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All Places > Keysight Blogs > General Electronics Measurement > Blog > Author: Seok Jin Wong

Traditionally, digital multimeters (DMMs) have been single-measurement instruments. When engineers want to measure more than one parameter on their signal, they need to have two multimeters to measure two different measurements at the same time. This is not time and cost efficient.

With the right architecture and design, DMMs can make multiple measurements. This advanced feature can help you save cost and analysis time and finish your analysis faster!


Two measurements in a single screen

Secondary measurements are defined as auxiliary measurements that augment information provided by a main primary measurement function. Depending on the function, you can measure complementary data that traditionally would have taken two different operations to acquire. The table below illustrates all secondary measurement capabilities of the Keysight’s Truevolt DMMs.


Primary measurement function

34460A secondary measurement function

34465A/70A secondary measurement function



ACV, peak, pre-math



DCV, frequency, pre-math

2-wire, 4-wire resistance





ACI, peak, pre-math



DCI, frequency, pre-math



Period, ACV, pre-math



Frequency, ACV, pre-math



Sensor, pre-math



Input/ref, pre-math











An example of a common secondary measurement would be the ability to measure the frequency of an AC signal, as shown in Figure 1.


Figure 1. AC voltage with frequency.


The secondary measurement provides more information than is possible with other digital multimeters because of the advanced secondary features in Keysight’s 34465A and 34470A DMMs. As an example, Figure 2 shows the primary measurement of DC voltage (DCV) with a secondary measurement of AC voltage (ACV). This is an especially important measurement if your signal has both an AC and DC component.


Figure 2. DC voltage (primary) and AC voltage (secondary) measurements.


In DCV mode, there are two additional secondary measurements that can be made to provide insight into your signal: Peak and Pre-Math. The Peak measurement, as shown in Figure 3, keeps track of the minimum and maximum DCV readings read by the DMM.


Figure 3. Peak measurement of DCV.


The Pre-Math is a very valuable measurement because it allows you to see modified readings and raw readings in one screen (Figure 4). You can also modify your primary display by applying useful math functions to your data (e.g., a null value or scaling) or filtering your data (Figure 5). See Adding Math Enables Faster Analysis section for more information on applying a math function.

Once you have applied the desired math function, the secondary display will display the raw reading without the math. This is useful for determining if the applied math is correct and if the readings are within the expected range.


-0.165 DB
Figure 4. A DCV signal with dB scaling with the Pre-Math measurement


Figure 5. A with the null value applied with the raw measurement on the secondary display.


You have seen the benefits of the secondary measurement capability. Below you will see how to do this without changing the instrument’s configuration.


Example 1

A test engineer wants to monitor the temperature inside of an environmental chamber and needs a high level of confidence that the measurements are accurate. A 34465A DMM is selected due to its ability to log data and provide simple trend charts. A 5-KΩ NTC thermistor is used to spot check for accuracy. The engineer notices that the thermistor has a temperature error of a few degrees. To understand the error, the secondary display on the Truevolt DMM is turned on and temperature and resistor readings are read at the same time. According to the datasheet for the thermistor, it should read 25 ºC at 5 KΩ. The engineer’s probe is put inside of a calibrated chamber set to 27 ºC, but the probe reads 25 ºC with a 5-KΩ resistance reading, a two-degree error. After a bit of characterization, the engineer decides that he can simply add an offset value to adjust for the offset of his thermistor.


Example 2

A system designed to apply a linear force to a small structure can provide an oscillating force with an AC signal and a constant force with a DC signal. The system designer wants to keep track of both signals concurrently to characterize how much force is being applied. Using a Truevolt DMM with its ability to make secondary measurements, he can read both the DC and AC components of his control signal at the same time.



Advanced dual-screen measurements not only allow you to get more information concurrently, they also allow you to check your raw data compared to your adjusted measurements. This saves you measurement time.

To learn more, download the Simultaneous Measurements with a Digital Multimeter application brief.

AC RMS is the most useful measurement for real-world waveforms because it does not depend on the shape of the signal. Most of the time, RMS measurement is described as a measure of equivalent heating value with a relationship to the amount of power dissipated by a resistive load driven by the equivalent DC value. For example, a 1Vpk sine wave will deliver the same power to a resistive load as a 0.707Vdc signal. A true RMS reading on a signal will give you a better idea of the effect the signal will have on your circuit.


If an AC RMS reading does not make sense, do not automatically assume there is something wrong with your circuit; the trouble might be with how you made the measurement. Study this list of five considerations that can affect your AC RMS measurement below:


  1. Take note on the measurement scale

Most meters specify AC inputs down to 5 or 10 percent of full scale, some even lower. For maximum accuracy, you need to measure as close to full scale as you can. In some cases, you might need to override auto scaling. Make sure the peak of the signal does not overload and saturate the meter’s input circuitry.

  1. Settling time consideration

RMS measurements require time-averaging over multiple periods of the lowest frequency being measured. Be sure to select your digital multimeter’s appropriate low frequency filter to allow for the fundamental to be captured. The lower the AC filter frequency is, the longer the settling time, and the longer it will take to make the measurement. Consequently, if you are not concerned about low frequencies in a measurement and your DMM has selectable averaging filters, switch to a faster filter.

  1. AC and DC coupling
    It is easy to overlook this simple issue when you are in a hurry. If your meter is AC coupled (or has selectable AC coupling), it inserts a capacitor in series with the input signal that blocks the DC component in your signal. Blocking the DC may not be desirable, depending on the signal and what you are trying to accomplish. If you are expecting to include the DC component, but the meter is AC coupled, the results can be dramatically wrong. As a side note, if you need to measure a small AC signal riding on a large DC offset but your meter doesn’t provide AC + DC directly, you can measure the AC component using AC coupling and measure the DC component separately, then square each
    and take the square root of the sum, sqrt(Vac^2+ Vdc^2).
  2. Low-level measurement errors

When measuring AC voltages less than 100 mV, be aware that these measurements are especially susceptible to errors introduced by extraneous noise sources. An exposed test lead will act as an antenna, and a properly functioning digital multimeter will measure the signals received. The entire measurement path, including the power line, acts as a loop antenna. Circulating currents in the loop will create error voltages across any impedances in series with the DMM’s input. For this reason, apply low-level AC voltages to the digital multimeter through shielded cables, and connect the shield to the input LO terminal. Connect the DMM and the AC source to the same electrical outlet whenever possible, and minimize the area of any ground loops that cannot be avoided.

  1. Bandwidth errors

Signals that are rich in harmonics can produce low-reading measurements if the more significant of these components are not included in the measurement. Check the instrument’s data sheet to find the bandwidth of your multimeter. Then make sure your signals do not exceed it.



Making accurate, true RMS AC measurements with modern digital multimeters is simple and straightforward. However, you need to avoid common traps and pay attention to details. To get accurate results, know your meter and its measurement capabilities.


To learn more, download the Make Better AC RMS Measurements With Your Digital Multimeter application note.

Have you ever encountered a scenario in which an AC voltage signal is measured on an electrical circuit that has been completely disconnected? Isn’t it confusing when voltage is measured in the dummy circuits?


Stray voltage, sometimes referred to as ghost voltage, is a voltage that appears in an electrical conductor such as a wire, even though the wire is disconnected from an electrical circuit. You may spend hours troubleshooting this circuit and end up realizing that it’s a stray voltage, even though all wires are disconnected!


Where do stray voltages come from?

It is very common for electricians and technicians to pull extra wire when facilities or buildings are built and wired. This is just like renovating your house - you will pull extra wire from the conduit for future usage. Normally, these wires are left unconnected. These are the areas where phantom voltage will appear in the circuits.

Wires left unconnected are most likely to be the areas where stray voltage will appear in your circuits. 


Why do stray voltages appear?

Stray voltage readings can be caused by capacitive coupling of energized conductors with nearby unused wire. This capacitance increases as the length of the conductor increases. The longer the wire, the more prevalent a stray voltage.


Current in an active circuit can also trigger a stray voltage reading; the higher the current in the active circuit, the higher the stray voltage. Stray voltage readings caused by active circuits can range from a few volts up to the voltage of the adjacent conductors. It should be noted that according to Underwriters Laboratories Inc. (UL), stray voltage is not real voltage and it cannot cause physical harm to a person. This is because, even though the voltages may be high, the amount of energy stored in the capacitive coupling is very low.


UL also states that care must be taken to ensure that the voltage reading is a stray voltage and not a result of a cable defect or improper installation; as such a situation may result in a shock hazard.


Here is an example that illustrates the overall situation. Imagine that you are installing low voltage lighting in a warehouse office, as shown in Figure 1. The warehouse is equipped with two wires running in parallel to the conduit. One is for light A, which is ON, and the other pair of wires will be used to install a new light using a new expansion cable that runs parallel with light A.


installation in building or facilityFigure 1: Installation of low voltage lighting in a warehouse office.


Before beginning the installation, you check the voltage on the wire using a normal handheld multimeter with high input impedance, and the measurement result shows as 40 volts even though the line is disconnected from the main switch. Now, you suspect that touching conductors has formed a short circuit, causing voltage to leak through the conductor’s insulation. You spend a lot of time troubleshooting and investigating. However, after a thorough investigation, you find that there is no short circuit to ground! The 40 volts displayed on the measurement reading is a phantom voltage reading formed by the unused wire. After all the hard work troubleshooting, you realize you have lost a lot of time troubleshooting a stray voltage.

From this example, we can conclude using a normal handheld digital multimeter to measure such circuit can make it difficult for you to differentiate ghost voltage reading from legitimate readings. Most handheld digital multimeters have high input impedance compared to the impedance of the circuit being measured. The handheld multimeters with high input impedance that is greater than 1 MΩ are designed to place very little load on the circuit under test. In this capacitive coupling situation, a phantom voltage reading is measured by this high input impedance multimeter.


In our example, if a low input impedance multimeter had been used to perform the AC voltage measurement, the electrician would have found virtually zero stray voltage. This is because stray voltage is a physical phenomenon involving very small values of capacitance; it cannot energize a load. Using a multimeter with low input impedance will short out the capacitive coupling effect, while using a high input impedance multimeter will not.

The solution

Certain models of Keysight’s handheld multimeters, for example, the U1242C, have a unique feature: a ZLow function (Figure 2) that allows you to switch from high input impedance mode to low input impedance mode to check for the presence of stray voltages. This solution eliminates the need to carry both a low impedance meter and a high impedance meter.

ZLOW function on U1242C handheld DMMFigure 2. U1242C handheld DMM with ZLow function


The ZLow function acts like a backup voltage indicator and eliminates the need to carry additional tools for troubleshooting. If a real voltage is measured using the ZLow function, the positive temperature coefficient (PTC) thermistor that is designed as an over current protection will ensure the multimeter always operates in high input impedance.

Use a multimeter with flexible input impedance

Now you know how to detect stray voltages efficiently and effectively using a handheld multimeter with low impedance mode. Keysight offers different handheld multimeters that come with ZLow function that can remove stray voltages from your measurements by dissipating the coupling voltage. Use ZLow to reduce the possibility of false readings in areas where the presence of stray voltages is suspected.

To learn more, download the Stray Voltage Testing Made Easy with U1272A application note.
Check out for more info about Keysight’s handheld digital multimeters.


Keysight handheld multimeters

Using a digital multimeter (DMM) to perform measurements is very common in today’s world. Technicians use multimeters for equipment servicing, engineers use multimeters to troubleshoot, students use multimeters for lab research, and so on. Since digital multimeters have many functions, you need to know how to properly use your multimeter. Most multimeter failures are caused by improper use.


In this blog, you will learn four tips to avoid damaging your digital multimeter:


  • Read the warning labels and specifications

Before you begin taking measurements with your digital multimeter, you should read the warning labels and specifications. Do not exceed the values provided in the specifications guide or as indicated by the yellow warning labels on the instrument. Always refer to the specification guide for conditions required to meet the listed specification.


 Example of the 34470A digital multimeter’s rear panel showing warnings on the maximum voltage input and maximum current input.

Figure 1. Example of the 34470A digital multimeter’s rear panel showing warnings on the maximum voltage input and maximum current input.


  •  Ensure proper grounding 

Always use the three-prong AC power cord supplied with the instrument. Proper grounding of the instrument will prevent a build-up of electrostatic charge that may be harmful to the instrument and you. Do not damage the earth-grounding protection by using an extension cable, power cable, or autotransformer without a protective ground conductor. It is good to check the AC power quality and polarity; Typical required AC voltage is 100 V, 120 V, 220 V ± 10%, or 240 V +5%/–10%. Typical expected grounding wire resistance is < 1 Ω; The voltage between neutral and ground line is < 1 V. If needed, you can install uninterruptible power supply [UPS] to power your meter.


  • Avoid overpowering the digital multimeter

You can avoid damaging your digital multimeter by anticipating the signal level you’ll measure and presetting the proper signal range on the DMM. Overpowering the digital multimeter can damage the components inside the meter. For example, Figure 2 shows that the maximum voltage input of a 34461A digital multimeter is 1000VDC and 700 VAC. Before turning on or off the connected equipment or the DUT, reduce the signal level to the minimum safety level. This will prevent unexpected voltage or current swell or sag from affecting the input or output of your instrument.


Keysight’s 34461A digital multimeter front panel.

Figure 2. Keysight’s 34461A digital multimeter front panel.


  • Check for proper temperature and humidity

You need to keep your multimeter in a clean and dry environment. Typical temperature for storage condition is between – 40 and 75 °C; Typical humidity is < 95% RH. The DMM’s optimal operating temperature should be from -5°C to 23° You also need to ensure proper ventilation among racks so the temperature does not go up if all instruments are in use at same time. You should also frequently inspect and clean the cooling vents and fans.


Take good care of your instrument!


Using your instrument properly helps you and your organization save on maintenance costs. Therefore, it is always good to know the basic tips mentioned above to prevent damage to your multimeter.


To learn more, download the Tips for Preventing Damage to Digital Multimeters application note.

Visit or more info about Keysight digital multimeters.

A typical part of an engineer’s job is to perform measurements with a multimeter. For example, if you are working as a building maintenance engineer or electrician, your daily routine may require you to measure power from AC mains or other high voltages. Before you start performing a measurement, what is the first thing that comes to your mind? Should you just grab the nearest handheld multimeter available? No!

Imagine you need to select a helmet. You need to select one with high quality to protect your head. But a helmet’s structure, design, and protective ability vary for different kinds of activities. A helmet designed for rock climbing needs to protect you against small rocks, falling objects, and sharp faces. A bicycle helmet needs to protect your head during impact to reduce the likelihood of injury in the event of an accident. A motorcycle helmet needs to protect you against high-speed impacts with the road and other vehicles. It’s important to choose the right helmet for your activity.

Likewise, different multimeters are designed with different levels of protection against common electrical hazards.


For your own safety and the safety of those near you, you must choose a multimeter that is designed and tested to protect you against electrical hazards you might encounter. Remember you only have one life, and there are no second chances. Here are some safety considerations that you need to take note of before you start making measurements with a multimeter.


1. Understand your multimeter’s safety indicators

Safety certification is important to ensure the multimeter is compliant with the relevant safety standards. Usually, the manufacturer of the multimeter will obtain safety certifications from third-party independent testing agencies, such as the Canadian Standard Association (CSA), to ensure the product has been tested and complied to the relevant safety standard.


Products that successfully pass the independent testing are labeled with the logo of the independent testing agency on the back of the multimeter.


The accessories used with a multimeter ─ like probes ─ should also be tested and marked with a third-party safety agency logo. Before you start measuring, be sure to select a multimeter and accessories that have passed these tests.

Some independent testing agency logos are as follows:

VDE certification markTuV technical inspection association certification mark CSA Group certification mark ETL SEMKO certification mark

Be cautious of the CE marking:

CE certification markThe “CE” marking is an abbreviation for "European Conformity" (from the French phrase “Conformité Européene”). The CE marking indicates the product's conformity to the applicable European Union safety, health, and environmental requirements, a mandatory conformity mark on all products sold in the European Union.


Manufacturers are permitted to self-certify. They must meet the standards, issue their own Declaration of Conformity, and mark the product “CE.” Therefore, the CE marking is not a guarantee of independent testing.


For safety purposes, you should not accept a multimeter that has only a CE mark unless you know the manufacturer to be trustworthy and you have reviewed the manufacturer’s Declaration of Conformity.


2. Pay attention to voltage rating of the measurement circuit and measurement limit of your multimeter

Before you start to perform a measurement, you need to understand the maximum voltage rating of your circuit. Use a multimeter that will be able to withstand the maximum voltage of the circuit. In general, manufacturers label the multimeter’s measurement limits on the front panel. A multimeter provides protection circuitry to prevent damage to the instrument and protect against the danger of electric shock, provided the measurement limits are not exceeded.


To ensure safe operations of the instrument, do not exceed the measurement limits shown on the front panel of the multimeter.

3. Take precautions when performing live measurements

Here are a few safety precautions you should take when making live measurements with a multimeter:

  • When measuring a live circuit, use insulated tools like safety glasses, insulated mats, and insulated gloves.

insulated gloves with handheld DMM

Figure 1. Wear insulated gloves when using a multimeter.


  • Inspect the test leads for damaged insulation or exposed metal. Check the test leads for continuity. If the test leads are damaged, replace them before you use the multimeter.
  • It is recommended that you disconnect circuit power and discharge all high-voltage capacitors before testing resistance, continuity, diodes, or capacitance.
  • When making measurements, always connect the common test lead before you connect the live test lead. When you disconnect the leads, disconnect the live test lead first. Avoid holding the test lead in your hands to minimize personal exposure if transients occur. Before use, verify the multimeter’s operation by measuring a known voltage.

Conclusion – Safety First!

Safety is always the top priority no matter what you are measuring, so there are a few precautions you should take. Always choose a multimeter with a voltage rating higher than the circuit you are measuring. Remember to check that the multimeters and probes you are going to use are marked with third-party safety agency logos such as CSA, ETL, TÜV, or VDE. Do not overlook the safety of the probes! With safety in mind, you will be assured that the high voltage goes into your measurement instrument instead of you!


To learn more, download the Think SAFETY when Selecting a Handheld Multimeter application note.

For more info about Keysight’s DMMs, visit

How can you determine which digital multimeter (DMM) is best for your application?

There are five key specs you need to consider before purchasing a digital multimeter to make sure you’re picking the right DMM for your testing needs: number of display digits, counts, range, resolution, and accuracy. Let’s look at each of these in more detail.


1. Display Digits

In general, most manufacturers specify a DMM display by digits of resolution. For example, the digits of resolution for Keysight digital multimeters goes from 3 ½ digits to 8 ½ digits.


A 5 ½ digit DMM, for example, has five full digits that display values from 0 to 9, and a fractional digit. Fractional digit is the most significant digit in the display. It is the ratio of the maximum value the digit can attain over the number of possible states. For example, a ½ digit has a maximum value of one and has two possible states (0 or 1). A ¾ digit has a maximum value of 3 with four possible states (0, 1, 2, or 3). The higher the digits of resolution, the more precise and accurate your measurements will be. It also means that a multimeter with higher digits of resolution will be more expensive.

2. Counts

Nowadays, manufacturers have started to specify the display in terms of “count” because “digits of resolution” often creates confusion. The count of a DMM refers to how large a number the multimeter can display before it changes the measurement ranges and how many digits it can show in total. This affects how precise a measurement the DMM can display. For example, a 4½ DMM can also be specified as a 19,999-display count or 20,000 display count multimeter.

3. Resolution

Resolution is defined as the smallest change in an input signal that produces a change in the output signal. Resolution will be improved when the DMM’s measurement range is reduced. Ultimately, you want the multimeter to display the best resolution of your measurement reading. You can play around with the multimeter and select the measurement range that gives you the reading with optimum resolution.


4. Range

Range is correlated to resolution, the resolution display on the multimeter will depend on the measurement range that you select.

To choose a digital multimeter, you need to know the minimum and maximum measurement range, and the resolution you require. This information is usually available in the DMM’s data sheet.


4.1 Autorange vs. Manual Range Multimeters

Most of the digital multimeters today offer both auto ranging measurements and manual ranging measurements.


4.1.1 Auto Ranging Measurements

For auto ranging measurements, all you have to do is select your desired measurement functions and let the multimeter automatically choose the best range. Normally, you should select auto ranging when you do not know the potential measurement reading range.

4.1.2 Manual Ranging Measurements

Manual ranging measurements are normally selected when the desired measurement value is known. Based on your measurement value, you can choose the desired measurement range. If the measurement value is unknown and you would like to use the manual range, it is recommended that you use the “step down” method. Start with the highest range and then step down to lower ranges to achieve the optimum resolution on the display.


5. Accuracy

The accuracy of a DMM is different from its display resolution. The accuracy is the maximum allowable limit of error in the readings. Normally, manufacturers display DC voltage (DCV) accuracy as a benchmark when comparing with other manufacturer specifications, as DCV has better accuracy compared to other functions. The accuracy specification is expressed as ±((% of reading + % of range).



There is a range of digital multimeters available on the market. To select the right multimeter, you need to know the target application, the measurement range, and required resolution. If you require a higher measurement accuracy, make sure to select a DMM with a higher resolution.


To learn more, download the How to Select a Handheld DMM That is Right for You application note.

For more information about Keysight’s DMMs, visit

Whether you use your frequency counter on the bench or in an automated test system, you want your measurements to be as accurate as possible. There’s a key component in your frequency counter that dictates its accuracy: the timebase oscillator.


Measurement accuracy in frequency counters begins with the timebase because it establishes the reference against which your input signal is measured. The better the timebase, the better your measurements can be. I said ‘can be’ because you need to do scheduled maintenance of your frequency counter. To maximize your counter’s accuracy, you need to calibrate it. Here’s why:


The frequency at which quartz crystals vibrate is heavily influenced by ambient temperature.


There are three main categories of timebase technologies, divided up based on how they address thermal behavior: room temperature crystal oscillators (RTCO), oven controlled crystal oscillators (OCXO) and temperature compensated crystal oscillators (TCXO).


Standard Timebase – A Room Temperature Crystal Oscillator


The first category is the standard timebase, also known as room temperature crystal oscillator (RTCO). A standard timebase doesn’t employ any kind of temperature compensation or control. While this has the advantage of being inexpensive, it also gives the largest frequency errors. The curve in Figure 1 shows the thermal behavior of a typical crystal. As the ambient temperature varies, the frequency output can change by 5 parts per million (ppm) or more. This works out to ± 5 Hz on a 1 MHz signal, so it can be a significant factor in your measurements.


Frequency Counter: The frequency output of an unprotected crystal can vary widely in response to ambient temperature. Putting the crystal in a controlled thermal environment (an oven) helps maintain a stable output frequency.

Figure 1. The frequency output of an unprotected crystal can vary widely in response to ambient temperature. Putting the crystal in a controlled thermal environment (an oven) helps maintain a stable output frequency.


The obvious solution to this temperature-induced variance is to control the temperature, which leads us to our next type of oscillator.


Oven-controlled Crystal Oscillators (OCXO)


For an OCXO, the crystal oscillator is housed in an oven that holds its temperature at a specific point in the thermal response curve. Surrounding the crystal with temperature-control circuitry gives it better timebase stability. Typical errors are as small as 0.0025 ppm (±0.0025 Hz on a 1 MHz signal). Additionally, oven-controlled timebases also help minimize the effects of crystal aging. This means you don’t have to calibrate your frequency counter as often.


OCXOs are very accurate but more expensive and have a bigger footprint than other timebase options. So, many engineers opt for temperature-compensation circuitry instead of temperature-controlled circuitry.


Temperature Compensated Crystal Oscillators (TCXO)


Temperature compensated crystal oscillators (TCXOs) are designed to account for temperature changes instead of trying to hold a fixed temperature. One method of compensating for frequency changes due to temperature variation is to add external components with complementary thermal responses.


This approach can stabilize the thermal behavior enough to reduce timebase errors by an order of magnitude relative to RTXO (approximately 1 ppm, ±1 Hz on a 1 MHz signal).


Choose a Timebase That’s Right for You


Now you have learned that different types of timebases bring a different response to a counter.


The quality of your frequency counter’s timebase will affect your measurement accuracy. Depending on the accuracy that you need, you can choose the right one.


It’s also worth pointing out that the timebase does not need to be housed within the frequency counter. You can connect a precision source or house-standard external source to the counter to improve measurement accuracy. You should also note that, no matter what timebase you select, leaving your timebase powered up will provide the most accurate results.


To learn more about frequency counter measurements, download the 10 Hints for Getting the Most from Your Frequency Counter application note.

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:


Frequency Counter: The number of digits displayed by a direct counter versus frequency (for a 1 second gate time).

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.


Frequency Counter: Comparing resolution for direct and reciprocal counters (for a 1 second gate time).

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).


Frequency Counter: The gate times needed to yield various resolutions with a 10 digits/second reciprocal counter.

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.