High Frequency Two-Counter Measurement Method

Use this high-frequency measurement method if you measure a digital frequency or period of a signal with a high frequency component. To perform measurements using this method in NI-DAQmx, a paired counter generates a pulse train with a period specified using the measurement time attribute/property. The measurement time is generally much larger than the period of the input signal being measured to reduce quantization error. However, the measurement time must be small enough to keep the counter from rolling over. The measurement counter counts the number of periods of the input signal that occur during the measurement time, averages the results, and returns the averaged value in the Read function/VI. The value returned is calculated as follows:

Period (in seconds) = Measurement Time / Number of Periods Counted

Frequency (in Hz) = Number of Periods Counted / Measurement Time

To determine if you should use the high-frequency measurement method, refer to the quantization error tables. If the quantization error listed for the one-counter method is too high, use the high-frequency measurement method instead.