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Raspberry Pi and Traffic Lights

Although we come across traffic lights almost every time we step out of our homes, we rarely stop to think about how they work. However, Gunnar Pelpman has done just that, and he has put the hugely popular single board computer, Raspberry Pi to good use. While most of the tutorials introduce turning on and off LEDs, he has prepared a somewhat more complex tutorial, one that teaches how to program traffic lights. Moreover, he has done this with the Raspberry Pi (RBPi) running the Windows 10 IoT Core.

Traffic Lights may look very complicated installations, but they are rather simple in operation. They mostly comprise a controller, the signal head, and the detection mechanism. The controller acts as the brains behind the installation and controls the information required to light up the lights through their various sequences. Depending on location and time of the day, traffic signals run under a variety of modes, of which two are the fixed time mode and the vehicle actuation mode.

Under the fixed time mode, the traffic signal will repeatedly display the three colors in fixed cycles, regardless of the traffic conditions. Although adequate in areas with heavy traffic congestion, this mode is very wasteful for a side road with light traffic—if for some cycles there are no waiting vehicles, the time could be more efficiently allocated to a busier approach.

The second most common mode of operation of the traffic signal is the vehicle actuation. As its name suggests, the traffic signal adjusts the cycle time according to the demands of vehicles on all approaches.

Sensors, installed in the carriageway or above the signal heads, register the demands of the traffic. After processing these demands, the controller allocates the cycle time accordingly. However, the controller has a preset minimum and maximum cycle time, and it cannot violate them.

The hardware for the project could not be simpler. Gunnar has used three LEDs—red, orange, and green—to represent the three in a traffic light. The LEDs have an appropriate resistor in series for current limiting, and three ports of the RBPi drive them on and off. The rest of the project is the software, for which Gunnar uses the UWP application.

According to Gunnar, there are two options for writing UWP applications—the first a blank UWP application and the second a background application for IoT—depending on your requirement. The blank UWP is good for trying things out as a start, as, at a later point of time, you can build a User Interface for your application.

After creating the project with the blank UWP application, Gunnar added a reference to Windows IoT Extensions for the UWP. Next, he opened the file MainPage.xaml and added his own code, which begins with a test for the wiring. He uses the init() function to initialize the GPIO pins and stop() to turn all LEDs off. Then the code turns on all LEDs for 10 seconds to signal everything is working fine.

According to Gunnar, the primitive code mimics the traffic lights. He uses a separate code for the cycling of the traffic lights, and another for blinking them on and off. He uses the play() function for running ten cycles of the traffic light.

USB Type C and USB 3.1 Gen 2 – What is the Difference?

With the need for increasing capabilities, USB technology has evolved and improved over several years. Recently, the USB Implementation Forum has released the specifications for the SuperSpeed+1 standard or USB 3.1 Gen 2 signal standard and the USB Type C connector. Data transfer rates have been increasing from USB 1.0, released in January 1996, with a full speed of 1.5 MB/s, to USB 2.0, released in April 2000, with full speed of 60 MB/s, and to USB 3.0, released in Nov 2008, with a full speed of 625 MB/s. The latest standard, USB 3.1 Gen 2 was released in Jul 2013, and has a full speed of 1.25 GB/s.

Confusion between USB Type C and USB 3.1 Gen 2

When discussing the relationship, people are often confused between the USB Type C and the USB 3.1 Gen 2 standard. The major point to note is the USB Type C standard defines the physical connector alone, whereas the USB 3.1 Gen 2 standard defines the electrical signal for communication.

Therefore, system designers have the freedom to select signals conforming to USB 3.1 Gen 2 to pass through USB Type C connectors and cables or through a connector that do not conform to the USB Type C specification. Designers can implement their own proprietary connector and still use the USB 3.1 Gen 2 signal standard in case they want to use their own hardware or to ensure their system remains isolated from other systems.

The reverse is also equally true and applicable. One can use the USB Type C connector to transmit and receive signals that do not conform to the USB signal standards. Although the implementation will benefit from the inexpensive and easily available USB Type C connectors and cables, the OEM must label it correctly, since the user will be at the risk of connecting the proprietary non-conforming system to a USB 3.1 Gen 2 standard system and damaging one or both the systems.

OEMs can also transmit legacy USB signaling configurations using the USB Type C connectors and cables. This is because the USB standard allows using pre-USB 3.1 Gen 2 on USB Type C connectors, as they have designed the standard to cause no damage to either system. However, the most optimum power and data transfer will occur only when both systems are negotiating a common power configuration and communication standard.

Why USB Type C

Compared to the older configurations, the use of the USB Type C connector offers several advantages. Apart from being a smaller package with more conductors, the USB Type C supports higher voltage and current ratings, while offering greater signal bandwidths.

Physically smaller, the USB Type C plugs and receptacles fit in a wide range of applications where space is restricted. Moreover, one can connect the plugs and receptacles any way—either right-side up or up-side down. This allows easier and faster insertions of plugs into their receptacles.

While USB Type A and B connectors can have a maximum of four or five conductors, there are 24 contacts within the USB Type C and it can carry 3 A at 5 V, or 15 W of power.

Condition Monitoring with MEMS Accelerometers

In the market today, several condition-monitoring products are available as easy to deploy and highly integrated devices. A vast majority of them contain a microelectromechanical system or MEMS accelerometer as their core sensor. Not only are these economical, they also help in reducing the cost of deployment and ownership. In turn, this expands the facilities and the number of equipment benefitting from a condition monitoring program.

Compared to the legacy mechanical sensors, solid state MEMS accelerometers offer several attractive attributes. So far, their low bandwidth had restricted their application for use in condition monitoring. For instance, the noise performance of MEMS accelerometers was found to be not sufficiently low to cater to diagnostic applications requiring low noise levels over bandwidths beyond 10 KHz and over high frequency ranges.

The above situation is changing. Although still restricted of a few KHz of bandwidth, MEMS accelerometers with low noise are now available allowing the designers of condition monitoring products to use them in their new product concepts. This is because the use of MEMS brings several valuable and compelling advantages to the designer.

For instance, the size and weight of the MEMS accelerometers are of the utmost importance to airborne applications in health and usage monitoring systems, especially as they employ multiple sensors on a platform. MEMS devices in surface mount packages in a triaxial formation provide very high performance, while their footprints are only 6 x 6 mm, and weigh less than one gram. This shrinks the final package, while the interface of a typical MEMS device uses a single supply, which makes it easier to use in digital applications by saving on cost and weight of cables.

The triaxial arrangement is simpler with solid-state electronics and the small size of the transducers. They offer a small form factor enabling mounting on a printed circuit board, with the assembly hermetically sealed in housing suitable for fitting on a machine. MEMS devices require very low levels of power from single voltage supply and simple signal conditioning electronics, suitable for battery-powered wireless products.

Designers are able to use MEMS accelerometers in industrial settings for easy transition to digital interfaces now common. This is because the topology of the signal conditioning circuit for MEMS devices is common with both analog and digital output variations, allowing them to adapt the sensors to a wider variety of situations.

For instance, designers can load open protocols such as the Modbus RTU into a micro-controller, while using them with easily available RS-485 transceiver chips. Using surface mount chips, designers can lay out the complete solution for a transmitter with small footprint and fit them within relatively small board areas. They can insert these assemblies into packages, hermetically sealing them for supporting intrinsically safe characteristics or for conforming to environmental robustness certifications.

Although the current generation of MEMS devices can safely withstand 10,000 g of shock according to their specifications, in reality they can tolerate much higher levels without affecting sensitivity specifications. For instance, automatic test equipment can trim the sensitivity of a high-resolution sensor to remain stable over time and temperature to 0.01°C.

How does LoRa Benefit IoT?

Cycleo, a part of Semtech since 2012, has developed and patented a physical layer with a modulation type, with the name LoRA or Long Range, where the transmission utilizes the license-free ISM bands. LoRa consumes very low power and is therefore, ideal for IoT for data transmission. Sensor technology is one possible field of application for LoRa, where low bit rates are sufficient, and where the sensor batteries last for months or even years. Other applications are in the industry, environment technology, logistics, smart cities, agriculture, consumption recording, smart homes, and many others.

LoRa uses wireless transmission technology, and consumes very low power to transmit small amounts of data over distances of nearly 15 Km. It uses CSS or Chirp Spread Spectrum modulation, originally meant for radar applications, and developed in the 1940s, with chirp standing for Compressed High Intensity Radar Pulse. The name suggests the manner of data transmission by this method.

Many current wireless data transmission applications use the LoRa method, owing to its relative low power consumption, and its robustness against fading, in-band spurious emissions, and Doppler effect. IEEE has taken up the CSS PHY as a standard 802.15.4a for use as low-rate wireless personal area networks.

A correlation mechanism, based on band spreading methods, makes it possible for LoRa to achieve the long ranges. This mechanism permits use of extremely small signals that can disappear in noise. De-spreading allows modulation of these small signals in the transmitter. LoRa receivers are sensitive enough to decode these signals, even when they are more than 19 dB below the noise levels. Unlike the DSSS or direct sequence spread spectrum that the UMTS or WLAN uses, CSS makes use of chirp pulses for frequency spreading rather than using the pseudo-random code sequences.

A chirp pulse, modulated by GFSK or FM, usually has a sine-wave signal characteristic along with a constant envelope. As time passes, this characteristic falls or rises continuously in frequency. That makes the frequency bandwidth of the pulse equivalent to the spectral bandwidth of the signal. CSS uses the signal characteristic as a transmit pulse.

Engineers use LoRaWAN to define the MAC or media access protocol and the architecture of the system for a WAN or wide area network. The special design of LoRaWAN especially targets IoT devices requiring energy efficiency and high transmission range. Additionally, the protocol makes it easier for communications with server-based internet applications.

The architecture of the LoRaWAN MAC is suitable for LoRa devices, because of its influence on their battery life, the network capacity, the service quality, and the level of security it offers. Additionally, it has a number of applications as well.

The LoRa Alliance, a standardization body, defines, develops, and manages the regional factors and the LoRa waveform in the LoRaWAN stack for interaction between the LoRa MAC. The standardization body consists of software companies, semiconductor companies, manufacturers of wireless modules and sensors, mobile network operators, testing institutions, and IT companies, all working towards a harmonized standard for LoRaWAN. Using the wireless technology of LoRa, users can create wireless networks covering an area of several square kilometer using only one single radio cell.

Why Use a Multi-Layer PCB?

Although a multi-layer PCB is more expensive than a single or double-layer board of the same size, the former offers several benefits. For a given circuit complexity, the multi-layer PCB has a much smaller size as compared to that a designer can achieve with a single or even a double-layer board—helping to offset the higher cost—with the main advantage being the higher assembly density the multiple layers offer.

There are other benefits of a multi-layer PCB as well, such as increased flexibility through reduced need for interconnection wiring harnesses, and improved EMI shielding with careful placements of layers for ground and power. It is easier to control impedance features in multi-layer PCBs meant for high-frequency circuits, where cross talk and skin effect is more prominent and critical.

As a result, one can find equipment with multi-layer PCBs in nearly all major industries, including home appliances, communication, commercial, industrial, aerospace, underwater, and military applications. Although rigid multi-layer PCBs are popular, flexible types are also available, and they offer additional benefits over their rigid counterparts—lower weight, higher flexibility, ability to withstand harsh environments, and more. Additionally, rigid flex multi-layer PCBs are also available, offering the benefits of both types in the same PCB.

Advantages of a Multi-Layer PCB

Compared to single or double-layer boards, multi-layer PCBs offer pronounced advantages, such as:

  • Higher Routing Density
  • Compact Size
  • Lower Overall Weight
  • Improved Design Functionality

Use of multiple layers in PCBs is advantageous as they increase the surface area available to the designer, without the associated increase in the physical size of the board. Consequently, the designer has additional freedom to include more components within a given area of the PCB and route the interconnecting traces with better control over their impedance. This not only produces higher routing density, but also reduces the overall size of the board, resulting in lower overall weight of the device, and improving its design functionality.

The method of construction of multi-layer PCBs makes them more durable compared to single and double-layer boards. Burying the copper traces deep within multiple layers allows them to withstand adverse environment much better. This makes boards with multiple layers a better choice for industrial applications that regularly undergo rough handling.

With the availability of increasingly smaller electronic components, there is a tendency towards device miniaturization, and the use of multi-layer PCBs augments this trend by providing a more comprehensive solution than single or double-layer PCBs can. As these trends are irreversible, more OEMs are increasingly using multi-layer boards in their equipment.

With the several advantages of multiple layer PCBs, it is imperative to know their disadvantages as well. Repairing PCBs with several layers is extremely difficult as several copper traces are inaccessible. Therefore, the failure of a multi-layer circuit board may turn out to be an expensive burden, sometimes necessitating a total replacement.

PCB manufacturers are improving their processes to overcome the increase in inputs and to reduce design and production times for decreasing the overall costs in producing multi-layer PCBs. With improved production techniques and better machinery, they have improved the quality of multi-layer PCBs substantially, offering better balance between size and functionality.

What are Multi-Layer PCBs?

Most electronic equipment have one or more Printed Circuit Boards (PCB) with components mounted on them. The wiring to and from these PCBs determines the basic functionality of the equipment. It is usual to expect a complex PCB within equipment meant to deliver highly involved performance. While a single layer PCB is adequate for simple equipment such as a voltage stabilizer, an audio amplifier may require a PCB with two layers. Equipment with more complicated specifications such as a modem or a computer requires PCB with multiple layers, that is, a PCB with more than two layers.

Construction of a Multi-Layer PCB

Multiple layer PCBs have three or more layers of conductive copper foil separated by layers of insulation, also called laminate or prepreg. However, a simple visual inspection of a PCB may not imply its multi-layer structure, as only the two outermost copper layers are available for external connection, with the inner copper layers remaining hidden inside. Fabricators usually transform the copper layers into thin traces according to the predefined electrical circuit. However, some of the layers may also represent a ground or power connection with a large and continuous copper area. The fabricator makes electrical interconnections between the various copper layers using plated through holes. These are tiny holes drilled through the copper and insulation layers and electroplated to make them electrically conducting.

A via connecting the outermost copper layers and some or all of the inner layers is a through via, that connecting one of the outermost layers to one or more inner layers is the blind via, while the one connecting two or more inner layers but not visible on the outermost layers is the blind via. Fabricators drill exceptionally small diameter holes using lasers to make vias, as this allows maximizing the area available for routing the traces.

As odd number of layers can be a cause of warping in PCBs, manufacturers prefer to make multiple layer boards with even number of layers. The core of a PCB is an insulating laminate layer with copper foils pasted on both its sides—forming the basic construction of a double-layer board. Fabricators make up further layers by adding a combination of prepreg insulation and copper layers on each side of the double-layer board—repeating the process for as many extra layers as defined by the design—to make a multi-layer PCB.

Depending on the electrical circuit, the designer has to define the layout of traces on each copper layer of the board, and the placement of individual vias, preferably using CAD software packages. The designer transfers the layered design output onto photographic films, which the fabricator utilizes to remove the excess metal from individual copper layers by the process of chemical etching, followed by drilling necessary holes and electroplating them to form vias. As they complete etching and drilling for each layer, the fabricator adds it on to the proper side of the multi-layer board.

Once the fabricator has placed all layers properly atop each other, application of heat and external pressure to the combination makes the insulation layers melt and bond to form a single multi-layer PCB.

Things Gateway Ties IoT Devices Together

Project Things from Mozilla is a framework of software and services. It helps to bridge the communication gap between IoT devices. Project Things does this by giving each IoT device a URL on the web. The latest version of the Things Gateway, also from Mozilla, can directly let you control your home over the web, and manage all your devices through a single secure web interface. Therefore, if you have several smart devices in your home, you will not need different mobile apps to manage each of them. The best part of the Things Gateway is you can easily build one on a single board computer and use the power of the open web to connect off-the-shelf smart home products immediately, even if they are from different brands.

DIY hackers will find many exciting new features in the latest version. It even includes a rules engine, where you can set ‘if this, then that’ style of scenarios for making up rules of how things should interact. Other features include a floor plan view for laying out the devices on a map of your house, an experimental voice control, and it supports several new types of IoT devices. If you have a new device requiring new protocols, there is a brand new add-ons system. Third party applications that want to access your gateway can now do so, as there is a new way to authorize them safely.

On the hardware side, you will need a single board computer. Although Mozilla recommends a Raspberry Pi 3, any single board computer will do, as long as it has Wi-Fi and Bluetooth support built-in. Access to GPIO ports is also necessary, as you will require direct hardware access. Although a laptop or desktop computer will also work here, using the single board computer will provide the best experience.

If your smart home devices use other protocols such as Zigbee or Z-Wave, you will also need a USB dongle. Things Gateway supports Zigbee with Digi Xstick and for Z-Wave you will have to use a dongle compatible with OpenWave. You will need the proper device suitable for your region, as Z-Wave operating frequencies vary for different countries.

For the software part, you will need at least a 4 GB micro SD card to flash the software. The Gateway already has support for several different smart sensors, plugs, and smart bulbs from various brands, which may be using Wi-Fi, Z-Wave, or Zigbee. The Wiki mentions all the tested parts, and you can contribute if you have tested other new devices. However, if you are not yet ready with the actual hardware of IoT devices, and want to try out the Gateway software, the Virtual Things add-on us your friend. Simply install it and start adding virtual IoT things to your Gateway.

Mozilla offers the Things Gateway software image for the Raspberry Pi, which you can download and flash onto the micro SD card. The safest way to do this is to use Etcher, a cross-platform image writer software, useful for Linux, Windows, and the Mac OS.

LTM2893 μModule isolator for ADCs

Analog to digital converters (ADCs) need to float to the common mode of the input signal to absorb the harsh voltage conditions and transients. The best way to do this is to place an isolation barrier between the ADC and the external signal. Even applications that perform under moderate conditions can benefit from the presence of an isolator. The LTM2893 from Linear Technology provides such isolation, improving on system safety, especially when reading from high-resolution successive approximation register type of ADCs.

Ideally, the isolator for an ADC should be near invisible. Its function would be to manage the control and data signals, maximizing the sampling rate, and minimizing the effects of jitter on the performance of signal to noise ratio. The LTM2893 μModule isolator from Linear Technology meets all the above criteria, achieving these for ADCs with SPI interfaces, offers a 1 Msps range, while supporting a 6K Vrms isolation rating.

Options that are more traditional exist, but provide limited functionality, especially when reading data from high-resolution successive approximation register (SAR) ADCs. Most traditional high speed digital isolators work maximum up to 25 MHz, with a few special devices reaching 40 MHz On the other hand, the LTM2893 can easily read data samples at rates up to 100 MHz. Additionally, it is flexible enough to be able to handle multiple ADCs. This effectively solves timing issues and other limitations of the standard digital isolator interfacing that SAR ADCs face.

Test and process equipment need isolation so that their inputs are not damaged if accidentally misconnected or from overvoltage events. Usually, engineers use an isolator as a high voltage level shifter for extending the common mode range thereby reducing the ground noise. The LTM2893 is intelligent enough to ignore transients events of the common mode type up to 50K V/μs, as this provides a low-capacitance isolation barrier along with fully differential data communication.

When dedicated SPI isolators and other general-purpose digital isolators isolate ADCs, they use multiple digital isolators for supporting signals such as busy status or conversion start signals. In addition, they offer a 3- or 4-wire SPI port. They also suffer from signal propagation delays, as the isolated SPI port must wait for the return of the acknowledgement signal before the next data latching can occur. Adding all the propagation and the response delays from the ADC SPI port, a single read may suffer a delay of about 35 ns. Therefore, although the initially rating of a digital isolator may be at 150 Mbps, in reality, the delays reduce the effective frequency to 25 MHz or even less.

Linear Technology has provided the LTM2893 with a dedicated master SPI engine on its isolated side, and a dedicated slave engine and a buffer on the logic side. The master SPI engine of the LTM2893 monitors the status signals from the ADC, fetching the data as soon as its BUSY signal goes low. There is no interaction with the logic side once the conversion has started.

The buffer register on the slave SPI engine on the logic side receives data from the isolated side via the isolated barrier. The two sides therefore, operate independently of each other.

What are Linear Actuators?

Any device creating motion in a straight line is a linear actuator. A vehicle is an excellent example of linear motion—the engine makes the car move forward or back in a straight line, unless the driver changes the direction. Other examples of this application are available in the process industries, material handling, food, and beverages processing industry, robotics, and many more.

The industry uses different types of linear actuators powered by pneumatics, hydraulic, and/or electric. As it is natural for pneumatic and hydraulic power to produce linear motion, they are simple devices and the industry often calls them cylinders. On the other hand, electric powered linear actuators almost always use a rotary electric motor. That means converting the rotary motion to a linear one through a belt or a screw/nut system. Although conversion does make the electric linear actuator somewhat more complex than pneumatic or hydraulic actuators are, using electricity can offer significant advantages in several applications.

Engineers must make a crucial decision when selecting the type of linear actuator they prefer for use in a specific application. For instance, although the pneumatic cylinder offers the advantages of lower cost and ease of use, the user often confronts potential compressed air leaks that reduce the efficiencies in operation. Similarly, although providing high-thrust capabilities, a hydraulic cylinder is prone to fluid leaks and they may not be very friendly to the environment.

On the other hand, electric linear actuators offer distinct benefits:

  • Ability to handle complex motion profiles
  • Ability to adapt quickly to changing needs
  • High efficiency, lower energy usage, and lower lifetime costs
  • Integrates easily into other electric production systems

With motion-control systems becoming increasingly more complicated, electric linear actuators provide precise control of force, deceleration, acceleration, and speed. Their ease of use allows them to outperform easily other technologies dependent on fluid power. Easily handling complex motion profiles, electric linear actuators offer infinite positioning capabilities with data feedback and high accuracy and repeatability.

It is easy to change the programming of an electric actuator. As parameters change, changing the program allows the actuator to adjust to the new specifications. Not only does this minimize downtime, it also reduces loss of productivity in the industry.

Compared to the 10-15% total system efficiency for pneumatic systems and 40-50% for hydraulic systems, electric powered linear actuators operate at 70-80%. Although the initial acquisition cost may be high, electricity powered linear actuators offer savings over their lower total life cycle cost, apart from the savings in energy use, efficiency, and reduced maintenance.

If all other equipment in the system operate on electricity, it is easy for users to integrate electric actuators also into the motion control system. Users can take advantage of integrating electric actuators in systems that use PLCs, HMIs and other similar devices for enhancing motion control, data collection, and diagnostics.

In the market, numerous actuator types/styles are available and they come in various degrees of precision and cost. For instance, one can have models that offer high repeatability, but with moderate accuracy. It is necessary to understand the requirements in the application for selecting a suitable actuator.

The Importance of Decibels in Electronics

Engineers use decibels everywhere for calculating power levels, voltage levels, reflection coefficients, noise figures, field strengths, and more. Most instruments use it, whether they are signal generators, spectrum analyzers, test receivers, power meters, network analyzers, or audio analyzers. Despite this, decibels remain a mystery to most people, sometimes including experienced engineers.

Engineers deal with numbers in all their professional activities, with some numbers being very large or very small. However, most of the time, rather than the numbers themselves, it is the ratio of two quantities that is more important. For instance, the base station of a mobile radio system many be transmitting 80 W of power, of which only 2×10-8 W or 2.5×10-8 percent reaches a mobile phone.

For dealing with very large or very small numerical figures, it becomes easier to use the logarithm of the numbers instead. For instance, the above base station transmits at +49 dBm, while the signal strength reaching the mobile phone is at -57 dBm. This makes the level difference between the two 106 dBm.

The main advantage in expressing ratios in decibels is they become far easier to manipulate. Adding and subtracting decibel values needs a much lower mental effort than to multiply or divide linear values.

Although a ratio cannot have dimensions, engineers use units of Bel to honor the inventor of the telephone, Alexander Graham Bell. The use of decibel makes the numbers more manageable, with decibel being one tenth of the Bel. Just as we multiply meters with 1000 to convert them to millimeters, we need to multiply Bel values by 10 to convert them to decibels. Therefore, dB represents ten times the ratio of two power values expressed as a logarithm to the base 10.

Since the decibel is a ratio, engineers must express arbitrary power levels with reference to a fixed quantity, as this allows proper comparison of the power levels. Telecommunication and radio frequency engineers most commonly use the reference quantity as the power of 1 mW into a 50-Ohm load. That is how in our example above, expressing 80 W of power in dBm becomes 10 x log (80/0.001) = 49 dBm. Although earlier, some engineers used the natural logarithm with a base of e, nowadays, engineers use the base 10 logarithm almost exclusively for calculating dBm.

Although the concept of decibels involves the ratio between two power levels, this can be easily converted to a ratio between two voltage levels, since the knowledge of power and resistance helps in finding the voltage across the resistance.

Apart from dBm, engineers use other reference quantities such as 1 W, 1 V, 1 µV, 1 A or 1 µA as well. In these cases, they express the dB quantities as dBW, dBV, dBµV, dBA, and dBµA. Similarly, for field strength measurements, these become dBW/m2, dBV/m, dBA/m and dBµA/m.

Engineers obtain absolute values when expressing power level ratios using the reference values above. Therefore, they call these absolute values as levels. For instance, a level of 10 dBm represents a value 10 dB above 1 mW, while a level of -20 dBµV represents a value 20 dB below 1 µV.