The dynamic averaging method for frequency and period measurements automatically configures the counter settings based on the range of frequencies to be measured before the start of an acquisition. During the acquisition, the counter dynamically adjusts the number of periods that are averaged to balance measurement accuracy and latency. The NI 9361 uses this method.
To ensure the correct behavior of the dynamic averaging method, the minimum and maximum values on the DAQmx Create Virtual Channel VI must be configured before the start of an acquisition. The divisor, measurement time and maximum measurable period will be automatically calculated and configured based on the input minimum and maximum frequency, and sample clock timing. The input divisor and measurement time settings wired into the DAQmx Create Virtual Channel VI are ignored.
During an acquisition, the dynamic averaging method will average between 1 period and divisor periods of the input signal to determine the input signal's frequency. The measurement time property sets an upper limit to the amount of time that is used for this averaging. As a result, the dynamic averaging method balances measurement accuracy vs. measurement latency throughout the input signal range as shown in the following table.
|Signal Frequency||Counter Behavior|
(Divisor*Input Signal Period < Measurement Time)
|Measures divisor periods of signal.|
(Input Signal Period < Measurement Time < Divisor*Input Signal Period)
|Measures between 1 and Divisor periods of the signal, depending on the number of periods that fit within the Measurement Time.|
(Input Signal Period >= Measurement Time)
|Measure 1 period of signal.|
| Very Low
(Input Signal Period > Maximum Measurable Period)
|Input signal is slower than the maximum measurable period. 0 Hz returned.|
The measurement time and divisor can also be set manually using the property node before the acquisition starts if the auto measurement settings are not desired. The measurement time or divisor can be disabled by setting it to zero.
|0||1||Measure 1 period of the input signal, similar to the 1 Counter (Low Frequency) method.|
|Input measurement time||0||Counts the number of periods of the input signal that occur during input measurement time, similar to the 2 Counters (High Frequency) method.|
|0||Input divisor||Counts how long it takes for the input divisor periods of the signal to elapse, similar to the 2 Counters (Large Range) method.|
You can view examples of the dynamic averaging method in the NI 9361 datasheet, which you can find at ni.com/manuals.