© National Instruments | 5-13NI cDAQ-9181/9184/9188/9191 User ManualYou can route the signal to measure to the Source input of Counter 0, as shown in Figure 5-14.Assume this signal to measure has frequency fx. NI-DAQmx automatically configures Counter0 to generate a single pulse that is the width of N periods of the source input signal.Figure 5-14. Large Range of Frequencies with Two CountersNext, route the Counter 0 Internal Output signal to the Gate input of Counter 1. You can route asignal of known frequency (fk) to the Counter 1 Source input. Configure Counter 1 to performa single pulse-width measurement. Suppose the result is that the pulse width is J periods of thefk clock.From Counter 0, the length of the pulse is N/fx. From Counter 1, the length of the same pulse isJ/fk. Therefore, the frequency of fx is given by fx = fk * (N/J).Sample Clocked Buffered Frequency MeasurementSample clocked buffered point frequency measurements can either be a single frequencymeasurement or an average between sample clocks. Use CI.Freq.EnableAveraging to set thebehavior. For buffered frequency, the default is True.A sample clocked buffered frequency measurement with CI.Freq.EnableAveraging set to Trueuses the embedded counter and a sample clock to perform a frequency measurement. For eachsample clock period, the embedded counter counts the signal to measure (fx) and the primarycounter counts the internal time-base of a known frequency (fk). Suppose T1 is the number ofticks of the unknown signal counted between sample clocks and T2 is the number of tickscounted of the known timebase as shown in Figure 5-15. The frequency measured is:fx = fk * (T1/T2)Source OutCounter 0SourceGateOutCounter 1Signal toMeasure (fx)Signal of KnownFrequency (fk)CTR_0_SOURCE(Signal to Measure)CTR_0_OUT(CTR_1_GATE)CTR_1_SOURCEIntervalto Measure0 1 2 3 … N
