Remote-Control Powered Trapdoor Lift

William Wachsmann, a retired electronic technologist from Canada, has more than 35 years of experience working with minicomputers, microcomputers, embedded systems, and programming in industries ranging from nuclear and aerospace to voicemail and transportation systems.

But despite the complexity of the work he has done over the years, when it came to building a remote-controlled, powered trapdoor lift system for his home, he had two priorities: simplicity and price.

“Although it can be fulfilling to design your own hardware, why reinvent the wheel if you don’t have to? Many reasonably priced modules can be wired together,” Wachsmann says in his article about the project, which appears in Circuit Cellar’s May issue. “Add some software to express the functionality required and you can quickly and inexpensively put together a project. Not only is this method useful for a homebuilt one-of-a-kind application, but it can also be used for proof-of-concept prototyping. It leaves you free to concentrate on solving the problems pertinent to your application.”

Wachsmann’s project relies on off-the-shelf modules for the electrical functions of a trapdoor lift system that provides access to his basement.

“Lifting the trapdoor was hard on my wife’s back,” he says. “If her arms were full when she came upstairs and the door was open, she had to twist her body to release the mechanical latching mechanism while simultaneously stopping the door from falling on her head.”

The multidisciplinary project includes mechanical, electronic, and software components. For the full details—including programming of the project’s Freescale Semiconductor FRDM-KL25Z microprocessor using the mbed online IDE—check out the May issue now available for membership download and single-issue purchase.

(And if you’re interested in other articles about remote-control projects, check out Raul Alvarez’s Home Energy Gateway system, which enables users to remotely monitor home energy consumption and monitor household devices. Alvarez’s project won second place in the 2012 DesignSpark chipKIT challenge administered by Circuit Cellar.)

Excerpts from Wachsmann’s article below describe his system’s mechanical and hardware elements.

I used a screw lift from an old treadmill. It has a 48-VDC motor that draws about 1 A under a 100-lb load. The screw mechanism has a 6” travel distance. Built-in limit switches shut off the motor when the screw reaches the end of its travel in each direction. The screw’s length is nominally 30” when closed and 36” when open. The length can be adjusted slightly by rotating the screw by hand, but the overall travel distance is still 6”.

A simple switch would have sufficed to control the screw lift’s DC motor, but I wanted it to be remotely controlled so the trapdoor could be raised and lowered from anywhere upstairs and from the basement. When someone is in the basement with the trapdoor closed there is no visible way for them to know if the door is obstructed. Initially, I was going to install a warning beeper that the door was opening, but that wouldn’t help if an inanimate object (e.g., a bag of groceries) was on top of the door. I needed to incorporate some form of sensing mechanism into the design.

Figure 1: This diagram represents the trapdoor mechanics. The arm’s down position is shown in red; the up position is shown in blue. Vertical red arrows are labeled with the downward force in pounds

Figure 1: This diagram represents the trapdoor mechanics. The arm’s down position is shown in red; the up position is shown in blue. Vertical red arrows are labeled with the downward force in pounds

I needed a levered system design that used a pivoted bar and the motorized screw lift. The lift also had to enable the door to be manually opened in case of a power failure.
I used IMSI/Design’s TurboCAD program for the mechanical design. By using CAD software, I could experiment with the pivot position, moment arms, and torque requirements to meet the mechanical constraints imposed by the screw lift and the trapdoor’s size and weight.

Photo 1: The screw lift and pivot arm mechanism with a spring assist are shown.

Photo 1: The screw lift and pivot arm mechanism with a spring assist are shown.

Figure 1 shows a diagram of the trapdoor, which is hinged on the left. The opposite side of the door exerts a 15.2-lb downward force. This means the torque (force × distance) required to open the door is 509.2 in-lbs. The pivot arm in red is the position when the door is closed. The blue pivot arm shows the position when the door is open to an 80° angle.

To keep within the 6” lift constraint, I used a 4.25” moment arm to pull down on the pivot arm. This left me with the force required to initially lift the door at 119.5 lb. Also this did not include the added torque due to the pivot arm’s weight.

After mulling this over for a couple of days (and nights) I had an idea. I realized that 119.5 lb is only needed when the door is completely closed. As the door opens, the torque requirement lessens. I incorporated a heavy spring (see Photo 1). When the door is closed the spring extension provides an additional downward force of about 35 lb. This is enough to lessen the load on the screw lift and to compensate for the pivot arm’s additional 2.2 lb. Using a screw lift meant the arm would not spring up if the door was manually opened.

I used an angle iron for the pivot arm. It is 28” long because it had to push up on the door to the right of the door’s center of gravity at 16.75” without adding too much additional torque. The roller is the type used as feet on beds. I used an arbor and 0.75”-diameter bolt through the floor joist for the pivot (see Photo 2).

An arbor is used as a bearing with a 0.75” bolt through the floor joist. The lift mechanism pivots at this point.

Photo 2: An arbor is used as a bearing with a 0.75” bolt through the floor joist. The lift mechanism pivots at this point.

I had set an arbitrary $100 limit for the rest of the system and I quickly realized I would easily come in under budget. I used a $24.25 two-channel RF wireless garage door remote-control receiver, which I purchased from eBay (see Photo 3). This controller can be used in a latched or an unlatched mode. The latched mode requires a momentary push of one of the buttons to cause one of the relays to switch and stay in the On position. When the controller is in unlatched mode, you must hold the button down to keep the relay switched.

Photo 3: The two-channel wireless remote control is shown with the cover removed from the receiver. It came with two keychain-style remotes, which I marked with Up and Down arrows.

Photo 3: The two-channel wireless remote control is shown with the cover removed from the receiver. It came with two keychain-style remotes, which I marked with Up and Down arrows.

Unfortunately, this remote control and any similar ones only come with single-pole double-throw (SPDT) relays. What I really wanted were double-pole double-throw (DPDT) relays to switch both sides of the motor to enable current reversal through the motor.
A remote control system with two remotes seemed ideal and was possible to design with SPDT, so I purchased the relays. Figure 2 shows the circuit using two bridge rectifier DC power supplies. It turns out there were problems with this approach.

SW1 and SW2 represent the Up and Down relays. In latched mode, the door would open when SW1 was energized using the A button on a remote. Pressing the A button again would stop the motor while the door was opening. So would pressing the B button, but then to continue opening the door you needed to press the B button again. Pressing the A  button in this state would cause the door to close because SW2 was still energized. Added to this confusion was the necessity of pressing the A button again when the door was fully opened and stopped due to the internal limit switches. If you didn’t do this, then pressing the B button to close the door wouldn’t work because SW1 was still energized.

Figure 2: It would be theoretically possible to use dual-power supplies and single-pole double-throw (SPDT) switches to control a motor in two directions. When SW1 (b,c) is connected, current flows through D2. When SW2 (b,c) is connected, current flows through D1 in the opposite direction.

Figure 2: It would be theoretically possible to use dual-power supplies and single-pole double-throw (SPDT) switches to control a motor in two directions. When SW1 (b,c) is connected, current flows through D2. When SW2 (b,c) is connected, current flows through D1 in the opposite direction.

I decided to just use the door in unlatched mode and continuously hold down the A button until the door was fully open. What was the problem with this? Noise! Interference from the motor was getting back into the control and causing the relay to frequently switch on and off. This is not good for mechanical relays that have a finite contact life.

After playing around for a while with both operation modes, I noticed that even in the latched mode the motor would sometimes stop and it would occasionally even reverse itself. This was really bad and it became worse.

If both SW1 and SW2 happened to switch at the same time and if the current was at a maximum and there was arcing at the terminal, there could conceivably be a momentary short through a diode in each of the bridge rectifiers that would burn them out. Arc suppression devices wouldn’t help because when active at high voltages, they would almost look like a short between the switch’s terminals A,C. I needed to step back and rethink this.

I found an $8.84 two-channel DPDT relay switch board module on eBay. The module enabled me to use a single-power supply and isolated the motor current from the remote-control board. These relays boards have TTL inputs, so it is tricky to use relays on the remote control board to control the relays on the second relay board. You have to contend with contact bounce. Even if I incorporated debounce circuits, I still didn’t have a way stop the door from opening if it was obstructed.

It was time to get with the 21st century. I needed to use a microcontroller and handle all the debounce and logic functions in firmware.

I bought a $12.95 Freescale Semiconductor FRDM-KL25Z development board, which uses the Kinetis L series of microcontrollers. The FRDM-KL25Z is built on the ARM Cortex-M0+ core. This board comes with multiple I/O pins, most of which can be programmed as required. It also has two micro-USB ports, one of which is used for downloading your program onto the microcontroller and for debugging (see Figure 3).

Figure 3: This is the system’s complete wiring diagram. On the left is a 48-V AC supply and an unregulated 12-V DC motor. A 2.7-Ω, 5-W resistor, which is used for current sensing, is in series with the motor.

Figure 3: This is the system’s complete wiring diagram. On the left is a 48-V AC supply and an unregulated 12-V DC motor. A 2.7-Ω, 5-W resistor, which is used for current sensing, is in series with the motor.

Low-Power Remote-Control Transceivers

LinxThe TT Series remote-control transceiver is designed for bidirectional, long-range, remote-control applications. The module includes an optimized frequency-hopping spread spectrum (FHSS) RF transceiver and an integrated remote-control transcoder.

The FHSS is capable of reaching more than 2 miles in typical line-of-sight environments with 0-dB gain antennas. An amplified version increases the output power from 12.5 to 23.5 dBm, boosting the range to more than 8 miles in line-of-sight environments with 0-dB antennas.

The TT Series transceiver features best-in-class receive sensitivity (up to −111 dBm) and low power consumption (only 19.2 mA in receive mode and 36 mA in transmit mode at 12.5 dBm). The initial version operates in the 902-to-928-Hz frequency band for North and South America.

The transceiver is housed in a compact reflow-compatible surface-mount technology (SMT) package. It doesn’t require any external RF components except an antenna, which simplifies integration and reduces assembly costs.

Programming is not required for basic operation. The transceiver’s primary settings are hardware-selectable, which eliminates the need for an external microcontroller or other digital interface. Eight status lines can be set up in any combination of inputs and outputs to transfer button or contact states. A selectable acknowledgement indicates that the transmission was successfully received. For advanced features, a UART interface provides optional software configuration.

A simple pairing operation configures two modules to operate together. A single button press on each side causes the modules to automatically swap their 32-bit addresses and store them in nonvolatile memory. It can be configured to automatically send an acknowledgement to the transmitting unit either after receiving a command or with external circuitry when an action has taken place. An optional external processor can send two data bytes with the acknowledgement.

The TT Series transceiver module is available as part of Linx Technologies’s master development system that comes with two development boards for benchmarking and prototyping. Each board is populated with a transceiver, two remote-control development boards, and programming boards. The system also includes antennas, a daughterboard with a USB interface, demonstration software, extra modules, and connectors.

Contact Linx Technologies for pricing.

Linx Technologies

A Wire Is an Inductor (EE Tip #126)

I’m confident you know that you should keep wires and PCB tracks as short as possible. But I’m also sure that you will underestimate this problem fairly frequently.

Remember that 1 cm of a 0.25-mm-wide PCB track is roughly equivalent to an inductance of 10 nH. If this 10 nH is paired with, say, a 10-pF capacitor, that gives a resonant frequency as low as 500 MHz, which is easily below the third or fifth harmonics of the clock frequencies commonly seen on modern high-speed digital boards. Similarly, a 1-cm-long track will jeopardize the performances of any RF system such as a 2.4-GHz transceiver. There is only one solution: keep tracks and wires as short as possible. If you can’t, then use impedance-matched tracks.

Remember this rule especially for the ground connections: any grounded pad of any part working in high frequencies should be directly connected by avia to the underlying ground plane. And this via must be as close as possible to the pad, not some millimeters away.

Just yesterday I did a design review of a customer’s RF PCB. A small 0402 inductance was grounded through a via that was 3 mm away. It was a bad idea because the inductance was as low as 1 nH. Those 3 mm changed its value completely.—Robert Lacoste, “Mixed-Signal Designs,” CC25:25th Anniversary Issue, 2013. 

Wireless Data Links (Part 2): Transmitters and Antennas

If you built your own ham radio “back in the day,” you’ll recall the frustration of putting it together with components that were basic at best.

But as columnist George Novacek points out in the second installment of his series examining wireless data links: “Today you can purchase excellent, reasonably priced low-power gear for data communications off the shelf.”

Transmitter and receiver

Photo 1: SparkFun Electronics’s WRL-10524 transmitter and WRL-10532 receiver are low cost, basic, and work well.

Part 2 of Novacek’s series, appearing in the March issue, looks at transmitters and antennas.

In one section, Novacek expands upon the five basic data-transmitter modules—a data encoder, a modulator, a carrier frequency generator, an RF output amplifier, and an antenna:

Low-power data transmitters often integrate the modulator, the carrier frequency generator, and the amplifier into one circuit. A single transistor can do the job. I’ll discuss antennas later. When a transmitter and a receiver are combined into one unit, it’s called a transceiver.

Modulation may not be needed in some simple applications where the mere presence of a carrier is detected to initiate an action. A simple push button will suffice, but this is rarely used as it is subject to false triggering by other transmitters working in the area in the same frequency band.

Digital encoder and decoder ICs are available for simple devices (e.g., garage door openers) or keyless entry where just an on or off output is required from the receiver. These ICs generate a data packet for transmission. If the received packet matches the data stored in the decoder, an action is initiated. Typical examples include Holtek Semiconductor HT12E encoders and HT12D decoders and Freescale Semiconductor MC145026, MC145027, and MC145028 encoder and decoder pairs. For data communications a similar but more advanced scheme is used. I’ll address this when I discuss receivers (coming up in Part 3 of this series).

Novacek’s column goes on to explain modulation types, including OOK and ASK modulation:

OOK modulation is achieved by feeding the Data In line with a 0-to+V-level  datastream. ASK modulation can be achieved by the data varying the transistor biasing to swing the RF output between 100% and typically 30% to 50% amplitude. I prefer to add a separate modulator.

The advantage of ASK as opposed to OOK modulation is that the carrier is always present, thus the receiver is not required to repeatedly synchronize to it. Different manufacturers’ specifications claim substantially higher achievable data rates with ASK rather than OOK.

For instance, Photo 1 shows a SparkFun Electronics WRL-10534 transmitter and a WRL-10532 receiver set for 433.9 MHz (a 315-MHz set is also available), which costs less than $10. It is a bare-bones design, but it works well. When you build supporting circuits around it you can get excellent results. The set is a good starting point for experimentation.

The article also includes tips on a transceiver you can purchase to save time in developing ancillary circuits (XBee), while noting a variety of transceiver, receiver, and transmitter modules are available from manufacturers such as Maxim Integrated, Micrel, and RF Monolithics (RFM).  In addition, the article discusses design and optimization of the three forms of antennas: a straight conductor (monopole), a coil (helical), and a loop.

“These can be external, internal, or even etched onto the PCB (e.g., keyless entry fobs) to minimize the size,” Novacek says.

Do you need advice on what to consider when choosing an antenna for your design?  Find these tips and more in Novacek’s March issue article.

Wireless Data Links (Part 1)

In Circuit Cellar’s February issue, the Consummate Engineer column launches a multi-part series on wireless data links.

“Over the last two decades, wireless data communication devices have been entering the realm of embedded control,” columnist George Novacek says in Part 1 of the series. “The technology to produce reasonably priced, reliable, wireless data links is now available off the shelf and no longer requires specialized knowledge, experience, and exotic, expensive test equipment. Nevertheless, to use wireless devices effectively, an engineer should understand the principles involved.”

Radio communicationsPart 1 focuses on radio communications, in particular low-power, data-carrying wireless links used in control systems.

“Even with this limitation, it is a vast subject, the surface of which can merely be scratched,” Novacek says. “Today, we can purchase ready-made, low-power, reliable radio interface modules with excellent performance for an incredibly low price. These devices were originally developed for noncritical applications (e.g., garage door openers, security systems, keyless entry, etc.). Now they are making inroads into control systems, mostly for remote sensing and computer network data exchange. Wireless devices are already present in safety-related systems (e.g., remote tire pressure monitoring), to say nothing about their bigger and older siblings in remote control of space and military unmanned aerial vehicles (UAVs).”

An engineering audience will find Novacek’s article a helpful overview of fundamental wireless communications principles and topics, including RF circuitry (e.g., inductor/capacitor, or LC, circuits), ceramic surface acoustic wave (SAW) resonators, frequency response, bandwidth, sensitivity, noise issues, and more.

Here is an article excerpt about bandwidth and achieving its ideal, rectangular shape:

“The bandwidth affects receiver selectivity and/or a transmitter output spectral purity. The selectivity is the ability of a radio receiver to reject all but the desired signal. Narrowing the bandwidth makes it possible to place more transmitters within the available frequency band. It also lowers the received noise level and increases the selectivity due to its higher Q. On the other hand, transmission of every signal but a non-modulated, pure sinusoid carrier—which, therefore, contains no information—requires a certain minimum bandwidth. The required bandwidth is determined by the type of modulation and the maximum modulating frequency.

“For example, AM radios carry maximum 5-kHz audio and, consequently, need 10-kHz bandwidth to accommodate the carrier with its two 5-kHz sidebands. Therefore, AM broadcast stations have to be spaced a minimum of 20 kHz apart. However, narrowing the bandwidth will lead to the loss of parts of the transmitted information. In a data-carrying systems, it will cause a gradual increase of the bit error rate (BER) until the data becomes useless. At that point, the bandwidth must be increased or the baud rate must be decreased to maintain reliable communications.

“An ideal bandwidth would have a shape of a rectangle, as shown in Figure 1 by the blue trace. Achieving this to a high degree with LC circuits can get quite complicated, but ceramic resonators used in modern receivers can deliver excellent, near ideal results.”

Figure 1: This is the frequency response and bandwidth of a parallel resonant LC circuit. A series circuit graph would be inverted.

Figure 1: This is the frequency response and bandwidth of a parallel resonant LC circuit. A series circuit graph would be inverted.

To learn more about control-system wireless links, check out the February issue now available for membership download or single-issue purchase. Part 2 in Novacek’s series discusses transmitters and antennas and will appear in our March issue.

Places for the IoT Inside Your Home

It’s estimated that by the year 2020, more than 30 billion devices worldwide will be wirelessly connected to the IoT. While the IoT has massive implications for government and industry, individual electronics DIYers have long recognized how projects that enable wireless communication between everyday devices can solve or avert big problems for homeowners.

February CoverOur February issue focusing on Wireless Communications features two such projects, including  Raul Alvarez Torrico’s Home Energy Gateway, which enables users to remotely monitor energy consumption and control household devices (e.g., lights and appliances).

A Digilent chipKIT Max32-based embedded gateway/web server communicates with a single smart power meter and several smart plugs in a home area wireless network. ”The user sees a web interface containing the controls to turn on/off the smart plugs and sees the monitored power consumption data that comes from the smart meter in real time,” Torrico says.

While energy use is one common priority for homeowners, another is protecting property from hidden dangers such as undetected water leaks. Devlin Gualtieri wanted a water alarm system that could integrate several wireless units signaling a single receiver. But he didn’t want to buy one designed to work with expensive home alarm systems charging monthly fees.

In this issue, Gualtieri writes about his wireless water alarm network, which has simple hardware including a Microchip Technology PIC12F675 microcontroller and water conductance sensors (i.e., interdigital electrodes) made out of copper wire wrapped around perforated board.

It’s an inexpensive and efficient approach that can be expanded. “Multiple interdigital sensors can be wired in parallel at a single alarm,” Gualtieri says. A single alarm unit can monitor multiple water sources (e.g., a hot water tank, a clothes washer, and a home heating system boiler).

Also in this issue, columnist George Novacek begins a series on wireless data links. His first article addresses the basic principles of radio communications that can be used in control systems.

Other issue highlights include advice on extending flash memory life; using C language in FPGA design; detecting capacitor dielectric absorption; a Georgia Tech researcher’s essay on the future of inkjet-printed circuitry; and an overview of the hackerspaces and enterprising designs represented at the World Maker Faire in New York.

Editor’s Note: Circuit Cellar‘s February issue will be available online in mid-to-late January for download by members or single-issue purchase by web shop visitors.

Amplifier Classes from A to H

Engineers and audiophiles have one thing in common when it comes to amplifiers. They want a design that provides a strong balance between performance, efficiency, and cost.

If you are an engineer interested in choosing or designing the amplifier best suited to your needs, you’ll find columnist Robert Lacoste’s article in Circuit Cellar’s December issue helpful. His article provides a comprehensive look at the characteristics, strengths, and weaknesses of different amplifier classes so you can select the best one for your application.

The article, logically enough, proceeds from Class A through Class H (but only touches on the more nebulous Class T, which appears to be a developer’s custom-made creation).

“Theory is easy, but difficulties arise when you actually want to design a real-world amplifier,” Lacoste says. “What are your particular choices for its final amplifying stage?”

The following article excerpts, in part, answer  that question. (For fuller guidance, download Circuit Cellar’s December issue.)

The first and simplest solution would be to use a single transistor in linear mode (see Figure 1)… Basically the transistor must be biased to have a collector voltage close to VCC /2 when no signal is applied on the input. This enables the output signal to swing

Figure 1—A Class-A amplifier can be built around a simple transistor. The transistor must be biased in so it stays in the linear operating region (i.e., the transistor is always conducting).

Figure 1—A Class-A amplifier can be built around a simple transistor. The transistor must be biased in so it stays in the linear operating region (i.e., the transistor is always conducting).

either above or below this quiescent voltage depending on the input voltage polarity….

This solution’s advantages are numerous: simplicity, no need for a bipolar power supply, and excellent linearity as long as the output voltage doesn’t come too close to the power rails. This solution is considered as the perfect reference for audio applications. But there is a serious downside.

Because a continuous current flows through its collector, even without an input signal’s presence, this implies poor efficiency. In fact, a basic Class-A amplifier’s efficiency is barely more than 30%…

How can you improve an amplifier’s efficiency? You want to avoid a continuous current flowing in the output transistors as much as possible.

Class-B amplifiers use a pair of complementary transistors in a push-pull configuration (see Figure 2). The transistors are biased in such a way that one of the transistors conducts when the input signal is positive and the other conducts when it is negative. Both transistors never conduct at the same time, so there are very few losses. The current always goes to the load…

A Class-B amplifier has more improved efficiency compared to a Class-A amplifier. This is great, but there is a downside, right? The answer is unfortunately yes.
The downside is called crossover distortion…

Figure 2—Class-B amplifiers are usually built around a pair of complementary transistors (at left). Each transistor  conducts 50% of the time. This minimizes power losses, but at the expense of the crossover distortion at each zero crossing (at right).

Figure 2—Class-B amplifiers are usually built around a pair of complementary transistors (at left). Each transistor conducts 50% of the time. This minimizes power losses, but at the expense of the crossover distortion at each zero crossing.

As its name indicates, Class-AB amplifiers are midway between Class A and Class B. Have a look at the Class-B schematic shown in Figure 2. If you slightly change the transistor’s biasing, it will enable a small current to continuously flow through the transistors when no input is present. This current is not as high as what’s needed for a Class-A amplifier. However, this current would ensure that there will be a small overall current, around zero crossing.

Only one transistor conducts when the input signal has a high enough voltage (positive or negative), but both will conduct around 0 V. Therefore, a Class-AB amplifier’s efficiency is better than a Class-A amplifier but worse than a Class-B amplifier. Moreover, a Class-AB amplifier’s linearity is better than a Class-B amplifier but not as good as a Class-A amplifier.

These characteristics make Class-AB amplifiers a good choice for most low-cost designs…

There isn’t any Class-C audio amplifier Why? This is because a Class-C amplifier is highly nonlinear. How can it be of any use?

An RF signal is composed of a high-frequency carrier with some modulation. The resulting signal is often quite narrow in terms of frequency range. Moreover, a large class of RF modulations doesn’t modify the carrier signal’s amplitude.

For example, with a frequency or a phase modulation, the carrier peak-to-peak voltage is always stable. In such a case, it is possible to use a nonlinear amplifier and a simple band-pass filter to recover the signal!

A Class-C amplifier can have good efficiency as there are no lossy resistors anywhere. It goes up to 60% or even 70%, which is good for high-frequency designs. Moreover, only one transistor is required, which is a key cost reduction when using expensive RF transistors. So there is a high probability that your garage door remote control is equipped with a Class-C RF amplifier.

Class D is currently the best solution for any low-cost, high-power, low-frequency amplifier—particularly for audio applications. Figure 5 shows its simple concept.
First, a PWM encoder is used to convert the input signal from analog to a one-bit digital format. This could be easily accomplished with a sawtooth generator and a voltage comparator as shown in Figure 3.

This section’s output is a digital signal with a duty cycle proportional to the input’s voltage. If the input signal comes from a digital source (e.g., a CD player, a digital radio, a computer audio board, etc.) then there is no need to use an analog signal anywhere. In that case, the PWM signal can be directly generated in the digital domain, avoiding any quality loss….

As you may have guessed, Class-D amplifiers aren’t free from difficulties. First, as for any sampling architecture, the PWM frequency must be significantly higher than the input signal’s highest frequency to avoid aliasing….The second concern with Class-D amplifiers is related to electromagnetic compatibility (EMC)…

Figure 3—A Class-D amplifier is a type of digital amplifier (at left). The comparator’s output is a PWM signal, which is amplified by a pair of low-loss digital switches. All the magic happens in the output filter (at right).

Figure 3—A Class-D amplifier is a type of digital amplifier. The comparator’s output is a PWM signal, which is amplified by a pair of low-loss digital switches. All the magic happens in the output filter.

Remember that Class C is devoted to RF amplifiers, using a transistor conducting only during a part of the signal period and a filter. Class E is an improvement to this scheme, enabling even greater efficiencies up to 80% to 90%. How?
Remember that with a Class-C amplifier, the losses only occur in the output transistor. This is because the other parts are capacitors and inductors, which theoretically do not dissipate any power.

Because power is voltage multiplied by current, the power dissipated in the transistor would be null if either the voltage or the current was null. This is what Class-E amplifiers try to do: ensure that the output transistor never has a simultaneously high voltage across its terminals and a high current going through it….

Class G and Class H are quests for improved efficiency over the classic Class-AB amplifier. Both work on the power supply section. The idea is simple. For high-output power, a high-voltage power supply is needed. For low-power, this high voltage implies higher losses in the output stage.

What about reducing the supply voltage when the required output power is low enough? This scheme is clever, especially for audio applications. Most of the time, music requires only a couple of watts even if far more power is needed during the fortissimo. I agree this may not be the case for some teenagers’ music, but this is the concept.

Class G achieves this improvement by using more than one stable power rail, usually two. Figure 4 shows you the concept.

Figure 4—A Class-G amplifier uses two pairs of power supply rails. b—One supply rail is used when the output signal has a low power (blue). The other supply rail enters into action for high powers (red). Distortion could appear at the crossover.

Figure 4—A Class-G amplifier uses two pairs of power supply rails. b—One supply rail is used when the output signal has a low power (blue). The other supply rail enters into action for high powers (red). Distortion could appear at the crossover.

2.4-GHz RF High-Power Amplifier


The SST12CP12 high-power amplifier

The SST12CP12 is a 2.4-GHz RF high-power amplifier that adds support for 256-QAM ultra-high data rate modulation. With its high linear output power, this amplifier significantly extends the range of IEEE 802.11b/g/n WLAN systems while providing excellent power at the maximum 256-QAM data rate. The amplifier is also spectrum-mask compliant up to 28.5 dBm for 802.11b/g communication and utilizes orthogonal frequency-division multiplexing (OFDM) to correct severe channel conditions without using complex equalization filters.

The SST12CP12 power amplifier has a 380mA at 23 dBm low operating current, which enables more transmission channels and a higher data rate for each system. The amplifier also features easy to use 50-Ω on-chip input match and simple output match. In addition, the integrated linear power detector provides temperature stability and immunity to voltage standing wave ratio (VSWR) radio-wave reflection to provide accurate output power control.

The SST12CP12 costs $0.97 each, in 10,000-unit quantities. It ships in a 3-mm × 3-mm × 0.55-mm, 16-pin QFN package.

Microchip Technology, Inc.

Q&A: Alenka Zajić, Communications Specialist

From building RF components for cell phones to teaching signal processing and electromagnetics at Georgia Institute of Technology’s School of Electrical and Computer Engineering, Alenka Zajić has always been interested in engineering and communications. Alenka and I discussed her fascination with a variety of communication technologies including mobile-to-mobile, computer system, energy-efficient, and wireless. She also described her current research, which focuses on improving computer communication.

Alenka Zajić

Alenka Zajić

NAN: Give us some background information. Where are you located? Where and what did you study?

ALENKA: I am originally from Belgrade, Serbia, where I got my BS and MS degrees at the School of Electrical Engineering, University of Belgrade.

After graduating with a BS degree, I was offered a design engineer job at Skyworks Solutions in Fremont, CA, where my job was to create passive RF components (e.g., antennas, filters, diplexers, baluns, etc.) for cell phones.

I was very excited to move to California, but was not sure if I would like to pursue an engineering career or a research/academic career. Since it took about six months to get an H1B visa, I decided to take all the required MS courses in Belgrade while waiting for the visa and all I had to do was finish the thesis while working in California. It was a bigger challenge than I expected, but it worked out well in the end.

While I enjoyed working in the industry, I was always more drawn to research than commercialization of products/innovations. I also enjoy “trying something new,” so it became clear to me that I should go back to school to complete my doctoral studies. Hence, I moved to Atlanta, GA, and got my PhD at the School of Electrical and Computer Engineering, Georgia Institute of Technology.

After graduation, I worked as a researcher in the Naval Research Laboratory (Washington, DC) and as a visiting assistant professor in the School of Computer Science, Georgia Tech, until last year, when I became the assistant professor at the School of Electrical and Computer Engineering, Georgia Tech.

NAN: How long have you been teaching at Georgia Tech? What courses do you currently teach and what do you enjoy most about teaching?

ALENKA: This is my second year at the School of Electrical and Computer Engineering. Last year, I taught introduction to signal processing and electromagnetics for undergraduates. This year, I am teaching electromagnetics for graduate students. One of the most rewarding aspects of university teaching is the opportunity to interact with students inside and outside of the classroom.

NAN: As an engineering professor, you have some insight into what interests future engineers. What are some “hot topics” that intrigue your students?

ALENKA: Over the years, I have seen different areas of electrical and computer engineering being “hot topics.” Currently, embedded programming is definitely popular because of the cell phone applications. Optical communications and bioengineering are also very popular.

NAN: You have contributed to several publications and industry journals, written papers for symposiums, and authored a book, Mobile-to-Mobile Wireless Channels. A central theme is mobile-to-mobile applications. Tell us what fascinates you about this topic.

ALENKA: Mobile communications are rapidly becoming the communications in most people’s minds because they provide the ability to connect people anywhere and at any time, even on the move. While present-day mobile communications systems can be classified as “fixed-to-mobile” because they enable mobility only on one end (e.g., the mobile phone) while the other end (e.g., the base station) is immobile, emerging mobile-to-mobile (M-to-M) communications systems enable mobile users or vehicles to directly communicate with each other.

The driving force behind M-to-M communications is consumer demand for better coverage and quality of service (e.g., in rural areas where base stations or access points are sparse or not present or in disaster-struck areas where the fixed infrastructure is absent), as well as increased mobility support, location-based services, and energy-efficient communication (e.g., for cars moving in opposite directions on a highway that exchange information about traffic conditions ahead, or when mobile devices “gang together” to reach a far-away base station without each of them expending a lot of power).

Although M-to-M is still a relatively young technology, it is already finding its way into wireless standards (e.g., IEEE 802.22 for cognitive radio, IEEE 802.11p for intelligent transportation systems, IEEE 802.16 for WiMAX systems, etc.).

Propagation in M-to-M wireless channels is different from traditional fixed-to-mobile channels. The quality of service, energy efficiency, mobility support, and other advantages of M-to-M communication all depend on having good models of the M-to-M propagation channels.

My research is focused on studying propagation and enabling communication in challenging environments (e.g., vehicle-to-vehicle wireless radio communications, underwater vehicle-to-underwater vehicle acoustic communications, and inside a processor chip). In each of these projects, my work aims not only to improve existing functionality, but also to provide highly useful functionality that has not existed before. Examples of such functionality include navigating people in a direction that will restore (or improve) their connection, voice, or even video between submerged vehicles (e.g., for underwater well-service operations), and use of on-chip transmission lines and antennas to achieve broadcast-type communication that is no longer feasible using traditional wires.

NAN: Your research interests include electromagnetics and computer system and wireless communications. How have your interests evolved?

ALENKA: My research was mostly focused on electromagnetics and its impact on wireless communications until I joined the School of Computer Science at Georgia Tech. Talking to my Computer Science colleagues, I have realized that some of the techniques developed for telecommunications can be modified to improve communication among processors, memory, racks in data centers, and so forth. Hence, I started investigating the problem of improving communication among computers.

NAN: What types of projects are you currently working on?


Two of Alenka Zajić's currrent projects are energy-efficient underwater acoustic communications and electromagnetic side channels in high-performance processors and systems.

Two of Alenka Zajićs currrent projects are energy-efficient underwater acoustic communications and electromagnetic side channels in high-performance processors and systems.

ALENKA: I have several projects and they all include theoretical and experimental work. Two of my current projects are energy-efficient underwater acoustic communications and electromagnetic side channels in high-performance processors and systems. I will provide a brief explanation of each project.

Energy-efficient underwater acoustic communications: Many scientific, defense, and safety endeavors require communications between untethered submerged devices and/or vehicles.

Examples include sensor networks for seismic monitoring, analysis of resource deposits, oceanographic and environmental studies, tactical surveillance, and so forth, as well as communications between unmanned or autonomous underwater vehicles (UUVs, AUVs) for deep-water construction, repairs, scientific or resource exploration, defense applications, and so forth. Such underwater sensing and vehicular applications will require energy-efficient underwater communications, because underwater sensor networks and AUVs are highly energy-constrained. They are typically powered by batteries that are very difficult to replace or recharge deep underwater. At the same time, existing wireless communication approaches still provide extremely low data rates, work over very limited distances, and have low energy efficiency. Radio signals and wireless optics have a very limited range underwater, so underwater wireless communications mostly rely on acoustic signals that can travel long distances in water.

Some of Alenka’s research focuses on electromagnetic side channels in high-performance processors and systems. This is a measurement setup.

Some of Alenka’s research focuses on electromagnetic side channels in high-performance processors and systems. This is a measurement setup.

Unfortunately, acoustic underwater communications have a narrow available spectrum—propagation delays that are orders-of-magnitude longer than in radio communications—and many sources of signal distortion that further reduce data rates and increase the required transmitted power when using simple modulations and coding. Hence, we are working on characterization of underwater acoustic channels and their implications for underwater-vehicle-to-underwater-vehicle communications and networking.

Electromagnetic side channels in high-performance processors and systems: Security of many computer systems relies on the basic assumption that data theft through unauthorized physical tampering with the system is difficult and easily detected, even when attackers are in physical proximity to systems (e.g., desktops in cubicles, laptops and smartphones used in public spaces, remote data centers used for cloud computing, remotely operated robotic vehicles, aircraft, etc.).

On the other hand, the motivation for attackers keeps expanding. Increasing use of electronic banking provides monetary incentives for successful attacks, while the trend toward computer-controlled everything (e.g., power plants, robotic weapons, etc.) can motivate terrorists and/or rogue states.

Although simple physical attacks (e.g., stealing the system or taking it apart to insert snooping devices) are relatively hard to carry out without significant risk of detection, more sophisticated physical attacks are likely to be explored by attackers as incentives for such attacks grow. Side-channel attacks are especially worrisome, because they circumvent traditional protection measures.

Most side-channel attacks (e.g., power analysis, timing analysis, or cache-based attacks) still require some degree of direct access (i.e., to attach probes, run processes, etc.) that exposes attackers to a significant risk of detection. However, attacks that exploit electromagnetic emanations from the system only require physical proximity. So, increasingly motivated attackers may be able to carry out numerous attacks completely undetected, and several other side channels (e.g., power, timing, memory use, etc.) can “spill over” into the electromagnetic side channel, turning electromagnetic emanations into a very information-rich side channel.

My work in this domain focuses on carrying out a systematic investigation of electromagnetic side channel data leakage, quantifying the extent of the threat, and providing useful insights for computer designers to minimize such leakage.

NAN: Is there a particular electronics engineer or academic who has inspired the type of work you do today?

ALENKA: I have been fortunate to have great mentors (Dr. Antonije Djordjević and Dr. Gordon Stüber) who taught me the importance of critical thinking, asking the right questions in problem-solving, and clearly and concisely stating my ideas and results.

ISM Basics (EE Tip #100)

The industrial, scientific, and medical (ISM) bands are radio frequency ranges freely available for industrial, scientific and medical applications, although there are also many devices aimed at private users that operate in these bands. ISM devices require only general type approval and no individual testing.

Source: Wolfgang Rudolph & Burkhard Kainka’s article, “ATM18 on the Air,” 080852, Elektor, 1/2009.

Source: Wolfgang Rudolph & Burkhard Kainka’s article, “ATM18 on the Air,” 080852, Elektor, 1/2009.

The radio communication sector of the International Telecommunication Union (ITUR) defines the ISM bands at an international level. Wi-Fi and Bluetooth operate in ISM bands, as do many radio headphones and remote cameras, although these are not usually described as ISM devices. These devices are responsible for considerable radio communications interference (especially at 433 MHz and at 2.4 GHz).

ITU-R defines the following bands, not all of which are available in every country:

  • 6.765 to 6.795 MHz
  • 13.553 to 13.567 MHz
  • 26.957 to 27.283 MHz
  • 40.66 to 40.70 MHz
  • 433.05 to 434.79 MHz
  • 902 to 928 MHz
  • 2.400 to 2.500 GHz
  • 5.725 to 5.875 GHz
  • 24 to 24.25 GHz

Some countries allocate further ISM bands in addition to those above. ISM applications have the lowest priority within any given band. Many bands available for ISM are shared with other spectrum users: for example the 433 MHz ISM band is shared with 70 cm amateur radio communications.

ISM users must not interfere with other users, but must be able to tolerate the interference to their own communications caused by higher-priority users in the same band. The band from 868 MHz to 870 MHz is often mistakenly characterized as an ISM band. It is nevertheless available to short-range radio devices, such as RFID tags, remote switches, remote alarm systems, and radio modules.

For more information, refer to Wolfgang Rudolph & Burkhard Kainka’s article, “ATM18 on the Air,” 080852, Elektor, 1/2009.

Issue 265: Embedded Systems Abound

I recently read on the transcript of an interview (May 9, 2002) with arachnologist Norman Platnick who stated: “You’re probably within seven or eight feet of spider no matter where you are. The only place on earth that has no spiders at all—as far as we know—is Antarctica.” It didn’t take long for me to start thinking about embedded systems and my proximity to them. Is the average person always within several feet of embedded systems? Probably not. But what about 50% or 60% of the time? E-mail me your thoughts.

Circuit Cellar 265, August 2012 - Embedded Development

Embedded systems are becoming ubiquitous. They’re in vehicles, mobile electronics, toys, industrial applications, home appliances, and more. If you’re indoors, the temperature is likely monitored and controlled by an embedded system. When you’re engaged in outdoor activities (e.g., hiking, golfing, biking, or boating), you probably have a few MCU-controlled devices nearby, such as cell phones, rangefinders, pedometers, and navigation systems. This month we present articles about how embedded systems work, and our authors also provide valuable insight about topics ranging from concurrency to project development.

Freescale’s Mark Pedley kicks off the issue with a revealing article about a tilt-compensating electronic compass (p. 16). Now you can add an e-compass to your next MCU-based project.

E-compass technology (Source: M. Pedley, CC265)

Turn to page 24 for an in-depth interview with Italy-based engineer Guido Ottaviani. His fascination with electronics engineering, and robotics in particular, will inspire you.

Have you ever come across a product that you know you could have designed better? Scott Weber had that experience and then acted on his impulse to build a more effective system. He created an MCU-based light controller (p. 32).

The MCU-based light controller is on the right (Source: S. Weber, CC265)

If you want to ensure a microcontroller works efficiently within one of your systems, you should get to know it inside and out. Shlomo Engelberg examines the internal structure of an I/O pin with a pull-up resistor (p. 40).

Bob Japenga continues his series “Concurrency in Embedded Systems” on page 44. He covers atomicity and time of check to time of use (TOCTTOU).

On page 48 George Novacek presents the second part of his series on project development. He covers project milestones and design reviews.

Ed Nisley’s June 2012 article introduced the topic of MOSFET channel resistance. On page 52 he covers his Arduino-based MOSFET tester circuitry and provides test results.

The MOSFET tester PCB hides the Arduino that runs the control program and communicates through the USB cable on the left edge. (Source: E. Nisley, CC265)

If you read Robert Lacoste’s June 2012 article, you now understand the basics of frequency mixers. This month he presents high-level design methods and tools (p. 58).

Jeff Bachiochi wraps up the issue with an examination of a popular topic—energy harvesting (p. 68). He covers PV cell technology, maximum power point tracking (MPPT), and charge management control.

A great way to investigate MPPT for your design is to use an STMicroelectronics evaluation board, such as this STEVAL-ISV006V2 shown in the top of the photo. The smaller cell in the center is rated at 165 mW (0.55-V output at 0.3 A) measuring 1.5” × 0.75”. At the bottom is a Parallax commercial-quality solar cell that is rated at 2.65 W (0.534-V output at 5.34 A) measuring 125 mm. (Source: J. Bachiochi, CC265)

Circuit Cellar 265 is currently on newsstands.

Elektor RF & Microwave App for Android

Elektor has an iPhone/iPad app for several months. And now Android users can have an Elektor app of their own. The Elektor RF & Microwave Toolbox app is perfect for engineers and RF technicians who need easy, reliable access to essential equations, converters, calculators, and tools.

A screenshot of the Elektor RF & Microwave app for Android

The app includes the following handy tools:

1.Noise floor (Kelvin,dBm)
2.Amplifier cascade (NF, Gain, P1db, OIP2, OIP3)
3.Radar equation (2-way path loss)
4.Radio equation (1-way path loss)
5.Power and voltage converter (W,dBm,V,dBµV)
6.Field intensity and power density converter (W/m2, V/m, A/m, Tesla, Gauss,dBm, W)
7.Mismatch error limits (VSWR, Return loss)
8.Reflectometer (VSWR, Return loss)
9.Mitered Bend
10.Divider and Couplers (Wilkinson, Rat race, Branchline , microstrip and lumped)
11.Balanced and und balanced PI and T attenuator
12.Skin depth (DC and AC resistance)
13.PCB Trace calculator (impedance/dimensions)
14.Image rejection (amplitude and phase imbalance)
15.Mixer harmonics (up and down conversion)
16.Helical antenna
17.Peak to RMS (peak, RMS, average, CF)
18.Air Core Inductor Inductance
19.Parallel plate Capacitor
20.PI and T attenuator
21.Ohm’s Law
22.Parallel LCR impedance/resonance
23.Series LCR impedance/resonance
24.Inductor impedance
25.Capacitance impedance
26.Antenna temperature (Kelvin)
27.Radar Cross Section (RCS) calculator (Sphere,Cylinder, flat plate, corners, dBsm)
28.Noise Figure Y-Factor Method
29.EMC (EIRP, ERP, dBµV/m)
30.Noise figure converter (dB, linear, Kelvin)
31.Frequency Band Designations
32.Resistor color code (reverse lookup, 3 to 6 band)
33.Filter Design (Butterworth, Chebyshev, prototype):
34.µ-Filter Design (microstrip, stripline)
35.PCB Trace Width and Clearance Calculator

Visit the Android Market for more information about the Elektor app.

Circuit Cellar does not yet have an app for Android. The Circuit Cellar iPhone/iPad app is available on iTunes.

Screenshots of the Circuit Cellar app

Elektor International Media is the parent company of Circuit Cellar.

RFI Bypasssing

With GPS technology and audio radio interfaces on his personal fleet of bikes, Circuit Cellar columnist Ed Nisley’s family can communicate to each other while sending GPS location data via an automatic packet reporting system (APRS) network. In his February 2012 article, Ed describes a project for which he used a KG-UV3D radio interface rigged with SMD capacitors to suppress RF energy. He covers topics such as test-fixture measurements on isolated capacitors and bypassing beyond VHF.

Photo 2 from the Febuary article, "RFI Bypassing (Part 1)." A pair of axial-lead resistors isolate the tracking generator and spectrum analyzer from the components under test. The 47-Ω SMD resistor, standing upright just to the right of the resistor lead junction, forms an almost perfect terminator. (Source: Ed Nisley CC259)

Ed writes:

Repeatable and dependable measurements require a solid test fixture. Although the collection of parts in Photo 2 may look like a kludge, it’s an exemplar of the “ugly construction” technique that’s actually a good way to build RF circuits. “Some Thoughts on Breadboarding,” by Wes Hayword, W7ZOI, gives details and suggestions for constructing RF projects above a solid printed circuit board (PCB) ground plane.

You can read this article now in Circuit Cellar 259. If you aren’t a subscriber, you can purchase a copy of the issue here.