Chapter 10 High-Precision Timing (Counters/Timers)

Measuring the Period and Frequency of Low-Frequency Signals

How you measure the period and frequency of low-frequency signals (lower than 1000 Hz) depends on which counter chip is on your DAQ device. If you are not sure which chip your DAQ device has, refer to your hardware documentation.

TIO-ASIC, DAQ-STC

Open the Measure Period (DAQ-STC) and Measure Period (NI-TIO) VIs, available in the examples\daq\counter library, and study the block diagrams.

You connect your signal of unknown period to the GATE of counter. The counter measures the period between successive rising edges of your TTL signal by counting the number of internal timebase cycles that occur during the period. The period is the count divided by the timebase. The frequency is determined by taking the inverse of the period. You must choose timebase such that the counter does not reach its highest value, or terminal count (TC).

Am9513

Refer to the Measure Period-Easy (9513) VI, in the examples\daq\ counter\Am9513.llb for an example of how to use the Easy VI, Measure Pulse Width or Period, available on the Functions»

Data Acquisition»Counter palette.

You connect your signal of unknown period to the GATE of counter. The counter measures the period between successive rising edges of your TTL signal by counting the number of internal timebase cycles that occur during the period. The period is the count divided by the timebase. The frequency is determined by taking the inverse of the period. The valid? output indicates if the period was measured without overflow. Overflow occurs when the counter reaches its highest value, or terminal count (TC). You must choose timebase such that it does not reach TC. With a timebase of 1 MHz, the Am9513 can measure a period up to 65 ms. With a timebase of 100 Hz, you can measure a period up to 655 seconds.