Antenna Measurement Made Easy

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Covering the Basics

If you’re doing any kind of wireless communications application, that probably means including an antenna in your design. The science of antennas is complex. But here Robert shows how the task of measuring an antenna’s performance is less costly and exotic than you’d think.

By Robert Lacoste

Now that wireless communications is ubiquitous, chances are you’ll be using Bluetooth, Wi-Fi, cellular, LoRa, MiWi or other flavor of wireless interface in your next design. And that means including an antenna. Unfortunately, antenna design is not an easy topic. Even very experienced designers sometimes have had to wrestle with unexpected bad performances by their antennas. Case in point: Google “iPhone 4 antenna problem” and you will get more than 3 million web pages! In a nutshell, Apple tried to integrate a clever antenna in that model that was threaded around the phone. They didn’t anticipate that some users would put their fingers exactly where the antenna was the most sensitive to detuning. Was it a design flaw? Or a mistake by the users? It was hotly debated, but this so-called “Antennagate” probably had significant impact on Apple’s sales for a while.

I already devoted an article to antenna design and impedance matching (“The Darker Side: Antenna Basics”, Circuit Cellar 211, February 2008). Whether you include a standard antenna or design your own, you will never be sure it is working properly until you measure its actual performance. Of course, you could simply evaluate how far the system is working. But how do you go farther if the range is not enough? How do you figure out if the problem is coming from the receiver, the transmitter, propagation conditions or the antenna itself? My personal experience has been that the antenna is very often the culprit. With that in mind, it really is mandatory to measure whether or not an antenna is behaving correctly. Take a seat. This month, I will explain how to easily measure the actual performance of an antenna. You will see that the process is quite easy and that it won’t even need costly or exotic equipment.

SOME ANTENNA BASICS

Let’s start with some basics on antennas. First, all passive antennas have the same performance whether transmitting or receiving. For this article, I’ll consider the antenna as transmitting because that’s easier to measure. Let’s consider an antenna that we inject with a given radio frequency power Pconducted into its connector. Where will this power go? First off, impedance matching should be checked. If the impedance of the antenna is not well matched to the impedance of the power generator, then a part of the power will be reflected back to the generator. This will happen in particular when the transmit frequency is not equal to the resonant frequency of the antenna. In such a case, a part of Pconducted will be lost.  That is known as mismatch losses: Pavailable= Pconducted – MismatchLosses. While that itself is a very interesting subject, I have already discussed impedance matching in detail in my February 2008 article. I also devoted another article to a closely linked topic: standing waves. Standing waves appear when there is a mismatch. The article is “The Darker Side: Let’s play with standing waves” (Circuit Cellar 271, February 2013).

For the purpose of discussion here, I will for now assume that there isn’t any mismatching—and therefore no mismatch loss. …

Read the full article in the October 327 issue of Circuit Cellar

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Numeric Precision vs. DDS Calculations

Using the full frequency resolution of a direct digital synthesizer chip outstrips the capabilities of floating point numbers. Ed takes a look at what’s needed for high-resolution frequency calibration and measurements.

By Ed Nisley

As you saw in my July article, the filter bandwidths and frequency resolution required to characterize low-frequency quartz resonators far exceeded the capabilities of my bench instruments. I decided to take a look at building a special-purpose resonator tester around a cheap direct digital synthesizer sine-wave source, because DDS generators have

PHOTO 1 A knockoff Arduino Nano controls a generic AD9850 direct digital synthesizer circuit, both plugged into standard 0.1 inch headers, with hand-wiring connections below the proto board. The SMA connector provides a mechanically rugged output from the board; the DDS frequencies don’t require its RF properties.

PHOTO 1
A knockoff Arduino Nano controls a generic AD9850 direct digital synthesizer circuit, both plugged into standard 0.1 inch headers, with hand-wiring connections below the proto board. The SMA connector provides a mechanically rugged output from the board; the DDS frequencies don’t require its RF properties.

advantages over traditional analog oscillators and frequency counters in computer-controlled measurement systems.

Of course, nothing is ever so simple as it seems. In this article, I’ll explain how numeric precision affects Direct Digital Synthesis (DDS) output frequency calculations, work through the effects of floating-point and fixed-point arithmetic, and show how a carefully tweaked DDS oscillator frequency varies with temperature.

DDS Calculations

You can think of a direct digital synthesizer as a lookup table holding the digitized values of an analog waveform, a counter addressing the table entries in ascending order, and a DAC converting the numbers to analog voltages. The Analog Devices AD8950 DDS chip in Photo 1 has the equivalent of a table with 232 10-bit entries defining a sine wave, a counter clocked at 125 MHz, and a differential output current-mode DAC. The PCB, complete with the DDS and a 125 MHz quartz oscillator, costs under $20 on eBay or Amazon. …

Read the full article in the September 326 issue of Circuit Cellar

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New Thermal Imaging Solution for Benchtop Electronics Testing

FLIR Systems recently launched the FLIR ETS320 thermal imaging solution for benchtop electronics testing. Well suited for testing and analyzing the thermal characteristics of electronic components and printed circuit boards (PCBs), the battery-powered FLIR ETS320 comprises a high-sensitivity thermal camera and an adjustable, hands-free table stand. With more than 76,000 points of temperature measurement, the rechargeable FLIR ETS320 enables you to monitor power consumption, detect hot spots, and identify potential points of failure during product development. The highly accurate camera can visualize small temperature differences so you can evaluate thermal performance, ensure environmental compatibility, and troubleshoot problems.FLIR ETS 320

 

The FLIR ETS320 ships fully assembled and ready to connect to a PC running FLIR Tools software for detailed data analysis, recording, and reporting. The integrated test stand and sliding mount design offer flexibility when imaging electronic components of various sizes.

The FLIR ETS320 costs $2499 and is available through established FLIR distribution partners.

Source: FLIR Systems

Arduino-Based Liquid Level-Sensing Hardware

SST Sensing and Sparkfun recently developed an easy-to-use solution for single-point liquid detection using infrared technology. Highly accurate and reliable, the solution features an Optomax Digital liquid level switch, which is connected to an Arduino board via the TTL output and powered by the 5-V source. SST Sparkfun

Whether you’re a professional engineer or DIYer, you’ll find it easy to program the Arduino’s LEDs to indicate when the sensor is immersed in liquid (and thus determine if the liquid level is too high or low). The compact switch lends itself will to space-constrained applications. With long cabling, you can place the sensor near a liquid without putting the other electronics at risk. Since this is an optical solution, you can avoid a variety of issues (e.g., jamming and wear and tear) and ensure long operational lifespan.

SST offers the liquid level switch in a robust housing tip with either a Polysulfone or Trogamid construction (depending on the particular application requirements). The complete solution has an operational temperature range of –25°C to 80°C.

Source: SST

New Sensor Technologies for Next-Gen Temperature Measurement

Melexis recently announced two new sensing technologies for next-generaration temperature measurement. The MLX90640 sensor array is an alternative to high-end thermal cameras. The MLX90342 is a quad thermocouple interface that addresses automotive sensing to 1300ºC.

The MLX90640 IR sensor arrays benefits, characteristics, and specs:

  • 32 × 24 pixels
  • –40° to 85°C operational temperature range; measures object temperatures from 240°C and 300°C
  • ±1°C target object temperature accuracy
  • Noise equivalent temperature difference (NETD) of 0.1K RMS at a 1-Hz refresh rate
  • Doesn’t require frequent recalibration
  • Field-of-view (FoV) options: 55° × 35° version and 110° × 75° wide-angle version
  • Compact, four-pin TO39 package incorporating the requisite optics
  • I2C-compatible digital interface
  • Target applications: fire prevention systems, HVAC equipment, smart buildings, and IP/surveillance systemsMLX90342 Melexis

The MLX90342 high-performance quadruple thermocouple interface benefits, characteristics, and specs:

  • Supports a –40° to 1300°C thermocouple temperature range
  • Operating temperature specification of –40° to 155°C
  • On-board cold junction compensation and linearization
  • Factory calibration; guaranteed intrinsic accuracy of ±5°C at 1100°C.
  • 26-pin 6 mm × 4 mm QFN package
  • 50-Hz Rapid refresh rate
  • Temperature data can be transmitted via a SENT Revision 3 digital interface
  • Target applications: turbo charger temperature control, exhaust gas recirculation, and diesel/gas particle filtering systems

Source: Melexis

10-Bit HDO9000 High Definition Oscilloscopes

Teledyne LeCroy recently launched the HDO9000, which uses HD1024 high-definition technology that automatically optimizes vertical resolution under each measurement condition to deliver 10 bits of vertical resolution. Featuring a bright 15.4” capacitive touch screen, the HDO9000 oscilloscopes offer 10-bit resolution, bandwidths of 1 to 4 GHz, and sample rates of 40 GS/s. The HDO9000 and MAUI with OneTouch enables you to perform all common operations with one touch of the display.Teledyne hdo9000

The HDO9000’s features, benefits, and specs:

  • HD1024 high-definition technology provides 10 bits of vertical resolution with 4-GHz bandwidth.
  • 15.4” high resolution capacitive touch screen
  • The mixed signal (-MS) models have 16 digital lines for trigger, decode, and measurements for analyzing timing irregularities or for general-purpose debugging.
  • Compatibility with the HDA125 High-speed Digital Analyzer, with 12.5 GS/s digital sampling rate on 18 input channels, and the revolutionary QuickLink probing solution
  • Several optional software packages are available to equip HDO9000 for all validation and debug requirements ranging from automated standards compliance packages to flexible debugging toolkits.
  • The HDO9000 is available in 1, 2, 3, or 4 GHz bandwidths

The HDO9000’s prices range from $21,250 to $37,400. -MS versions of each model are available with 16 digital channel sampling at 1.25 GS/s for an additional $3,000.

Source: Teledyne LeCroy teledynelecroy.com

Build an Accurate Milliohm Meter

A milliohm meter is a handy benchtop tool for measuring small electrical resistance values. In this article, Mark Driedger details how to build a microcontroller-based milliohm meter that accurately measures DC resistance from 10 mΩ to 10 kΩ.


I built an Arduino-based milliohm meter that accurately measures DC resistance from 10 mΩ to 10 kΩ. I used careful design techniques to cancel many error sources rather than resort to costly components. The milliohm meter is useful for tasks such as measuring transformer and inductor winding resistance, ammeter current shunts, and resistance of PCB tracks.

The finished milliohm meter

The finished milliohm meter

Measurement Method

The milliohm meter calculates the value of the resistor under test (Rx) by measuring the voltage across it and the voltage across a series-connected, known reference resistor (Rr) when driven by a test current. The measured resistance is simply: Rx = Vx/Vr × Rr.

A technique called synchronous rectification (also known as a lock-in amplifier) is used to enhance accuracy. The direction of the test current is alternated and measurements of Vx and Vr are made synchronously with the change in direction of the test current. As we will see, this cancels a number of error sources and is easy to implement on the Arduino.

Synchronous rectification can be thought of as narrowband filter at the switching frequency, implemented using a mixer (multiplier) at the switching frequency followed by a low-pass filter at DC (averaging). Normally, the switching frequency would be high enough (say, 1 kHz) to allow AC-coupled, high-gain amplifiers to be used and to move the filter passband well away from induced 60-Hz AC line voltages. In this implementation, the relatively slow ADC conversion speed prevents us from using a high switching frequency. However, we retain many other benefits of synchronous rectification with regard to reducing measurement error and we gain accuracy improvement in other ways.

Implementation

An Arduino is used to control the synchronous rectification, read voltages Vx and Vr, and then compute and display the test resistor value. The test current is derived by paralleling four I/O pins through current-limiting resistors for each of the source and sink legs.

sad dsg

The circuitry

This increases the test current to roughly 100 mA, which is still well within the 40 mA/pin and 200 mA/chip limits of the Arduino processor, and the 150 mA limit of the Pro Mini’s onboard voltage regulator. The source and sink legs are alternately driven high and low to produce the test current.

sdfg sdgf

A look inside the meter

Measurement of Vx and Vr is made with an Analog Devices ADS1115 ADC, which has two differential inputs, a programmable gain amplifier (PGA) with 16× maximum gain, and 16-bit accuracy in a very small 10 MSOP package. The device costs between $10 and $15 on a small PCB module. Series resistors and film capacitors on the analog inputs provide some overload protection and noise filtering. At the highest gain, the meter resolution is approximately 75 µΩ/bit. Each measurement consists of two cycles of synchronous rectification, with 100 samples per cycle for a total of 200 samples.

An OLED module with an I2C interface is used for the display, although other options can be substituted with corresponding code changes. The meter is powered by a 9-V battery. Battery voltage is read through one of the analog input ports. Measurements are initiated with the push of the test switch to maximize battery life and minimize self-heating errors in the reference resistor. Each measurement takes roughly 2 s. Purchased modules are used for the Arduino, ADS1115 ADC, and the 64 × 128 OLED display, making it very easy to build.

OLED

OLED for displaying data

Construction

The meter is built using purchased modules and a small piece of protoboard for the shield. The ADC and display modules are available from multiple sources, and you can use any Arduino module of your choosing. (The photos and layout are for the Pro Mini.) Keep the ADC analog input wiring short and away from the processor. Use a four-wire connection to the reference resistor. Solder the drive leads farthest from the body, and the sense leads closer. The display module is mounted on the reverse side of the protoboard. The SDA/SCL I2C connections are brought from the Arduino module to the protoboard with a short cable and connector since they are not on the regular 0.1” grid.

dsf dsf

Protoboard layout

The ADS1115 module includes the pull-ups that are needed on the I2C interface lines (SDA, SCL).  I used a six-pin GX-16-6 connector for the probes. The additional two pins were used to close the battery circuit on the ground side, turning the meter on and off when the probes are connected.


The complete article appears in Circuit Cellar 314 (September 2016).

Mark Driedger has been experimenting with tube audio and electronics for over 35 years. His earned a BSc and MSc in Electrical Engineering in his native Canada. Mark has worked in the telecom industry for the past 28 years in various technical, business, and executive roles. He is currently COO for Procera Networks and lives in Dallas, TX.

High-Performance 100-kHz Handheld LCR Meter

B&K Precision recently announced the availability of a 100-kHz handheld LCR meter that includes features usually found only in bench-top meters. You can use the portable 880 model LCR meter to measure inductance, capacitance, and resistance with 0.1% basic impedance accuracy. Its provides test frequencies up to 100 kHz, selectable test signal levels, and four-terminal measurement capabilities.BK-LCR-Meter

Key features and specs include:

  • A fast auto-ranging function, convenient single-push auto detect mode, and versatile functions such as data recording, tolerance sorting, and relative mode
  • Four-terminal shielded configurations
  • A dual display with 40,000-count and 10,000-count resolution for primary and secondary measurements, respectively.
  • DC resistance measurement capability
  • Standard accessories such as an AC adapter with rechargeable 9-V battery, a mini USB cable, a shorting plate, banana-to-alligator test leads, Kelvin clip test leads, and an additional tweezer tool for measuring SMD components.

The 880 LCR meter comes with a three-year warranty and costs $399.

Source: B&K Precision

The Future of Electronic Measurement Systems

Trends in test and measurement systems follow broader technological trends. A measurement device’s fundamental purpose is to translate a measurable quantity into something that can be discerned by a human.  As such, the display technology of the day informed much of the design and performance limitations of early electronic measurement systems. Analog meters, cathode ray tubes, and paper strip recorder systems dominated.  Measurement hardware could be incredibly innovative, but such equipment could only be as good as its ability to display the measurement result to the user. Early analog multimeters could only be as accurate as a person’s ability to read to which dash mark the needle pointed.ipad_hand

In the early days, the broader electronics market was still in its infancy and didn’t offer much from which to draw. Test equipment manufacturers developed almost everything in house, including display technology. In its heyday, Tektronix even manufactured its own cathode ray tubes. As the nascent electronics market matured, measurement equipment evolved to leverage the advances being made. Display technology stopped being such an integral piece. No longer shackled with the burden of developing everything in house, equipment makers were able to develop instruments faster and focus more on the measurement elements alone. Advances in digital electronics made digital oscilloscopes practical. Faster and cheaper processors and larger memories (and faster ADCs to fill them) then led to digital oscilloscopes dominating the market. Soon, test equipment was influenced by the rise of the PC and even began running consumer-grade operating systems.

Measurement systems of the future will continue to follow this trend and adopt advances made by the broader tech sector. Of course, measurement specs will continue to improve, driven by newly invented technologies and semiconductor process improvements. But, other trends will be just as important. As new generations raised on Apple and Android smartphones start their engineering careers, the industry will give them the latest advances in user interfaces that they have come to expect. We are already seeing test equipment start to adopt touchscreen technologies. This trend will continue as more focus is put on interface design. The latest technologies talked about today, such as haptic feedback, will appear in the instruments of tomorrow. These UI improvements will help engineers better extract the data they need.

As chip integration follows its ever steady course, bench-top equipment will get smaller. Portable measurement equipment will get lighter and last longer as they leverage low-power mobile chipsets and new battery technologies. And the lines between portable and bench-top equipment will be blurred just as laptops have replaced desktops over the last decade. As equipment makers chase higher margins, they will increasingly focus on software to help interpret measurement data. One can imagine a subscription service to a cloud-based platform that provides better insights from the instrument on the bench.

At Aeroscope Labs (www.aeroscope.io), a company I cofounded, we are taking advantage of many broader trends in the electronics market. Our Aeroscope oscilloscope probe is a battery-powered device in a pen-sized form factor that wirelessly syncs to a tablet or phone. It simply could not exist without the amazing advances in the tech sector of the past 10 years. Because of the rise of the Internet of Things (IoT), we have access to many great radio systems on a chip (SoCs) along with corresponding software stacks and drivers. We don’t have to develop a radio from scratch like one would have to do 20 years ago. The ubiquity of smart phones and tablets means that we don’t have to design and build our own display hardware or system software. Likewise, the popularity of portable electronics has pushed the cost of lithium polymer batteries way down. Without these new batteries, the battery life would be mere minutes instead of the multiple hours that we are able to achieve.

Just as with my company, other new companies along with the major players will continue to leverage these broader trends to create exciting new instruments. I’m excited to see what is in store.

Jonathan Ward is cofounder of Aeroscope Labs (www.aeroscope.io), based in Boulder, CO. Aeroscope Labs is developing the world’s first wireless oscilloscope probe. Jonathan has always had a passion for measurement tools and equipment. He started his career at Agilent Technologies (now Keysight) designing high-performance spectrum analyzers. Most recently, Jonathan developed high-volume consumer electronics and portable chemical analysis equipment in the San Francisco Bay Area. In addition to his decade of industry experience, he holds an MS in Electrical Engineering from Columbia University and a BSEE from Case Western Reserve University.

Keysight Announces New Precision SMU Series Software Control Options

Keysight Technologies now offers a variety of different software control options for its B2900A Series Precision Source/Measure Units (SMUs). With the low-cost or free software options, you can access a variety of capabilities to support basic voltage and current sourcing up through full characterization of devices and materials using an intuitive GUI.Keysight b2900 smu

With a B2900A software control option, useyou don’t need to create a software measurement environment. This reduces development and evaluation times, making the B2900A SMUs well suited university educators, circuit designers, and R&D engineers.

The software control options for the B2900A SMUs include:

  • EasyEXPERT group+, which provides powerful IV parametric characterization for a wide range of devices and materials. The software is currently utilized in Keysight’s high-end precision current-voltage analyzer products (e.g., the B1500A, B1505A and E5270B/E526xA).
  • BenchVue, which enables benchtop integration of B2900A SMUs (as voltage/current sources) with a wide variety of other Keysight instruments, such as oscilloscopes and meters.
  • B2900A Quick I/V Measurement software, which permits easy measurement setup and execution on a Windows-based PC via a user-friendly GUI. This control option supports all B2900 precision instrument family products, including SMUs, low-noise sources and electrometers, and works on multiple interfaces (LAN, USB and GPIB).
  • Graphical Web Interface, which allows any Java-enabled web browser (e.g., Internet Explorer) to control B2900A SMUs over the LAN. Because special software is not required, this control option enables quick measurements on the fly.

The new control options for the B2900A SMUs are now available. The basic one-channel precision SMU model for the benchtop (B2901A) starts at $5,000.

Source: Keysight Technologies

Time-of-Flight IC for Distance Measurement & Object Detection

Intersil Corp.’s low-power ISL29501 time-of-flight (ToF) signal processing IC is an object detection and distance measurement solution when combined with an external emitter (e.g., LED or laser) and photodiode. Intended for Internet of Things (IoT) applications and consumer mobile devices, the ISL29501 offers precision long-range accuracy up to 2 m in both light and dark ambient light conditions. You can select an emitter and photodiode and then configure a custom low-power ToF sensing system. intersil ISL29501

The ISL29501’s on-chip emitter DAC with programmable current up to 255 mA enables you to select the desired current level for driving the external infrared (IR) LED or laser. This feature enables optimization of distance measurement, object detection, and power budget. In addition, the ISL29501 can perform system calibration to accommodate performance variations of the external components across temperature and ambient light conditions.

The ISL29501’s main specs and features:

  • On-chip DSP calculates ToF for accurate proximity detection and distance measurement up to 2 m
  • Modulation frequency of 4.5 MH
  • On-chip emitter DAC with programmable current up to 255 mA a
  • On-chip active ambient light rejection
  • Programmable distance zones
  • Automatic gain control
  • 2.7 to 3.3 V Supply voltage range
  • I2C interface supports 1.8- and 3.3-V bus

The ISL29501 is available in a low profile 4 mm x 5 mm, 24-lead TQFN package for $4.87 USD in 1,000-piece quantities. The ISL29501-ST-EV1Z reference design board costs $250.

http://www.intersil.com/en

New Software to Obtain Measurement Results without MIPI or Arbitrary Waveform Generator Expertise

Keysight Technologies recently announced introduced a software plug-in for the M8070A system software for M8000 Series BER test solutions. The M8085A MIPI C-PHY receiver test solution is designed for conformance and margin tests.Keysight-M8085

The MIPI C-PHY 1.0 standard supports camera and display applications. The standard comprises multilevel non-NRZ non-differential signaling. The Keysight M8190A arbitrary waveform generator (AWG) is the right instrument to generate such signals. The M8085A easy-to-use editor option enables you to set up the parameters and pattern content of test signals for turn-on and debug interactively from the GUI in familiar, application terms. During parameter adjustments, the software controls the AWG hardware to maintain uninterrupted signal generation.

In addition, the M8085A software provides the industry’s first complete and standard-conformant routines for calibration of signal parameters and physical layer (PHY) receiver tests. Thus, you can achieve results without expertise in the MIPI standard or with arbitrary waveform generators.

The software plug-in provides several options for selecting the error-detecting device. You can connect to the built-in detector in the device under test via the IBERReader interface, which transfers the test result to the M8085A software and displays the result in the GUI. Plus, it enables fully automated unattended tests.

The M8085A C-PHY software with various options is now available.

Source: Keysight 

New RF Signal Generator

RIGOL Technologies recently expanded its portfolio of RF Test solutions with the launch of the DSG800 Series RF Signal Generator. The series—which is targeted at engineers implementing Bluetooth, Wi-Fi, and other RF interfaces in embedded systems—covers output frequencies from 9 kHz to 3 GHz. It provides maximum output power up to 20 dBm and low SSB phase noise of –105 dBc/Hz, amplitude accuracy of ±0.5 dB, and frequency resolution 0.01 Hz at any frequency. An oven-controlled crystal oscillator timebase provides less than 5 ppb temperature stability and less than 30 ppb/year aging stability.DSG800 Rigol

The DSG800 RF signal generators provide:

  • Conventional sweep functions (step, list, logarithmic, and linear)
  • Analog modulation functions including amplitude modulation (AM), frequency modulation (FM), phase modulation (ΦM), and pulse modulation
  • An optional pulse train generation capability for translating serial data onto an RF link

    There are two models in the DSG800 series. THe DSG815 (9 kHz to 1.5 GHz) costs $1,999. The DSG830 (9 kHz to 3 GHz) costs $3,599.

Source: RIGOL Technologies

Build a Three-in-One Measurement System

No home electronics lab is complete without a signal generator, logic analyzer, and digital oscilloscope. But why purchase the measurement devices separately, when you can build one system that houses all three? The process is easier than you’d expect.

Hand-soldering a package this size is tough work. The signal-generator filter has bulky coils. In contrast, the MSP430F149’s PQFP64 is tiny.

Photo 1: Hand-soldering a package this size is tough work. The signal-generator filter has bulky coils. In contrast, the Texas Instruments MSP430F149’s PQFP64 is tiny.

Salvador Perdomo writes:

I’ve built an inexpensive and versatile measurement system that contains a signal generator, logical analyzer, and digital oscilloscope. If you build your own, you’ll be able to address many of the problems typically encountered on test benches.

The system is not PC-bus connected. Instead, it’s external to the computer, making use of the RS-232 serial port shown in Figure 1. Also, it doesn’t have a power supply input, so the same serial cable feeds it. Because the computer’s serial connection provides limited power, low power consumption is a fundamental requirement.

It is of interest to have your test benches as clear as possible to search for the faulty part of your design. So, a small measurement system is highly recommended. It’s better if it isn’t connected to the mains.

Figure 1: It is of interest to have your test benches as clear as possible to search for the faulty part of your design. So,a small measurement system is highly recommended. It’s better if it isn’t connected to the mains.

The low-power goal is achieved with a small number of components—the fewer the better. So, I quickly became interested in the Texas Instruments MSP430F149, which is a highly integrated device with low power consumption. Note that everything is integrated except the oscilloscope analog chain (coupling and programmable amplifier), part of the trigger circuit, and the input buffer for the logic analyzer. The microcontroller works with an 8-MHz crystal oscillator.

This application uses the register bank, the entire RAM (2 KB), and nearly all of the peripherals. The peripherals used include the 16-bit TimerA and B, ADC, analog comparator, multiply accumulate, and one USART with modulation capability. Only the second USART is spared.

The system has several main features. You can control and display on the PC by running software implemented on LabWindows/CVI. In addition, it has a signal generator based on the direct digital synthesis method and a frequency of up to 6 kHz with 0.3-Hz resolution. The output voltage reaches a peak of 1.3-V (±2 dB) fixed amplitude. The signal generator works simultaneously with the oscilloscope and logic analyzer (but not these two).

I included a digital oscilloscope with two channels that have 1-MHz bandwidth, 8 bits of resolution, and 401 words of memory per channel. There are 10 amplitude scales from 5 mV to 5 V per division and 18 timescales from 5 μs to 2.5 s per division. Note that there are four working modes: Auto, Normal, Single, and Roll.The logic analyzer has eight channels, 1920 words of memory per channel, and sampling from 1 to 100 kS/s. It is trigger-delay selectable between 0, 50, and 100% of memory length.

Looking at Photo 1, you see that the system’s hardware consists of two separate boards that are attached to each other. Photo 2a shows the tops of the boards, and Photo 2b shows the bottoms.

a—You can replace the relays in the coupling section and the driver circuit with solid-state relays if you can find ones with low leakage current. b—The op-amp’s SMD packages are best viewed from the bottom. The larger board is populated on both sides. Note the importance of the parasitic coupling of the PWM D/A outputs to the input of the amplifiers.

Photo 2: a—You can replace the relays in the coupling section and the driver circuit with solid-state relays if you can find ones with low leakage current. b—The op-amp’s SMD packages are best viewed from the bottom. The larger board is populated on both sides. Note the importance of the parasitic coupling of the PWM D/A outputs to the input of the amplifiers.

The larger board contains the oscilloscope analog chain: BNC connectors, relays (and circuit controller) for DC-GND-AC in the coupling section, and the digital programmable attenuator/amplifier. The top board contains the DC/DC converter power supply, charge-pump inverter, serial communication driver, low-pass filter, trigger (real and equivalent time sampling) circuit, channel-trigger selector, and the microcontroller.

Download the entire article.

Quad Bench Power Supply

The need for a bevy of equipment for building and testing presents a problem: how to deliver an adequate power supply while keeping workbench clutter to a minimum. Brian decided to tackle this classic engineering conundrum with a small, low-capacity quad bench power supply.

To the right of the output Johnson posts are the switches that set the polarity of the floating supplies—as well as the switch that disconnects all power supply outputs—while leaving the unit still powered up.

To the right of the output Johnson posts are the switches that set the polarity of the floating supplies—as well as the switch that disconnects all power supply outputs—while leaving the unit still powered up.

In “Quad Bench Power Supply,” Millier writes:

I hate to admit it, but my electronics bench is not a pretty sight, at least in the midst of a project anyway. Of course, I’m always in the middle of some project that, more often than not, contains two or three different projects in various stages of completion. To make matters worse, most of my projects involve microchips, which have to be programmed. Because I use ISP flash memory MCUs exclusively, it makes sense to locate a computer on my construction bench to facilitate programming and testing. To save space, I initially used my laptop’s parallel port for MCU programming. It was only a matter of time before I popped the laptop’s printer port by connecting it to a prototype circuit with errors on it.

Fixing my laptop’s printer port would have involved replacing its main board, which is an expensive proposition. Therefore, I switched over to a desktop computer (with a $20 ISA printer port board) for programming and testing purposes. The desktop, however, took up much more room on my bench.

You can’t do without lots of testing equipment, all of which takes up more bench space. Amongst my test equipment, I have several bench power supplies, which are unfortunately large because I built them with surplus power supply assemblies taken from older, unused equipment. This seemed like a good candidate for miniaturization.

At about the same time, I read a fine article by Robert Lacoste describing a high-power tracking lab power supply (“A Tracking Lab Power Supply,” Circuit Cellar 139). Although I liked many of Robert’s clever design ideas, most of my recent projects seemed to need only modest amounts of power. Therefore, I decided to design my own low-capacity bench supply that would be compact enough to fit in a small case. In this article, I’ll describe that power supply.

MY WISH LIST

Even though I mentioned that my recent project’s power demands were fairly modest, I frequently needed three or more discrete voltage levels. This meant lugging out a couple of different bench supplies and wiring all of them to the circuit I was building. If the circuit required all of the power supplies to cycle on and off simultaneously, the above arrangement was extremely inconvenient. In any event, it took up too much space on my bench.

I decided that I wanted to have four discrete voltage sources available. One power supply would be ground referenced. Two additional power supplies would be floating power supplies. Each of these would have the provision to switch either the positive or negative terminal to the negative (ground) terminal of the ground-referenced supply, allowing for positive or negative output voltage. Alternately, these supplies could be left floating with respect to ground by leaving the aforementioned switch in the center position.

This arrangement allows for one positive and two positive, negative or floating voltage outputs. To round off the complement, I added Condor’s commercial 5-V, 3-A linear power supply module, which I had on hand in my junk box. Table 1 shows the capabilities of the four power supplies.

I wanted to provide the metering of voltage and current for the three variable power supplies. The simultaneous voltage and current measurement of three completely independent power supplies seemed to indicate the need for six digital panel meters. Indeed, this is the path that Robert Lacoste used in his tracking lab supply.

As you can see, there are four power supplies. I’ve included all of the information you need to understand their capabilities.

As you can see, there are four power supplies. I’ve included all of the information you need to understand their capabilities.

I had used many of these DPM modules before, so I was aware of the fact that the modules require their negative measurement terminal to float with respect to the DPM’s own power supply. I solved this problem in the past by providing the DPM module with its own independent power source. Robert solved it by designing a differential drive circuit for the DPM. Either solution, when multiplied by six, is not trivial. Add to this the fact that high-quality DPMs cost about $40 in Canada, and you’ll see why I started to consider a different solution.

I decided to incorporate an MCU into the design to replace the six DPMs as well as six 10-turn potentiometers, which are also becoming expensive. In place of $240 worth of DPMs, I used three inexpensive dual 12-bit ADCs, an MCU, and an inexpensive LCD panel. The $100 worth of 10-turn potentiometers was replaced with three dual digital potentiometers and two inexpensive rotary encoders.

Using a microcontroller-based circuit basically allows you to control the bench supply with a computer for free. I have to admit that I decided to add the commercial 5-V supply module at the last minute; therefore, I didn’t allow for the voltage or current monitoring of this particular supply.

THE ANALOG CORE

Although there certainly is a digital component to this project, the basic power supply core is a standard analog series-pass regulator design. I borrowed a bit of this design from Robert’s lab supply circuit.

Basically, all three power supplies share the same design. The ground-referenced power supply provides less voltage and more current than the floating supplies. Thus, it uses a different transformer than the two floating supplies. The ground-referenced supply’s digital circuitry (for control of the digital potentiometer and ADC) can be connected directly to the MCU port lines. The two floating supplies, in addition to the different power transformer, also need isolation circuitry to connect to the MCU.

Figure 1 is the schematic for the ground-referenced supply. As you can see, a 24VCT PCB-mounted transformer provides all four necessary voltage sources. A full wave rectifier comprised of D4, D5, and C5 provides the 16 V that’s regulated down to the actual power supply output. Diodes D6, R10, C8, and Zener diode D7 provide the negative power supply needed by the op-amps. …

The ground-referenced power supply includes an independent 5-V supply to run the microcontroller module.

The ground-referenced power supply includes an independent 5-V supply to run the microcontroller module.

MCU AND USER INTERFACE

As with every other project I’ve worked on in the last two years, I chose the Atmel AVR family for the MCU. In this case, I went with the AT90S8535 for a couple of reasons. I needed 23 I/O lines to handle the three SPI channels, LCD, rotary encoders, and RS-232. This ruled out the use of smaller AVR devices. I could’ve used the slightly less expensive AT90LS8515, but I wanted to allow for the possibility of adding a temperature-sensing meter/alarm option to the circuit. The ’8535 has a 10-bit ADC function that’s suitable for this purpose; the ’8515 does not.

The ’8535 MCU has 8 KB of ISP flash memory, which is just about right for the necessary firmware. It also contains 512 bytes of EEPROM. I used a small amount of the EEPROM to store default values for the three programmable power supplies. That is to say, the power supply will power up with the same settings that existed at the time its Save Configuration push button was last pressed.

To simplify construction, I decided to use a SIMM100 SimmStick module made by Lawicel. The SIMM100 is a 3.5″ × 2.0″ PCB containing the ’8535, power supply regulator, reset function, RS-232 interface, ADC, ISP programming headers, and a 30-pin SimmStick-style bus. I’ve used this module for prototypes several times in the past, but this is the first time I’ve actually incorporated one into a finished project. …

eded to operate the three SPI channels and interface the two rotary encoders come out through the 30-pin bus. As you now know, I designed the ground-referenced power supply PCB to include space to mount the SIMM100 module, as well as the IsoLoop isolators. The SIMM100 mounts at right angles to this PCB; it’s hard-wired in place using 90° header pins. The floating power supplies share a virtually identical PCB layout apart from being smaller because of the lack of traces and circuitry associated with the SIMM100 bus and IsoLoop isolators.

The SIMM100 module has headers for the ISP programming cable and RS-232 port. I used its ADC header to run the LCD by reassigning six of the ADC port pins to general I/O pins.

When I buy in bulk, it’s inevitable that by the time I use the last item in my stock, something better has taken its place. After contacting Lawicel to request a .jpg image of the SIMM100 for this article, I was introduced to the new line of AVR modules that the company is developing.

Rather than a SimmStick-based module, the new modules are 24- and 40-pin DIP modules that are meant to replace Basic Stamps. Instead of using PIC chips/serial EEPROM and a Basic Interpreter, they implement the most powerful members of Atmel’s AVR family—the Mega chips.

Mega chips execute compiled code from fast internal flash memory and contain much more RAM and EEPROM than Stamps. Even though flash programming AVR-family chips is easy through SPI, using inexpensive printer port programming cables, these modules go one step further by incorporating RS-232 flash memory programming. This makes field updates a snap. …

The user interface I settled on consisted of a common 4 × 20 LCD panel along with two rotary encoders. One encoder is used to scroll through the various power supply parameters, and the other adjusts the selected parameter. The cost of LCDs and rotary encoders is reasonable these days. Being able to eliminate the substantial cost of six DPMs and six 10-turn potentiometers was the main reason for choosing an MCU-based design in the first place.

Brian Millier’s article first appeared in Circuit Cellar 149.