The RF Engineer's Essential Guide to Frequency Counters

Frequency counters must be an essential instrument on every RF engineer and precision electronic engineer's desk. In this article, learn:

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• The necessity of a frequency counter
• The different types of counters
• The industries that use them
• Their construction and working principle
• Tips to select and use them effectively

What Is a Frequency Counter?

A frequency counter accurately measures the frequencies of signals whose waveforms are sinusoidal, square, triangle, sawtooth, or other regular, repetitive patterns.

These instruments can measure frequencies and timing characteristics of both analog and digital signals.

Do You Need Frequency Counters? Frequency Counters vs. Oscilloscopes and Other Devices

Other instruments like oscilloscopes, spectrum analyzers, and frequency meters (like old analog-style needle gauges) can also measure frequency.

However, what sets frequency counters apart is their extremely high accuracy and precision.

While other instruments use software to measure frequency and other characteristics from digitized data, frequency counters use carefully engineered hardware circuits to precisely measure them directly on the input signal.

In addition, remember the following while choosing an instrument:

• They offer high precision and resolution even at very high gigahertz frequencies.
• They're better for continuous wave signals while software-based measurements work better for pulse signals.
• You can reliably use them for long-term frequency measurements.
• They are simpler and faster to use compared to oscilloscopes or spectrum analyzers.

What Are the Different Types of Frequency Counters?

Frequency counters come with a wide variety of capabilities and form factors. We first look at the different types based on capabilities.

Types of Frequency Counters by Capability

The primary method of categorization is based on their target industries and capabilities.

1. Universal Counters

Instead of pure frequency counters, many customers prefer all-in-one universal counters that support common ancillary measurement capabilities, like signal timing, along with frequency measurement. They can typically measure:

• Frequency: This is the number of full cycles completed in unit time.
• Period: This is the time interval of a full cycle of the signal.
• Frequency ratio: It measures the frequency ratio of two input signals.
• Time interval: This measures the time delay between events of two input signals.
• Time interval error: This measures the variation in timing relative to an expected or ideal value.
• Pulse width: This measures the time taken by a pulse, typically the high of a digital signal.
• Rise and fall times: These are the times taken by a digital signal to go from low to high states and back.
• Phase angle: This measures the phase difference between two input signals.
• Duty cycle: This is the percentage of a full cycle for which the signal, typically a square wave, is high.
• Voltages: The maximum, minimum, and peak-to-peak voltages of a signal are reported.
• Signal power: Some universal counters include built-in power meters to report the signal power.

In addition, these instruments support higher-level statistical and analytical functions like:

• Histogram plots to view the distribution of frequency measurements and detect stability or noise issues
• Trend plots of frequencies to detect drift and similar problems
• Statistics of frequency measurements to detect jitter
• Totalized metrics of frequency readings
• Allan deviation, a measure of frequency stability

2. Radio Frequency Counters

Radio frequency (RF) counters, or microwave frequency counters, are heavily used in RF engineering. Their measurable frequencies range from a few kilohertz (kHz) to hundreds of megahertz (MHz) and even tens of gigahertz (GHz). For example, the Keysight Auniversal counter/timer base model supports up to 350 MHz while an advanced model supports up to 15 GHz.

RF counters are built differently because digital circuit elements like flip-flops are too slow to keep up with the high-speed transitions of GHz-range radio signals. So RF counters use prescalers or other frequency downscaling components to first reduce the frequencies of input signals to the MHz range.

3. Counter Timers

These are also called time interval analyzers and are optimized for precise time interval measurements.

4. Modulation Domain Counters

These counters are specialized for analyzing modulated signals where the carrier signal is periodic but that pattern's obfuscated by a frequency modulation scheme.

Types of Frequency Counters Based on Form Factors

Frequency counters can also be classified based on their form factors and interfaces into:

1. Benchtop instruments
2. Handheld instruments
3. Hardware modules that plug into modular test equipment
4. Embedded blocks in application-specific integrated circuits
5. Logic blocks in field-programmable gate arrays

Which Industries Use Frequency Counters?

The different types of frequency counters are used wherever very precise frequency and timing measurements are required, like the industries below.

Electronic Component Manufacturing

Precise frequency and timing measurement are essential for the quality control of a variety of electronic components like:

• Oscillators
• Phase-locked loops
• Clock modules
• Analog to digital converters
• Timer circuits
• Frequency dividers

Frequency counters are used as testers in these industries.

Defense

RF counters are used for the quality control of critical radar and sonar systems.

Global Navigation Satellite Systems (GNSS)

GNSS constellations like the Global Positioning System heavily rely on precision in timing and frequency characteristics.

Telecommunication Systems

Similarly, frequency counters are used to achieve precise timing and frequency characteristics in wireless telecommunication equipment like 5G base stations.

How Does a Frequency Counter Measure Frequency?

Frequency measures the number of complete cycles of a signal in unit time. There are two frequency measurement approaches.

The first, called the direct frequency measurement technique, just counts the number of cycles completed in a fixed time interval called the gate time. This works as follows:

• A very precise clock called a timebase does the gating, which is the process of measuring the number of cycles completed during a fixed gate time interval.
• It reports the frequency as the number of cycles measured divided by the gate time.

Some disadvantages are obvious here:

• The gate time need not coincide with the signal's cycle, leading to inaccurate measurement of the number of cycles.
• For low-frequency signals, each cycle may take a long time. The counter has to set a long gate time to measure the cycles accurately.

An alternative and better approach is the reciprocal measurement technique. Instead of counting the number of cycles completed in a fixed gate time:

• It waits for a fixed (internally configured) number of cycles to complete.
• It precisely measures the time taken for these many cycles to complete, using a high-precision internal or external timebase. That time is its gate time.
• It calculates frequency by dividing the number of cycles by the measured gate time; hence the name reciprocal because it's measuring the time per cycle and then taking its inverse to calculate frequency.

The reciprocal technique's advantages include:

• The same number of digits of frequency resolution regardless of the input frequency, be it hertz or gigahertz
• Very accurate at very low and very high frequencies

However, reciprocal frequency counters are more expensive to build.

Contact us to discuss your requirements of Portable Frequency Counter Dealers. Our experienced sales team can help you identify the options that best suit your needs.

What Are the Key Components of a Frequency Counter?

The precision of frequency counters is because of the components that constitute them as explained in the sections below.

1. Precision Timebase

The timebase is an internal clock and gating circuit used for precise measurement of cycles or timing based on the measurement technique.

It's typically one of the following crystal oscillator circuits:

• Room temperature crystal oscillator: It uses a simple quartz crystal that maintains a fairly stable frequency at typical room temperatures and is suitable for non-critical uses.
• Temperature-compensated crystal oscillator: It's able to maintain stable frequencies by compensating for temperature fluctuations due to operating conditions or environmental changes.
• Oven-controlled crystal oscillator (OCXO): Also called a high-stability oven timebase, the oscillator is housed in a mini-oven that maintains a constant temperature to achieve extremely high stability and accuracy.

Timebases are prone to timebase errors from aging and temperature effects. To maintain the highest stability and accuracy over time, ensure meticulous maintenance and regular calibration.

2. Input Amplifier, Attenuator, and Prescaler

These components modify the input signal to match the counter's circuitry. They boost weak signals, reduce strong signals, block high-frequency noise, and provide impedance matching.

RF counters must ensure that the high input signal frequencies are downscaled so that the digital circuits downstream can keep up with their transitions. So they use high-speed prescalers, transfer oscillators, or heterodyne converters.

3. Trigger or Comparator Circuit

The trigger circuit determines exactly when each cycle of the input wave starts and ends.

4. Counter Circuit

This is where the actual counting of cycles and measurement of time occurs. It consists of digital logic circuits with components like flip-flops and NAND gates that are actuated by the input signal whenever it crosses the trigger levels.

5. Microprocessor

A microprocessor:

• Controls the frequency counter's operations
• Stores and transfers data
• Performs statistical and other analyses
• Communicates with users or other devices

6. Power Supply

A frequency counter uses a high-quality power supply with very low ripple and noise.

7. Interface Circuit and External Interfaces to Other Equipment

Most modern frequency counters can interface with personal computers and automated test equipment through the following input/output interfaces:

• General Purpose Interface Bus (GPIB): This is a hardware and data exchange standard for attaching programmable instruments to a computer. It includes the Standard Commands for Programmable Instrumentation (SCPI) standard that specifies a set of commands and semantics for each instrument class.
• Universal serial bus (USB): Frequency counters can transfer the measured data to USB storage devices.
• RS-232: This serial bus can be used to transfer data to a computer in custom binary formats.
• LAN extensions for instruments (LXI-C): This is an SCPI over ethernet protocol that enables browser-based control of instruments.

8. Display Circuit

If the frequency counter is a standalone instrument, it's typically equipped with a liquid crystal display (LCD) and a display circuit to show measurements, analysis results, and graphical outputs like histograms.

What Are the Key Specifications of Frequency Counters?

Review these key specifications when you choose a frequency counter:

• Number of input channels
• Frequency range and maximum frequency of each channel
• Frequency resolution, which is the precision of the frequency expressed as the number of digits
• Time resolution, which is the smallest measurable time difference between two events in a single shot and expressed in nanoseconds or picoseconds
• Phase resolution in degrees
• Timebase error, timebase aging coefficient in parts per year or parts per million, and the timebase uncertainty
• Data transfer rate to external interfaces

Tips for Achieving High Accuracy and Precision

Follow our tips for accurate measurements usingfrequency counters:

1. Avoid automatic arming mode because it's the least accurate. Both the time resolution and the systematic uncertainty, which contribute to the overall measurement error, can be improved by using one of the other arming modes ' external, time, or digit.
2. Keep the timebase warm by always having it powered up. This gives an order of magnitude improvement in the annual aging rate compared to regularly powering down.
3. Use the best available timebase. A high-stability oven timebase doesn't require frequent calibration, but it's recommended for less stable ones.
4. Pay attention to trigger levels and trigger level timing errors, especially when the signal is noisy.
5. Lock all timebases to a single clock to avoid jitter due to independent timebases.
6. Set the trigger level so that it intersects the signal at the point of maximum slew rate to minimize the amount of time taken for the trigger condition.
7. Don't set the trigger band so narrowly that any noise gets counted as zero crossings.

We implement all these measures in our frequency counters products described next.

Keysight Frequency Counters

Keysight's Series RF and universalfrequency counters offer the following state-of-the-art features:

• One to three input channels
• High-quality temperature-compensated timebases with just one part per million annual aging
• Optional high-stability oven timebases with as low as 50 parts per billion annual aging
• Extremely high, 12 digits frequency resolution
• Single-shot time resolution as low as 20 picoseconds
• Built-in advanced data analysis and graphing capabilities

Learn more through our Frequency Counter page or datasheets of our three models for very detailed specifications and accuracy calculations:

• A 350MHz universalfrequency counter
• A 350MHz universalfrequency counter
• A 350MHz RF counter

Achieve Unbeatable Accuracy With Keysight Frequency Counters

Frequency Counter Recommendation

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Post by jimfowler » Thu May 23, 3:11 pm

Search no worky, so I'm probably asking something that's been answered before but what are your recommendations for accurate-enough-for-analog-oscillator-calibration frequency counters? Unfortunately, neither my fluke handheld nor my siglent scope go low enough (sub 10 Hz) to accurately set lowest frequencies on the things I'm building.

- Jim

Re: Frequency Counter Recommendation

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Post by Graham Hinton » Thu May 23, 3:55 pm

jimfowler wrote:Search no worky, so I'm probably asking something that's been answered before but what are your recommendations for accurate-enough-for-analog-oscillator-calibration frequency counters? Unfortunately, neither my fluke handheld nor my siglent scope go low enough (sub 10 Hz) to accurately set lowest frequencies on the things I'm building.

You shouldn't be looking for a frequency counter/meter, but a universal counter instead. The difference is in the measurement technique and is easy to spot. Frequency meters count the input frequency in a gate period (usually 1 or 10 seconds) and display the result which is often jerky. They can't display fractional counts apart from by using longer gate times and moving the decimal place which makes them slower to update.

Universal counters use a reciprocal technique. They measure the period to microsecond accuracy and then compute the frequency and display that, often with a running average. The display is to many significant figures as is needed for tuning measurements. Even though A = 440Hz you still need to know that it is 440.Hz and not 440Hz +/-1Hz.

Many multimeters and scopes have this built in now or look for an old HP/Agilent Universal Counter. Just look for the subtle difference in displaying the frequency. Avoid anything with Nixie tubes (however cool you think they are) they went out years before reciprocal counting came in.

For very low frequencies you would measure the period anyway.

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Post by jimfowler » Thu May 23, 4:45 pm

Thanks for the tips and advice. I always appreciate your input, Graham.

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Post by Noodle Twister » Thu May 23, 6:58 pm

'Many frequency counters have precise settable gate times during which the number of transitions that take place on the input are counted. So if the meter reports that 10 transitions took place during a one second gate time, we know that the frequency is about 10Hz.

If the meter reports that ten transitions took place during a 10 millisecond gate time then we know the frequency is about 1Hz.

So if you want to look at a frequency that is below 1Hz, get ready to wait while the counter counts the transitions during the successive 10 second gate times. The prize for your patience is 100ms accuracy.'



Quote from Make:analog synthesizers - Ray Wilson.

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Post by Altitude909 » Fri May 24, 6:58 am

Look for an old HP on ebay, should be able to find one ~ $130

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Post by Graham Hinton » Fri May 24, 7:06 am

Noodle Twister wrote:'The prize for your patience is 100ms accuracy.'

That is a booby prize and why you shouldn't believe everything you read on the web.
Frequency meters are fine above 10kHz, so-so for audio and useless for tuning calibration.

This is one Universal Counter that can be found for around £100:



Beware of the almost identical looking HPA which is a frequency meter.

Re: Frequency Counter Recommendation

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Post by e-grad » Fri May 29, 9:07 am

Thanks to Graham I've learned that I will not need a freq. counter but an universal counter.

However, unforetuntely some freq. counters are labeled as universal counters. For example the HPA - which is to avoid - is described as universal counter by Keysight themselves:
A Universal counter

Sadly the recommended HPA is not that cheap in my part of the world. My budget is around 100 EUR.

Any other universal counter to recommend?

Does the HPB feat. reciprocal technique? Avatar: A lusona by the Chokwe people.

elektrograd

Re: Frequency Counter Recommendation

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Post by EATyourGUITAR » Fri May 29, 1:27 pm

Frequency counters are more useful when you need to calibrate a counting machine. We had something at work that takes current measurements and integrates them in analog. Then it sends a pulse when that cap is fully charged and it resets. The cmos part was an up counter that goes to or pulses. This allowed us to calibrate the machine against a reference current and stop watch. The measurement was not done in analog. We counted to to see if it was . By this technique, we were able to calibrate a current integral measurement device without ever taking current measurements. You probably don't have a use case that specific in synthesizers. The level of precision can vary a lot but I think there are multimeters that do continuous averaging even though they advertise it as a frequency counter. My cheap digital oscilloscope has this fancy version of a frequency counter but only 440.0Hz decimals. YMMV

Re:

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Post by e-grad » Sat May 30, 5:23 am

Thanks for your swift respond!

Still not sure which device to acquire. If I consider Graham's hint:

Graham Hinton wrote: 'Fri May 24, 7:06 amFrequency meters are fine above 10kHz, so-so for audio and useless for tuning calibration.

I think I should be fine for tuning with an ordninary freq. counter. Eventhough a freq. counter easily covers the audio range I should go for an univ. counter. to calibrate an VCO.

If it comes to tuning in microscales such as quarter tone scales even eight tone scales I would need the accuracy of an univ. counter.

Are both of my assumptions right? Avatar: A lusona by the Chokwe people.

elektrograd

Re: Frequency Counter Recommendation

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Post by EATyourGUITAR » Sat May 30, 7:05 am

you actually don't need any of that at all if you have a reference oscillator 440.000Hz. you can use your ears. save $.

Re: Frequency Counter Recommendation

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Post by Graham Hinton » Sat May 30, 7:18 am

For more Portable Frequency Counter Factoryinformation, please contact us. We will provide professional answers.

e-grad wrote: 'Fri May 29, 9:07 am However, unforetuntely some freq. counters are labeled as universal counters. For example the HPA - which is to avoid - is described as universal counter by Keysight themselves:

Keysight have lost the plot.

If it can display an audio frequency to many decimal places it's reciprocal counting.

Sadly the recommended HPA is not that cheap in my part of the world. My budget is around 100 EUR.

https://www.ebay.de/itm/HPA-Universal-Counter-Timer-100MHz-Frequency

Any other universal counter to recommend?

Lots of scopes and DMMs have this feature now.

Does the HPB feat. reciprocal technique?

I think it does, but it's massive, look at the depth in the pictures.

I think I should be fine for tuning with an ordninary freq. counter. Eventhough a freq. counter easily covers the audio range I should go for an univ. counter. to calibrate an VCO.

If it comes to tuning in microscales such as quarter tone scales even eight tone scales I would need the accuracy of an univ. counter.

Are both of my assumptions right?

Frequency meters are always +/-1Hz which is useless for tuning. If you only want tuning get a tuning meter. For calibration I use both a tuning meter (Korg DTR-2) and a universal counter (HPA). Tuning meters don't work above about 4kHz.