Operational "How to" Guides

Summary

How Does a Frequency Counter Work?

Description

Frequency counters are widely used to accurately measure the frequency of repetitive signals. There are two basic types of frequency counters:

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• 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.

See Also

What is a Frequency Counter

Frequency counters are test instruments used to provide very accurate measurements of the frequency of a signal.

Frequency Counter Timer Tutorial Includes:
Frequency counter     How does a frequency counter work     Interval timer     How to use a counter     Specifications     Accuracy    

Frequency counters are test instruments used in many applications associated with radio frequency engineering to measure the frequency of signals very accurately.

These frequency counters and counter timers are widely used within a variety of areas of electronics test to measure the frequency of repetitive signals, and also for measuring the time between edges on digital signals.

Whilst the actual requirements and applications for RF frequency counters and timers are different, they use the same basic circuitry, with some simple internal reconfiguration and as a result sometimes RF frequency counters are also able to act as timers. Typically the very high RF frequency counters will not incorporate the timer capability.

These test instruments are widely available, and can often be bought for very competitive prices. However, remember that cost is not everything, and a low cost item of test equipment may give poor performance.

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Introduction of the digital frequency couner

Prior to the introduction of the digital frequency counter, the measurement of frequencies was significantly more involved, and must less accurate. The RF test equipment used was far more rudimentary.

A number of approaches were used. The most simple was called an absorption wavemeter. This piece of test equipment was just a tuned circuit onto which was connected a diode rectifier and a meter. Essentially this detected the high power transmissions and gave a broad indication of their frequency as the meter deflected.

If more accurate frequency measurements were required, a unit called a heterodyne frequency meter or wavemeter was used.

This is a form is test equipment that uses a crystal oscillator to provide a calibration signal - typically with 1 MHz and 100 kHz crystals that enable the internal oscillator to be calibrated every 100 kHz.

The wave meter can be used to create a signal that can check the frequency of a receiver. Alternatively when used with a transmitter, the transmitted signal will be picked up by the wave meter and heard as a heterodyne within the wavemeter headphones. Again the accurate oscillator used in the wavemeter ensures the frequency of the signal is accurately known.

The first frequency counters introduced used nixie tubes as indicators, but were able to provide a very much faster means of frequency measurement than was previously possible with the heterodyne frequency meters.

Digital frequency counters quickly developed as technology improved increasing the maximum frequency of operation, improving he triggering, and providing more easily visible forms of display.

What is an RF frequency counter: basics

In essence a frequency counter is an electronics test instrument that operates by counting the number of times a signal passes a give voltage point - trigger point - in a given time.

Some frequency counters will have trigger points that can be set, but most automatically set the trigger - often around the zero crossing point.

To illustrate the operation, if the time for which the frequency counter is set to count is a second, i.e. a gate time of a second, and the waveform crosses the trigger point a hundred times, there will be a hundred repetitions of the waveform in a second, i.e. its frequency is 100 Hertz.

If the same waveform was used, but the gate time is reduced to a tenth of a second, then only ten repetitions would be seen. The circuitry can easily accommodate this and the circuit can deduce that in a tenth of a second ten repetitions are seen, then the waveform has a frequency of 100 Hz.

There is a balance between getting an accurate count and the length of time of the gate. With a tenth of a second gate time and a 100Hz signal, only ten crossings will be counted, whereas with a gate time of a second 100 crossings will be counted. Dependent upon where the gate time falls with respect to the incoming signal, it can be seen that the longer gate time will be more accurate.

The issue with the longer gate time is that the update rate is not as fast, but in many situations this may not be an issue.

The length of the gate time is critical. Normally the signal for the gate is taken from a crystal controlled oscillator of some form to ensure an accurate time. Often TCXOs (temperature controlled crystal oscillators), or OCXOs (oven controlled crystal oscillators) are used to ensure the best accuracy.

Another issue with these frequency counters can be that noise on the signal will cause false counts to be registered. Often careful design of the input circuitry can help ensure that these false counts only rarely occur.

RF frequency counter applications

RF frequency counters are used in very many applications where the frequencies of radio frequency or even audio frequency signals are to be measured. Some applications may include:

• Measuring the frequency of a transmitter carrier.
• Measuring the frequency of an oscillator in a circuit.
• Measure the frequency of a signal on a line
• Any application where the frequency of a steady repetitive signal needs to be measured.

To be able to measure the frequency of an RF signal using a frequency counter there are a few prerequisites.

• Frequency must be steady, i.e. not varying.
• The signal should not have modulation applied as this will prevent proper counting.
• Signal must have sufficient amplitude ' typically signals over about half a volt are suitable.
• Signals should not be so large that they overload the input - check the manufacturer's specification if in doubt.

Types of frequency counter

Although frequency counters may appear to be very straightforward, there are several different types. Each one is applicable to its own situation and capable of providing very good service.

• Bench frequency counter:   The bench frequency counter is the most commonly used format for this type of electronics test equipment.
• PXI frequency counter :   Along with many other items of test equipment, it is possible to obtain frequency counters in a PXI format. The PXI system provides a rack system for test and control systems. Test instruments in the form of a PXI card, and int his case a PXI frequency counter or timer can be slotted into the PXI rack. In this way a PXI frequency counter could be part of a sophisticated overall test system.
• Handheld frequency counter:   Frequency counter technology does not need to be large and several handheld frequency counters are available.
• Frequency counter in a digital multimeter:   Some digital multimeters incorporate a frequency counter function. When included in a DMM, normally it will be relatively basic. Typically no coaxial RF connection is available and this will mean that the test probes will need to be used and this may result in stray pick up and false count triggers which will make the reading less accurate. Additionally there will not be an accurate time base within the DMM, and this will also mean that results are not particularly accurate. That said, they will be more than adequate for many readings, especially when fault finding, or generally wanting to look at general readings of frequency.
• Panel meter:   Frequency counters and timers are available in panel mount modules. These can be incorporated into larger items of equipment where they can be used for counting frequency or time intervals. These frequency counter timers can be obtained relatively cheaply.

Whatever the actual format of the PXI frequency counter, the same basic techniques are used, and the frequency counter timer will operate in basically the same way.

RF frequency counters are a widely used piece of electronics test instrumentation. They are used for many RF measurements. Although Spectrum analyzers are also able to make frequency measurements, and todays analyzers can take very accurate readings, RF frequency counters are relatively cheap and provide measurements that are equally accurate or more accurate. They are widely used as bench test equipment.

Counter timers are also widely used test instruments, used for measuring intervals, and they can be used for very accurate measurements.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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