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March 2014

RIGOL Makes Waves

Bob Bluhm Vice President of Rigol Technologies

in the Test & Measure Industry USB Compliance Testing

Charge Measurement Tecniques


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Modern Test & Measure

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CONTENTS

TECH ARTICLE

Good Measurement Practices Part 2: Charge Measurement Techniques

Adaptive Filter TECH ARTICLE Is There An

By Alan Lowne

In Your Future?

Making Sense of Probe Terminology CEO, Saelig Company Inc.

COVER INTERVIEW

Bob Bluhm - Vice President of RIGOL Technologies

Linear filters are to be found everyw of-band noise. High frequency noise o with a low-pass filter. A typical low-p source such as a speech or music sign frequency within the bandwidth of th a notch filter to remove it? This woul signal. If the interference is more bro of the filter required to remove it wou situation that a linear filter could not the spectrum of the audio source, su

MAKING SENSE O PROBE TERMINO

As oscilloscope users, we know that the probe is a critical element in getting signals from the device under test into the instrument. The ideal probe would have perfectly flat magnitude response and perfectly linear phase response across its entire frequency range Unfortunately, that probe, though striven for by al oscilloscope manufacturers, does not exist.

RIGOL’S NEW DG1000Z SERIES OF DIGITAL SCOPES

22

PRODUCT OVERVIEW

26

TECH ARTICLE IS YOUR NEXT PET PROJECT

Make Measurable Improvements The test and measure industry is saturated with different types of oscilloscopes and waveform generators, making it difficult for engineers to select the right one for their particular application. With that in mind, Rigol Technologies developed a product line that makes the selection easier, more economical, and with higher performance—the DG1000Z Series. EEWeb spoke with Chris Armstrong, General Manager at Rigol Technologies, about the new product line and why it’s such an important release for the company.

RIGOL DG1000Z Series of Waveform Generators

Starting a USB PET Project

A USB BATTERY CHARGING DESIGN?

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Modern Test & Measure

Good Measurement Practices Essential to Characterizing Charge Accurately

2

PART

Charge Measurement Techniques 4


TECH ARTICLE

By Jonathan L. Tucker Keithley Instruments, Inc.

Unlike a voltage measurement, a charge measurement is a destructive process. In other words, making the measurement may remove the charge stored in the device under test. When measuring the charge on a device such as a capacitor, first disable the zero check of the electrometer, and then connect the capacitor to the high impedance input terminal. Zero check is a process where the input amplifier of the electrometer is reconfigured to shunt the input signal to low. Otherwise, some of the charge will be lost through the zero check impedance and won’t be measured by the electrometer. That’s because when zero check is enabled, the input resistance of the electrometer is about 10 mega-ohms. Opening the zero check switch produces a sudden change in charge reading known as “zero hop.” To eliminate the effects of zero hop, take a reading just after the zero check is disabled, then subtract this value from all subsequent readings. An easy way to do this is to enable the REL function after zero check is disabled, which nulls out the charge reading caused by the hop.

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Modern Test & Measure

The charge measurement range of most electrometers can be extended through the use of the external feedback mode, which allows using an external device as the electrometer’s feedback element. Placing the electrometer in the volts mode and then enabling external feedback switches the feedback circuit from an internal network to a feedback circuit connected to the preamp output. To extend the coulombs ranges, an external capacitor is used as the feedback element. As illustrated in Figure 1, an external feedback capacitor is placed between the preamp output terminal and the HI input terminal of the electrometer. To prevent electrostatic interference, the capacitor is placed in a shielded test fixture.

For example, using an external feedback capacitor of 10 micro-farads and measuring 5 volts on the display of the electrometer, the calculated charge is 50 micro-coulombs. The capacitance of the feedback element should be at least 10 pico-farads to avoid errors due to stray capacitance and noise gain. To ensure low leakage current and low dielectric absorption, the feedback capacitor should be made of a suitable dielectric material such as polystyrene, polypropylene, or Teflon. Several other elements of good measurement practice are critical to measuring charge accurately with electrometers, including making proper connections, minimizing electrostatic interference, and minimizing the impact of environmental factors. Making Connections To avoid measurement errors, it’s critical to make proper connections from the electrometer to the device under test. Always connect the high resistance terminal of the meter to the highest resistance point of the circuit under test.

When in external feedback mode, the electrometer will display the voltage across the feedback element. The unknown charge can be calculated from the following formula:

Q = CV Q = charge (coulombs) C = capacitance of the external feedback capacitor (farads) V = voltage on display of electrometer (volts)

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It’s critical to make proper connections from the electrometer to the device under test.

Figure 1. Connections for using an external feedback capacitor to extend an electrometer’s charge measurement range.


TECH ARTICLE

Electrostatic coupling or interference occurs when an electrically charged object approaches the input circuit under test. At low impedance levels, the effects of the interference aren’t noticeable because the charge dissipates rapidly. However, high resistance materials don’t allow the charge to decay quickly, which may result in unstable measurements. The erroneous readings may be due to either DC or AC electrostatic fields, so electrostatic shielding will help minimize the effects of these fields.

DC DC Fields

DC fields can produce noisy readings or undetected errors. These fields can be detected when movement near a test setup (such as the movement of the person operating the instrument or others in the immediate vicinity) causes fluctuations on the electrometer’s display. To perform a quick check for interference, all you have to do is place a piece of charged plastic, such as a comb, near the circuit. A large change in the meter reading indicates insufficient shielding.

AC AC Fields

AC fields can be equally as troublesome. These are caused most often by power lines and RF fields. If the AC voltage at the input is large, then part of this signal is rectified, producing an error in the DC signal being measured. This can be checked by observing the analog output of the electrometer with an oscilloscope. A clipped waveform indicates a need to improve electrostatic shielding. AC electrostatic coupling occurs when an electrostatic voltage source in the vicinity of a conductor, such as a cable or trace on a PC board, generates a current proportional to the rate of change of the

At low impedance levels, the effects of the interference aren’t noticeable because the charge dissipates rapidly. However, high resistance materials don’t allow the charge to decay quickly, which may result in unstable measurements.

Electrostatic Interference and Shielding

voltage and of the coupling capacitance. This current can be calculated with the following equation:

i = C dV/dt + V dC/dt For example, two conductors, each with 1cm2 area and spaced 1cm apart by air, will have almost 0.1 pico-farad of capacitance. With a voltage difference of 100 volts between the two conductors and a vibration causing a change of capacitance of 0.01 pico-farad/ second (a 10% fluctuation between them), a current of 1pA AC will be generated. To reduce the effects of the fields, a shield can be built to enclose the circuit being measured. The easiest type of shield to make is a simple metal box or meshed screen that

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Modern Test & Measure

encloses the test circuit. Shielded boxes are also available commercially. Made from a conductive material, the shield is always connected to the low impedance input of the electrometer.

Environmental Factors

The cabling between the HI terminal of the meter and the device under test also requires shielding. Capacitive coupling between an electrostatic noise source and the signal conductors or cables can be greatly reduced by surrounding those conductors with a metal shield connected to LO. With this shield in place, the noise current generated by the electrostatic voltage source and the coupling capacitance flows through the shield to ground rather than through the signal conductors.

Temperature and temperature stability

To summarize, follow these three guidelines to minimize error currents due to electrostatic coupling:

 eep all charged objects (including K people) and conductors away from sensitive areas of the test circuit. Avoid movement and vibration near the test area. When measuring currents of less than 1 nano-amp, shield the device under test by surrounding it with a metal enclosure and connect the enclosure electrically to the test circuit common terminal.

Although the word shielding usually implies the use of a metallic enclosure to prevent electrostatic interference from affecting a high impedance circuit, guarding implies the use of an added low impedance conductor, maintained at the same potential as the high impedance circuit, which will intercept any interfering voltage or current. A guard doesn’t necessarily provide shielding.

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A stable test environment is essential for making accurate low level measurements of all types.

Varying temperatures can affect low level measurements in several ways, including causing thermal expansion or contraction of insulators and producing noise currents. Also, a temperature rise can cause an increase in the input bias current of the meter. As a general rule, JFET gate leakage current doubles for every 10°C increase in temperature, but most electrometers are temperature compensated to minimize input current variations over a wide temperature range. To minimize errors due to temperature variations, operate the entire charge measurement system in a thermally stable environment. Keep sensitive instruments away from hot locations (such as the top of a rack) and allow the complete system to achieve thermal stability before making measurements. Use the instrument’s zero or suppress feature to null offsets once the system has achieved thermal stability. Repeat the zeroing process whenever the ambient temperature changes. To ensure optimum accuracy, zero the instrument on the same range as that to be used for the measurement. Ionization Interference Current measurements made at very low levels (<100fA) may be affected by ionization interference from sources such as alpha particles. A single alpha particle generates a track of from 30,000 to 70,000 positive and negative ions per cm, which may be polarized and moved about by ambient electric fields. Also, ions that strike a current-sensing node may generate a “charge hop” of about 10fC per ion. There are several ways to minimize test system noise due to ionization interference. First, minimize the volume of air inside the shield around sensitive input nodes. Also, keep sensitive nodes away from high intensity electric fields.


TECH ARTICLE

Excess humidity can reduce insulation resistance on PC boards and in test connection insulators. A reduction in insulation resistance can, of course, have a serious effect on high impedance measurements. In addition, humidity or moisture can combine with any contaminants present to create electrochemical effects that can produce offset currents. To minimize the effects of moisture, reduce the humidity in the environment (ideally <50%). Be sure all components and connectors in the test system are clean and free of contamination. When cleaning, use only pure solvents to dissolve oils and other contaminants, then rinse the cleaned area with fresh methanol or deionized water. Allow cleaned areas to dry for several hours before use. To minimize errors due to temperature variations, operate the entire charge measurement system in a thermally stable environment. Keep sensitive instruments away from hot locations (such as the top of a rack) and allow the complete system to achieve thermal stability before making measurements. Use the instrument’s zero or suppress feature to null offsets once the system has achieved thermal stability. Repeat the zeroing process whenever the ambient temperature changes. To ensure optimum accuracy, zero the instrument on the same range as that to be used for the measurement.

To minimize errors due to temperature variations, operate the entire charge measurement system in a thermally stable environment.

Humidity

is identified, the RF energy may be reduced or eliminated by shielding and adding snubber networks or filters at appropriate points. Part 3 of this series outlines techniques for optimizing some typical charge measurement applications. Meanwhile, please contact me with any questions you may have about Part 2 at jtucker@keithley.com ■

To read the first installment of the series, click the image below:

RFI Interference from radio frequency sources can affect any sensitive electrometer measurement. This type of interference may be indicated by a sudden change in the reading for no apparent reason. A non-linear device or junction in the input circuit can rectify the RF energy and cause significant errors. Sources of such RFI are nearby transmitters, contactors, solenoid valves, and even cellular telephones and portable two-way radios. Once the source

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Modern Test & Measure

MAKING SENSE OF PROBE TERMINOLOGY

by David Maliniak Technical Marketing Communication Specialist

As oscilloscope users, we know that the probe is a critical element in getting signals from the device under test into the instrument. The ideal probe would have perfectly flat magnitude response and perfectly linear phase response across its entire frequency range. Unfortunately, that probe, though striven for by all oscilloscope manufacturers, does not exist.

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TECH ARTICLE What does exist is a good deal of confusion about what can be done to coax probes to behave more like that ideal probe. We often hear terms such as “calibration,” “correction,” “compensation,” and “de-embedding” tossed around, often interchangeably. All of them do involve how the measurement system accounts for the probe’s effect on the signal under test. But let’s take a look at them and see how they actually differ in practice. One possible approach is to employ a calibrated, precision instrument—usually a vector network analyzer (VNA)—to measure the probe’s actual magnitude and phase response. With those measurements in hand, one might build a correction filter that removes the undesired effects of the probe’s frequency response characteristics from the acquired signal. Using a precision instrument to measure a probe’s performance and making some adjustment (the correction) to its output to nudge it toward ideal performance is, by definition, a calibration. Why? Because it involves measurements made to traceable standards. The correction is but a part of this overall calibration process. Then there’s compensation, which should be familiar to anyone who’s used a common bench oscilloscope and a passive probe. The probes usually come with a little plastic screwdriver. When you connect the probe to the oscilloscope, you use the little screwdriver to turn a trimmer capacitor on the probe’s plug end. In doing so, you adjust the probe’s output impedance to match the input impedance of the preamplifier on the input channel you’ve plugged it into.

“With high-bandwidth oscilloscopes, the compensation process is a little different in that the probe correction and scopechannel correction are convolved together.” With high-bandwidth oscilloscopes, the compensation process is a little different in that the probe correction and scopechannel correction are convolved together. As a result, the entire measurement system behaves in a very controlled fashion across the instrument’s full rated bandwidth and even beyond. These days, one will come across the concept of probe de-embedding. which involves accounting for reflections from the probe tip along the transmission line that it’s connected to. Using models of the probe tip’s loading impedance, the impedance profiles of the transmission line, and components in the circuit, one may account for reflections from components that travel back to the probe tip and affect measurements. Usually, the probe loading is enough that reflections from the probe are minimal, making the need for probe de-embedding a relatively rare one. In later posts, we’ll take a closer look at probe loading, a topic that engenders many questions for oscilloscope users as well as potential misunderstanding. ■

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Transforming Test & Measure Interview with Bob Bluhm Vice President of RIGOL Technologies

RIGOL Technologies is an industry-leading supplier of test and measurement equipment. With headquarters in Beijing, China, RIGOLâ&#x20AC;&#x2122;s steady growth has prompted company branch expansion to North America and Munich to supply their growing global demand. The customer draw comes from their fullportfolio test equipment that offers accurate and reliable testing results at an affordable price. We spoke with Bob Bluhm, Vice President of RIGOL Technologies, about the biggest challenges the company faces, the plan for consistent growth, and how they plan on transforming the entire industry through innovation.

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COVER INTERVIEW

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Modern Test & Measure

â&#x20AC;&#x153;Many of our customers are surprised by the quality that extends not just from the packaging of our product but to the fit-and-finish, feature set, ease of use, capabilities and low price compared to the key brands in the marketplace.â&#x20AC;?

What products and services does Rigol offer for engineers? RIGOL is a full-portfolio test and measurement supplier. Our portfolio includes RF products, time-domain products, power supply products, and precision measurement products. We offer an oscilloscope line that ranges from 50 MHz to 1 GHz which includes Mixed Signal Oscilloscopes (MSOs) and integrated arbitrary waveform generators. We have two spectrum analyzer lines that range from 9 kHz to 3 GHz that include tracking generators and options for measuring filters, amplifiers, etc. The portfolio includes function and arbitrary waveform generator lines from 20MHz to 350MHz, RF generators that range from 3 GHz to 6 GHz output frequency, two series of DC power supplies that range from 80 W to 195 W, and a Digital Multimeter (DMM) line. In addition to our hardware portfolio we offer PC SW packages that extend our analysis capabilities for RF, Power and Time measurements as well as remote instrument control.

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What is your strategy in fitting in the Test and Measure industry since there are a lot of companies that specialize in high performance test equipment? All of the customers we have spoken to in the US and worldwide are looking for a greater return on their R&D budgets. Customers have critical time to market needs with heavily scrutinized and shrinking test budgets. This dynamic has driven more alternatives as customers are looking for the lowest cost, reliable alternative to solve their technical issues. There is commoditization happening within in the test and measurement market. This is where RIGOL fits nicely in the market as customers are looking for ways to reduce their overall cost of test and meet their testing/design challenges. Our instrument lines offer customers all of the advanced features they are looking for but are priced anywhere from 30-50% percent below the traditional incumbents. We offer a 30 day no questions asked return policies with 3 year standard warrantees.


COVER INTERVIEW We have historically targeted the low-end general-purpose side of the market but with our new capabilities and expanding portfolio we are moving into the midrange, commercial markets.

unprecedented value to the customer by bringing more capabilities, performance and more advanced features at a significantly lower price. We are increasing the value that the customer gets for their T&M budget.

How do you differentiate Rigolâ&#x20AC;&#x2122;s price or value from other companies that offer test and measurement products?

In terms of the industry and products that you offer, what kind of innovations is Rigol doing?

I would define value in the form of an equation. If you define value as the sum of features, capabilities, performance, and quality, divided by price you arrive at a â&#x20AC;&#x153;valueâ&#x20AC;? a product brings to a customer. Each customer places different weighting on parts of this equation with price being just one element of that equation.

RIGOL is bringing increased technical capability at entry level pricing. Take our UltraVision technology for example; it is a hardware architecture that enables very fast waveform capturing rates of up to 140,000 acquisitions per second, intensity-graded display, long record length capabilities and hardware waveform recording, replaying and analysis. This technology greatly increases the ability for the customer to find infrequent events in their systems and quickly manage and analyze large amounts of acquisition data. Ultravision starts in our new DS1000Z Series with a price under 600 US Dollars.

Consumers today are demanding to have the features, capability, quality, and performance in their products and want that at lower prices than they have historically gotten. This is our value proposition, we offer

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Modern Test & Measure

“In the last six months RIGOL has introduced six new products and we will introduce almost that many more in the next six months.”

Talking about connectivity, RIGOL is a member of the LXI Consortium. We have LXI connectivity in addition to USB connectivity. We put a lot of effort and investment into introducing new products. In the last six months RIGOL has introduced six new products and we will introduce almost that many more in the next six months. The other thing that we are bringing to the marketplace in terms of innovation is the response time to our customer’s needs. One of our key values is our focus on customer’s needs and we have turned custom features for customers in weeks to help them solve their problems.

What kind of feedback do you get from your customers about Rigol’s products that they use? Many of our customers are surprised by the quality that extends not just from the packaging of our product but to the fit-and-

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COVER INTERVIEW finish, feature set, ease of use, capabilities and low price compared to the key brands in the marketplace. I was just at DesignCon and it was exciting to hear the feedback from delighted and excited customers who had purchased a RIGOL product and were looking at other lines to fill out their bench. Other customers, who were seeing the products for the first time, were in disbelief with the capability that the products offer for a low price. One of the most exciting parts of the role I am in right now is getting these products in front of customers and hearing their feedback. RIGOL is committed to the continuous improvement of our products.

What are some of the biggest challenges that you face right now in trying to move the company forward? Number one is brand recognition. Second is managing the speed at which new products are being introduced and managing the growth. We continue to grow our commercial team (Sales and Marketing) to handle the growth and be able to keep up with customer interest, demands and feedback. The third challenge is perception. The price can fool many people into making false assumptions about the quality of the products. The perception issue is a challenge that customers will overcome as they try the instrumentation. The last is channel development and adding strong partners to our great base.

Talking about growth in your business, how are you distributed across the globe and in the market center? The US represents roughly 20 percent of the worldwide sales, so from the international perspective, ~80 percent of our business would be considered international. We are really just getting started in the N. American channel. The company was founded in China, and it has had more time to establish its channels throughout Asia.

What is the culture like in Rigol? The culture at RIGOL is one of the reasons why I joined the company. If I had to define it in a few words, I would say it’s young, entrepreneurial, energetic, and customer-centric. The decisionmaking is very fast and there are no “sacred cows” that we have to work around. This company is not a public one, so we do not have to answer to shareholders every ninety days. We have long term strategies and can stay focused on the development and investment of that vision. There are four elements of RIGOL values, and RIGOL is a company that truly “walks its talk.” Since I first met the RIGOL management team in 2007 I have witnessed how they have held true to these values as they’ve grown. The values are simple. First, we should always be creating and delivering customer value. Second, we create platforms targeted

RIGOL’s DG1032Z Arbitrary Waveform Function Generator

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Modern Test & Measure

towards people development. Focusing on the passion and growth of an individual brings out the best in them. Third, we should always be innovating. This extends past products and into how we run every part of the company. Lastly, we will be persistent in all that we do. The company is quite new, and we recognize that there are a lot of incumbents that have a fiftyyear advantage on us, so we need to keep that persistence and focus on customers to continue innovating for our customers and partners. These values and the culture around them resonate with me and my beliefs.

What excites you the most about the future of Rigol Technologies? For RIGOL, the growth potential and our ability to transform the T&M industry is what most excites me. I have spent my whole career in Test and Measurement and love the extreme challenges that it brings to the designer. Designing products that can measure microvolts to kilovolts and capture events that range from nanoseconds to days presents such design challenges. My passion is helping to re-define expectations around the value of test and measurement equipment. To be in a company that shares this passion, core set of values and vision is what I love doing.

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Is there anything about Rigol that you would like to share with the readers? There are a couple of other facts about RIGOL that might be interesting to your readers. RIGOL has its biomedical line of equipment that is sells in China. There is over 800,000 square feet in the Beijing facility, and in May of this year, we will be opening a new, 380,000 square foot manufacturing facility in Suzhou. At that time that we introduce a new manufacturing facility and we will also be creating another new R&D design center in addition to the R&D teams in Beijing. The growth rate of the company continues to exceed my expectations—past what I thought would be possible at the early stages and these changes are another step in keeping up with that growth. There is no shortcut to developing a brand and there is no shortcut to developing trust with engineers and customers. It is a continuous process to develop innovative products that bring value to our customers, continuously improve our quality, and respond to customer’s needs. RIGOL’s company’s vision, values and people are aligned to make this happen. ■


COVER INTERVIEW

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Modern Test & Measure

RIGOL’S NEW DG1000Z SERIES DIGITAL WAVEFORM GENERATORS Make Measurable Improvements The test and measure industry is saturated with different types of oscilloscopes and waveform generators, making it difficult for engineers to select the right one for their particular application. With that in mind, Rigol Technologies developed a product line that makes the selection easier, combining value with performance —the DG1000Z Series. EEWeb spoke with Chris Armstrong, General Manager at Rigol Technologies, about the new product line and why it’s such an important release for the company.

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PRODUCT HIGHLIGHT

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Modern Test & Measure

“The exciting thing about the DG1000Z is that we’ve redone the foundation of the mainstream waveform generator,” Armstrong explained. Rigol added new, innovative technology called SiFi that dramatically improves signal fidelity—adding greater precision and accuracy to the created waveform. The result is an instrument with incredibly deep memory for advanced applications at a fraction of the price.

she wants.” Armstrong explained. “They don’t need to trim or re-sample it. They can actually set the output sample rate of the instrument so it’s much easier to work with large sets of data. Consequently, they get the best signal fidelity and best quality out of our generator.”

“It comes standard with eight million points, with the capability of getting up to 16,” Armstrong noted, “this gives you the ability to create detailed, long duration arbitrary waveforms.” Armstrong said there is an issue associated with arbitrary waveforms in a traditional, simple, waveform generator. One of these main problems is that the engineer has to trim the arbitrary waveforms and sample them just right to get them to output correctly.

There are two different models in the DG1000Z family— the DG1032Z and the DG1062Z. They have a sine wave capability of 30MHz and 60MHz, respectively. Both models have two independent channels that can be synchronized and can also change phase between them. The user can perform different outputs on them and different arbitrary waves as well. These signal generators can also do different waveforms including sine, square, ramp, and pulse. Plus, arbitrary waves are completely independent.

To remedy this, Rigol’s DG1000Z product line has an output sample rate that can be dynamically set. “The engineer can take whatever arbitrary waveform he or

Like Rigol’s other products, the DG1000Z series includes a USB in front and back, allowing the engineer to use a memory stick to move waves around.

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Product Line Up

Features and Functions Like Rigol’s other products, the DG1000Z series includes a USB in front and back, allowing the engineer to use a memory stick to move waves around. Instruments can be connected to a PC over USB as well as Ethernet programmability. A USB dongle can be hooked up allowing the engineer to control it over GPIB if needed. Three BNC plugs are available on the front of each generator. One is a trigger and a sync output, while the other two are channels. Armstrong said, “Traditionally, if you’re doing RF work and want to create noise or test immunity of your system, several steps are involved to do that in a traditional arbitrary signal. You need to create the signal in RF space, convolve the data in the


PRODUCT HIGHLIGHT

time domain, and then load the arbitrary wave while being careful of the sample rate and number of points. That is one frustrating experience.” But with the DG1000Z Series, those complex tasks are made easy. For example, Armstrong explained, “You can say, I want my base frequency to be 10 MHz; I want peaks at the third and fifth harmonic; and I want it 90 degrees out of phase and 10 dB down. Basically, you can massage the look of the signal you want while thinking about it in RF space to do the kind of things you need.” Armstrong also calls attention to the amount of modulation options that include AM, FM, Pulse, and Frequency Shift Keying (FSK). Functions like these are FPGA-based allowing Rigol to include a number of features in them. “We tend to incrementally add and build capabilities like the harmonics,” Armstrong explained. “This is now the fourth or fifth generation waveform generator for Rigol and there is a lot of capability built into it. Cost keeps coming down and there are more features every time. It is an exciting process to be part of.”

“This is now the fourth or fifth generation waveform generator for Rigol and there is a lot of capability built into it.” Product Price Points DG1000Z Series Waveform Generators models with Si-Fi technology enabling dynamic output sampling are in the $600 to $800 range, which, according to Armstrong, “is really an exciting value especially due to the deep memory capability our products offer.” Some models are in stock and available immediately, others may have up to a several week lead-time. Contact Rigol for additional information, www.Rigolna.com.

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Modern Test & Measure

IS YOUR NEXT PET PROJECT

A USB BATTERY CHARGING DESIGN?

Packet-Master USB-PET Compliance Tester for OTG 2.0 and BC 1.2

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TECH ARTICLE

Alan Lowne

CEO Saelig Co. Inc.

USB ports are everywhere, and are rapidly becoming the accepted standard as a universal charging means. But challenges await the designer who wants to make use of USB ports as a convenient charging source. USB-connected device designs are often subject to various constraints such as cost, physical size, and charge time, but yet must work almost anywhere. Many existing dedicated chargers have offered a USB-compliant physical connection but lacked a USB charging compliance program. This has led to many products having characteristics incompatible with complete USB specifications. And even though PC host ports go through an extensive certification process, PCs that claim a USB compliant Charging Port in their feature list are now required to pass USB-IF compliance checks.

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Modern Test & Measure

The USB-IF Battery Charging Specification, Revision 1.2 offers a good basis for building interoperable and compatible USB devices from the battery-charging standpoint. This specification was created to try to unify battery-charging attributes for USB2.0 – one reason being to minimize the number of cellphone chargers ending up in landfills. The standard also insures that device manufacturers can deliver their product without a dedicated charger, allowing recharging through any USB port. The USB-IF specification also allows compatibility between computer/hub USB ports and a “USB charger” (an AC-to-DC or DCto-DC adapter with one or more USB sockets.) There has been a lot of confusion as to what is possible or permissible in the USB charging world. Many chargers have just put a voltage on the D+ and D- pins of the USB port. Since traditional data communication on the USB is based on 3.3V for USB1.1 and 300mV for USB 2.0, putting a different voltage on these lines removes the possibility for enumeration and communication. If a mobile device has Charging Port Detection capability, it will start a charging session when attached to a USB port and the voltage on VBUS will be greater than its internal session valid threshold. If charging conditions are met, the device will then start the charger detection algorithm of USB-IF BC Rev.1.2. Some USB devices require you to download device-specific software when you first plug the device into the host’s USB port. Some cell phones are like this for syncing, and some facilitate charging at a higher current while communicating. In the BC 1.2 specification,

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there is a mode referred to as Charging Downstream Port (CDP) that allows for data and higher charging currents. If a voltage between 0.4V and 0.8V is sensed on D+ of a host or hub device, then D- should respond with 0.5V to 0.7V. Timing details associated with this specification that can be found in the specification. Once CDP has been established, peripheral devices are allowed to draw up to 1.5A and simultaneously communicate data. A dedicated charging port is a downstream USB port on a device that outputs power through a USB connector but is not capable of enumerating a device connected to it. Examples include plug top chargers which connect to a mains socket and chargers which connect to the low voltage power sockets in automobiles. All USB power ports, when active (and “not suspended,” in USBspeak) are classified as either “Low Power” (100mA) or “High Power” (500mA). Any port could also be “suspended,” which means almost off but still able to supply 2.5mA. Ports on PCs, laptops, and powered hubs are mostly “High Power,” while ports on hubs that receive no power other than that supplied by the upstream USB host are considered “Low Power.” Once plugged in, a device is allowed initially to draw up to 100mA when enumerating and negotiating the current budget with its host. Subsequently it might be allowed to raise its drain to 500mA, or it might be held at 100mA. This is detailed in the USB Serial Bus Specification Rev 2.0, section 7.2.1.4.


TECH ARTICLE USB Source Types Three different USB source types are defined in the USB-IF spec: Standard downstream port (SDP) - This is the typical USB port found in desktop and laptop PCs. The maximum load current is 2.5mA when suspended, 100mA when connected and not suspended, and 500mA (max) when configured for that current. Because SDP only supplies a maximum of 7.5W/500 mA, it belongs to low-speed charging port. A device can recognize an SDP with hardware by detecting that the USB data lines (D+ and D-) are each grounded through 15kohms, but enumerate to be USB compliant. In USB 2.0, it is not correct to draw power without enumerating. However, many USB products do in violation of the spec. Charging downstream port (CDP) - A higher current USB port for PCs, laptops, and other hardware which can supply up to 1.5A. This current can be supplied before enumeration. A USB device can recognize a CDP via a hardware handshake on the D+ and D- lines. (See USB Battery Charging Specification, section 3.2.3.) The hardware test occurs prior to returning the data lines to a communication function. So a CDP can be detected and charging starts before enumeration. Dedicated charging port (DCP) - These are power sources like wall chargers and car adapters that do not enumerate so charging can start without enumeration. DCPs can supply up to 1.5A and are identified by a short between D+ to D-. These adapters allow any USB cable to be used for charging USB devices. Detecting the Source Any device that connects to a USB receptacle and uses that power to run itself or charge a battery needs to know how much current it can draw. Trying to draw 1A from a source capable of supplying only 500mA could create problems like blown fuses or shutdowns. USB products can be designed to be compliant with BC1.2, compliant only with USB2.0, or noncompliant. BC1.2-compliant designs must be able to sense and limit input current for all USB source types, including legacy USB 1 and 2.0 ports. USB2.0-designed products can

charge from SDPs after enumeration, but may not recognize CDPs and DCPs. If they donâ&#x20AC;&#x2122;t recognize a CDP, they can still charge but only after enumeration, as with an SDP. A device can implement port detection using its own software, or can employ a charger or interface IC that detects by interacting with the USB D+ and D- data lines without relying on system resources. USB Connection Terminology USB terms that might need some explanation include the following: Attach: Physically plugging in a USB cable. Connect: When the attached device connects a 1.5kohm pull-up to the D+ or D- data lines. Enumerate: This is the initial host/data information exchange to identify device type. Configure: Setting the device parameters.

USB Car Charging Adapter During the enumeration and configuration information exchange, a USB device decides how much current a USB port can source. In addition to the USB 2.0 options, BC1.2 also allows for charging to take place without enumeration. Insuring Charging Compliance In order to create a USB charging design that is reliable, the USB-IF recommends the use of a test device called the USB-PET (Protocol and Electrical Tester) for comprehensive compliance testing. Made by European USB experts MQP, this equipment is capable of emulating and measuring all the electrical conditions and protocol requirements of a

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Modern Test & Measure

Script-based Tests USB host, a USB device, an OTG device, an Embedded Host, an OTG Peripheral-Only device, a Charging Downstream Port, or a Dedicated Charger Port. It can also perform a series of compliance tests on a Micro-ACA or Standard-ACA. The PET is controlled by a script, which is flexible enough to allow complete emulation as a host or peripheral. A set of standard scripts is provided with the PET for confirming the operation of devices designed to meet the OTG 2.0 and/ or Battery Charging 1.2 specifications. With BC 1.2, a Portable Device can get more power and the battery can be charged faster. The PET can be used to verify that a Portable Device complies with the BC 1.2 specification while communicating with a Charging Downstream Port and identifying a Dedicated Charger, and insuring that it continues to operate as a functional USB device.

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The PET (Protocol and Electrical Tester) is best described in terms of a number of functional blocks: 1.1 Serial Interface Engine (SIE) A fully functional SIE, with both host and peripheral capabilities, connected via a PHY to the UUT micro-AB receptacle on the front panel. It is under the control of the Script Processor. 1.2 Electrical Test Board (ETB) This contains circuitry to allow control and measurement of the electrical parameters for USB, OTG and BC specifications. It includes VBUS Generator, ID pin circuitry, data line test mode circuitry, VBUS current and voltage loads, and a variety of voltage and current measuring blocks. Extra connections are provided on the front panel to enable the testing of Accessory Charger Adapters (ACAs).


TECH ARTICLE 1.3 Script Processor Scripts are downloaded to this processor to control the sequence of operations required for a particular test. The processor controls the SIE and ETB as required by the operator. Scripts for all the OTG and BC compliance tests are provided by the GraphicUSB application which accompanies the PET. GraphicUSB also supports user-written scripts, to allow particular test situations to be set up. 1.4 High Speed/Full Speed/Low Speed USB Analyzer The PET also provides full USB2.0 analyzer functionality. Of particular interest is the fact that this analyzer has zero impact on the data line transmission quality. Messages generated by a running script can be inserted into the analyzer capture. The analyzer also displays a continuous accurate monitoring of the VBUS voltage value. The PET is supplied with MQP’s Windows application GraphicUSB for generating the test reports, and also analyzer-style data captures. It also provides automated compliance testing of both the data protocol and the electrical and timing requirements of the ‘USB On-The-Go and Embedded Host Automated Compliance Plan for the On-TheGo & Embedded Host Supplement Revision 2.0’. The suite of test scripts provided was

“The Packet-Master USBPET has the valuable capability of being able to capture an analysis of the activity on the bus during the compliance test.” developed in co-operation with the USB-IF USB On-The-Go Work Group. All aspects of On-The-Go, Embedded Hosts and Peripheral Only Devices are tested, including VBUS voltage and current performance, and SRP, HNP and ADP protocols. As well as compliance testing, the PacketMaster USB-PET has the valuable capability of being able to capture an analysis of the activity on the bus during the compliance test. This allows rapid investigation of protocolbased test failures. A range of optional Test Fixtures is also available to speed up electrical measurements, including provision for the Inrush Test. Special fixtures are required because of the unique test cable arrangement with the extra ID pin controlling signal. Script-based Tests USB-IF Compliance Test Scripts supplied with the PET include: OTG 2.0 A-UUT - for testing OTG devices in their OTG-A mode, and Embedded Hosts. OTG 2.0 B-UUT - for testing OTG devices in their OTG-B mode, and Peripheral-Only Devices. High Speed Electrical - invoke particular High Speed Electrical Test modes, for use with third party test equipment. Optional MQP Test Fixtures are available to simplify test-ups.

Typical PET Test Fixture

BC1.2 Portable Device (Weak Battery) - for testing Portable Devices (PDs) in situations where the charging requirements of the PD may affect the outcome of the test.

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Modern Test & Measure

MQP PET1

BC1.2 Portable Device (Good Battery) - tests aspects of Portable Devices (PDs) where it is undesirable that the PD should be drawing significant charging current.

BC1.2 Multiple Role Port (MRP) - for testing that Multiple Role Ports (i.e. ports which change between CDP, CDP and SDP) perform suitable handshaking.

BC1.2 Portable Device (Dead Battery Provision) - tests Portable Devices’ (PDs) adherence to the Dead Battery Provision.

BC1.2 ACA-Dock - for testing ACA-Docks.

BC1.2 Micro-ACA (Separate Charger) - for testing Micro-Accessory Charger Adapters (Micro-ACAs) which have a port for a separate charger BC1.2 Micro-ACA (Combined Charger) - for testing Micro-Accessory Charger Adapters (Micro-ACAs) which have a combined charger. BC1.2 Standard-ACA (Separate Charger) - for testing Standard-Accessory Charger Adapters (Standard-ACAs) which have a port for a separate charger. BC1.2 Standard-ACA (Combined Charger) tests Standard-Accessory Charger Adapters (Standard-ACAs) which have a combined charger. BC1.2 DCP - for testing Dedicated Charging Ports (DCPs). BC1.2 CDP - used for testing Charging Downstream Ports (CDPs). BC1.2 SDP - tests Multiple Role Ports when configured as Standard Downstream Ports (SDPs).

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Conclusion The PET tester hardware was designed in close co-operation with the USB-IF USB On-The-Go Work Group, and the Battery Charging Work Group, and designers of USB devices that feature charging capabilities would benefit from using the MQP USB-PET Protocol and Electrical Tester; it performs the suite of tests defined in the USB-IF specifications for OTG 2.0 and Battery Charging. The USB-PET is in use worldwide by compliance test houses and manufacturers and developers of OTG Devices, Embedded Hosts and other Portable Devices. ■ Reference Documents • Battery Charging 1.2 Specification http://www.usb.org/developers/devclass_ docs/USB_Battery_Charging_1.2.pdf • Universal Serial Bus Specification Revision 2.0 Specification http://www.usb.org/developers/docs/usb20_ docs/


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