 2018

    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.

    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.

    Figure 1. Large Range of Frequencies with Two Counters

    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).

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