Choosing a Counter Frequency Measurement Method

Overview

There are five methods to perform frequency or period measurements. The best method depends on several factors including the expected frequency of the signal to measure, the desired accuracy, how many counters are available, how long the measurement can take and even, whether the device supports a specific measurement method. 

Contents

Low Frequency with 1 Counter

Measures one period of the input signal using a known timebase. The frequency of the signal is the inverse of its period. Low frequency with 1 counter is a good method for many applications. It has the lowest measurement time but also the lowest accuracy, which further decreases as the signal frequency increases.

High Frequency with 2 Counters

With the two counter, high frequency method, the second counter provides a known measurement time. The 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. This method is accurate for high frequency signals. However, the accuracy decreases as the frequency of the signal to measure decreases.

Large Frequency Range with 2 Counters

The two counter, larger range measurement is the same as a one counter measurement, but now the user has an integer divide down of the signal. The second counter uses the input signal to create pulse that is measured with a known timebase. Results are an average of the divisor set in the VI. This method measures high and low frequency signals accurately. However, it requires two counters and it has a variable sample time and variable error % dependent on the input signal.

Sample Clocked

For each sample clock period, an embedded counter counts the signal to measure and the primary counter counts a known frequency timebase. The results can either be a single frequency measurement per sample clock or an average between sample clocks. This method is only supported by CompactDAQ, X Series, USB-powered M Series, NI 6612/6614 and NI 6738/6739 devices. Refer to Sample Clock Timing Support for Time-Based Measurements for more information. This method uses only one counter and is good for high and low frequencies and synchronizing with other measurements. Unlike all other methods, the measurement time and error do not change with the input signal frequency.

Dynamic Averaging

This method 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. Only the NI 9361 C Series Counter Input Modules and the NI 9326 Frequency Input Modules supports this method. The NI 9361 only supports this method. Dynamic Averaging provides the best tradeoff when it comes to accuracy versus latency for a quickly changing signal. 

Key Factors to Consider

You need to first eliminate any options that are not compatible with your hardware and if you have enough counters for your application, you simply need to calculate the error for the expected frequency for each available option, considering the measurement time (if applicable). The best method will be the one with the lowest error at a given frequency and measurement time (if applicable). 

If you want to find a numeric value for the error within each method, the Quantization Error topic from the NI-DAQmx Help shows the error at different input frequencies for the Low Frequency with 1 Counter, High Frequency with 2 Counters, Large Frequency Range with 2 Counters and Dynamic Averaging methods for a quick comparison. You can use the formulas included in the help topic or refer to the User Manual from your device for further information. 

It is also possible to use multiple counter tasks if your card contains multiple counters. This can allow you to meet the needs for response time and accuracy at different frequencies. 

Note: The Sample Clocked method is not included in the Quantization Error topic because it does not depend on the input frequency. If you need to compare a Sample Clocked Counter Frequency measurement with any other method, refer to the User Manual from your device to calculate the Sample Clocked method error.