Period-averaging measurements

Use PeriodAvg() to measure the period (in microseconds) or the frequency (in Hz) of a signal on a single-ended channel. For these measurements, the data logger uses a high-frequency digital clock to measure time differences between signal transitions, whereas pulse-count measurements simply accumulate the number of counts. As a result, period-average measurements offer much better frequency resolution per measurement interval than pulse-count measurements. See also Pulse measurements.

U terminals on the data logger are configurable for measuring the period of a signal.

The measurement is performed as follows: low-level signals are amplified prior to a voltage comparator. The internal voltage comparator is referenced to the programmed threshold. The threshold parameter allows referencing the internal voltage comparator to voltages other than 0 V. For example, a threshold of 2500 mV allows a 0 to 5 VDC digital signal to be sensed by the internal comparator without the need for additional input conditioning circuitry. The threshold allows direct connection of standard digital signals, but it is not recommended for small-amplitude sensor signals.

A threshold other than zero results in offset voltage drift, limited accuracy (approximately ±10 mV) and limited resolution (approximately 1.2 mV).

TIP:

Both pulse count and period-average measurements are used to measure frequency output sensors. However, their measurement methods are different. Pulse count measurements use dedicated hardware - pulse count accumulators, which are always monitoring the input signal, even when the data logger is between program scans. In contrast, period-average measurements use program instructions that only monitor the input signal during a program scan. Consequently, pulse count scans can occur less frequently than period-average scans. Pulse counters may be more susceptible to low-frequency noise because they are always "listening", whereas period-averaging measurements may filter the noise by reason of being "asleep" most of the time.

Pulse count measurements are not appropriate for sensors that are powered off between scans, whereas period-average measurements work well since they can be placed in the scan to execute only when the sensor is powered and transmitting the signal.