Frequency counters are widely used to accurately measure the frequency of repetitive signals. There are two basic types of frequency counters:
- Direct counting frequency counters
- Reciprocal counting frequency counters
Understanding the effects of these two different counters will help you choose the best counter for your needs and use it correctly. Today, we’ll look at the basics of direct counters and reciprocal counters.
How a Direct Counting Frequency Counter Works
Direct counters simply count cycles of a signal over a known period of time. This period is known as the gate time. The resulting count is sent directly to the counter’s readout for display. This method is simple and inexpensive. But it means that a direct counter’s resolution is fixed in Hertz and the count accuracy is lower than a reciprocal frequency counter.
For example, with a 1 second gate time, the lowest frequency the counter can detect is 1 Hz (since 1 cycle of the signal in 1 second is 1 Hz).
Thus, if you are measuring a 10 Hz signal, the best resolution you can expect for a 1 second gate time is 1 Hz (or 2 display digits). For a 1 kHz signal and 1 second gate, you get 4 digits. For a 100-kHz signal, 6 digits, and so on, as shown in Figure 1 below:
Figure 1. The number of digits displayed by a direct counter versus frequency (for a 1 second gate time).
How a Reciprocal Counting Frequency Counter Works
Reciprocal counters measure the input signal’s period and then reciprocate it to get frequency. Because of this architecture, the counter’s resolution is always the full number of display digits.
In other words, a reciprocal frequency counter will always have same number of digits of resolution regardless of the input frequency. You’ll see the resolution of a reciprocal counter specified in terms of the number of digits for a specific gate time, such as “10 digits per second.”
By looking at the frequency resolution specification, you can determine whether a counter is a direct counter or reciprocal counter. If it specifies resolution in Hertz, it’s a direct counter. If it specifies resolution in digits-per-second, it’s a reciprocal counter.
Figure 2 compares the resolution of direct and reciprocal counters. In the lower frequency spectrum, reciprocal counters have a substantial advantage over direct counters. We can see that the reciprocal counter has a constant resolution, whereas the direct counter has less resolution for lower frequencies.
Figure 2. Comparing resolution for direct and reciprocal counters (for a 1 second gate time).
As an example, at 1 kHz, a direct counter gives a resolution of 1 Hz (4 digits). A 10 digit/second reciprocal counter gives a resolution of 1 μHz (10 digits).
If precision resolution is not a priority, a reciprocal counter still offers a significant speed advantage. The reciprocal counter will give 1 mHz resolution in 1 ms, while a direct counter needs a full second to give you just 1 Hz resolution (Figure 3).
Figure 3. The gate times needed to yield various resolutions with a 10 digits/second reciprocal counter.
Should You Use a Direct or Reciprocal Frequency Counter?
The choice comes down to cost versus performance. If your resolution requirements are flexible and you aren’t too concerned with speed, a direct counter is the economical choice. However, many cases require a reciprocal counter for faster, higher resolution measurements. Reciprocal counters also offer continuously adjustable gate times (not just decade steps), so you can get the resolution you need within the minimum amount of time.
To learn more, download the 10 Hints for Getting the Most from Your Frequency Counter application note.