© National Instruments | 5-15NI cDAQ-9138/9139 User ManualChoosing a Method for Measuring FrequencyThe best method to measure frequency depends on several factors including the expectedfrequency of the signal to measure, the desired accuracy, how many counters are available, andhow long the measurement can take. For all frequency measurement methods, assume thefollowing:fx is the frequency to be measured if no errorfk is the known source or gate frequencymeasurement time (T) is the time it takes to measure a single sampleDivide down (N) is the integer to divide down measured frequency, only used inlarge range two countersfs is the sample clock rate, only used in sample clocked frequencymeasurementsHere is how these variables apply to each method, summarized in Table 5-2.• One counter—With one counter measurements, a known timebase is used for the sourcefrequency (fk). The measurement time is the period of the frequency to be measured, or1/fx.• Two counter high frequency—With the two counter high frequency method, the secondcounter provides a known measurement time. The gate frequency equals 1/measurementtime.• Two counter large range—The two counter larger range measurement is the same as aone counter measurement, but now the user has an integer divide down of the signal. Aninternal timebase is still used for the source frequency (fk), but the divide down means thatthe measurement time is the period of the divided down signal, or N/fx where N is the dividedown.• Sample clocked—For sample clocked frequency measurements, a known timebase iscounted for the source frequency (fk). The measurement time is the period of the sampleclock (fs).
