January 2014

A Case for Adaptive Filters

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Low-Cost Portable

Analyzers

Mario Narduzzi Marketing Manager of

Modular Solutions at Agilent Technologies

Moving Towards

ModularAgilent’s unique solutions for evolving test and measurement challenges.

Modern Test & Measure CONTENTS

4 TECH ARTICLEIs There an Adaptive Filter in Your Future?

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Switches and switch matrices are widely used for the routing of RF and microwave signals in high-speed ATE systems. As today’s devices and products become more complex, there is an increasing need for the speed and flexibility of switching systems capable of connecting multiple signals or instruments to multiple devices-under-test (DUTs).

On a smaller scale, the same is true in R&D and design validation when developing RF and microwave products or when creating test routines that will be used in manufacturing. Even on the bench or in a small system, dependable switching can improve efficiency and save time. For day-to-day testing, USB-based single-pole / double-throw (SPDT) coaxial switches provide a solution that offers excellent RF performance and convenient connectivity.

Two-Port USB Switches

By Kai-Nian CheahAgilent Technologies, Inc.

Simplifying RF & Microwave Measurements with

10 TECH ARTICLEGood Measurement Practices Part 1: Sources of Error

14 COVER INTERVIEWMario Narduzzi - Marketing Manager of Modular Solutions at Agilent

22 FEATURED ARTICLELow-Cost, Portable Logic Analyzers

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Linear filters are to be found everywhere and very useful for general purpose elimination of out-of-band noise. High frequency noise outside the frequencies of interest is relatively easy to remove with a low-pass filter. A typical low-pass filter is shown in Fig 1. But a trickier problem is a broadband source such as a speech or music signal that is degraded by narrowband interference with a frequency within the bandwidth of the signal – for example, a whistle. How can this be fixed? Design a notch filter to remove it? This would work, but the filter would inevitably remove some of the signal. If the interference is more broadband in nature—such as engine noise—then the bandwidth of the filter required to remove it would suppress most of the signal, which is unacceptable. Another situation that a linear filter could not handle would be narrowband interference that drifted across the spectrum of the audio source, such as a varying pitch tone.

Is There An

Adaptive Filter In Your Future?By Alan Lowne

CEO, Saelig Company Inc.

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Solving the Impossible

These problems appear intractable, but they can be solved with the use of a special class of filter called the adaptive filter. As its name suggests, the filter adapts to the input signals and learns to distinguish between the noise and the wanted input, so it can be used to recover a narrowband signal degraded by broadband noise, or vice versa, or non-stationary noise or a combination of these problems.

An adaptive filter self-adjusts its transfer function using an optimization algorithm driven by an error signal, matching changing parameters to eliminate or greatly reduce noise that is corrupting the desired signal.

Adaptive filters are most useful when applied in real-time situations, and they are appearing in mobile phones, digital cameras, and medical monitoring equipment. For instance, when observing a heart beat ECG corrupted by power supply noise, using a simple notch filter at 60 Hz would remove all the frequencies in the vicinity of 60 Hz. This would significantly degrade the quality of the ECG waveform since it would contain frequency components in the rejected range. But using an adaptive filter, utilizing the input both from the patient and from the interfering power supply would allow tracking the noise frequency as it fluctuates. This adaptive method permits a filter with a smaller rejection range, and hence the output signal quality is more accurate for medical diagnostic purposes.

Another real-life situation might be a helicopter pilot communicating with air traffic control, using a helmet-mounted microphone. The pilot may have great difficulty communicating in such a noisy environment. Inevitably, the microphone will pick up not just the pilot’s voice, but also noise from the engine and the rotor blades. A true, dual input adaptive filter would be ideal here. One input to the adaptive filter would be the pilot’s microphone (the “desired signal”). A second input would be from a microphone outside of the helmet, sensing the engine and rotor ambient noise. A sophisticated algorithm running on a real-

time DSP device can perform the required adaptive filtering to produce much improved communication. But the task of creating this DSP algorithm has been a difficult task up until now.

Enter The Wizard!

The Signal Wizard 2.5 is a unique, integrated system created by the UK-based University of Manchester for quickly designing, downloading and running very high performance filters in real-time with little or no training. It includes a high-level PC-based software interface that designs the filter according to the user’s requirements, a hardware module based on an advanced digital signal processor and a low-level firmware operating system that implements the filtering operations. Once designed, an integrated software interface is used to download the filter to the hardware module via a serial link where it is executed on demand. Most importantly, the system requires no knowledge of digital signal processing (DSP) theory on the part of the user, or of the mathematics associated with digital filter design.

Signal Wizard 2.5 almost automatically programs a Freescale (Motorola) DSP to filter unwanted interference from analog input signals. Conditioned signals are then converted to an analog output in real time. The filter is specified using an easy-to-use visual software interface where the user simply enters the required frequency cut-off points and downloads them to the DSP chip. The response shape can be tailored by changing the window type and number of coefficients. At the other end of the scale, the software can handle complex input functions. With each change, an up-to-date response is displayed. Once a satisfactory output is obtained, the filter can be downloaded to the DSP chip for immediate use.

Signal Wizard 2.5 has a true dual input adaptive filter capability. The PC control software appears as in Fig 3. The hardware requires two inputs, the degraded signal and the noise reference. With the previous a helicopter pilot situation, one input for the Signal Wizard adaptive filter is connected to the pilot’s microphone (degraded signal). The second input is connected to a microphone outside the helmet, listening to the engine/rotor noise (noise reference). No frequency

response needs to be specified; the user only needs to set the learning rate and the number of coefficients. However, since these are already pre-optimized by the software, very little adjustment is necessary in practice. The engine/rotor noise will not be identical in each microphone, with respect to both magnitude and timing. The greater the disparity, the longer the filter will take to converge, but matters have to be very extreme before the filters fails.

But Wait, There’s More…

In addition to producing adaptive filters, Signal Wizard 2.5 can create much sharper filters than any analog counterpart, or mimic equivalent analog filter responses with 24-bit, 48 kHz sampling accuracy. Low pass, high bass, multiple band-stop/band-pass filters may be combined to produce very complex filters for frequencies up to 24 kHz, as well as standard infinite impulse response (IIR) designs. The Windows-based software

Fig. 1 Low-pass Filter

Fig. 2 Adaptive filter principle

The Signal Wizard 2.5 almost automatically programs a Freescale

(Motorola) DSP to filter unwanted

interference from analog input signals.

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can also accept measured responses to define and recreate a filter template. In fact, it is a simple matter to produce filters with completely arbitrary frequency magnitude and phase characteristics using the finite impulse response (FIR) method, with no phase distortion, no matter how sharp the filter is – try doing that with an analog filter! Because the DSP processing module is so fast, it is possible to design filters with responses far beyond what is possible with traditional analog techniques.

Running under most Windows flavors, Signal Wizard 2.5 can quickly create dual input adaptive filters with 24-bit resolution for broadband or narrowband noise cancellation, but it can also be used to make other filter types, including some that are unrealizable with discrete components: FIR filters (multiple pass, stop or arbitrary filters); impulse or frequency response import mode; Butterworth and Chebyshev filters (all major types) to any order; Rectangular, Bartlett, Hamming, Hanning, Blackman or Kaiser Window functions; IIR filters: Butterworth and Chebyshev filters (all major types) and many more. And for phased-array sensor processing, an eight-channel version Signal Wizard 3.0 is also available. This has been put to good use in simplifying the complex task of focusing or “beam-steering” audio and sonar signals.

Conclusions

Signal Wizard 2.5 is a user-friendly filter design tool that de-mystifies the process of specifying a filter. The filter design process simply becomes one of describing the desired frequency response. Frequency responses can be plotted as magnitude, dB, square, root, real, imaginary or phase; log or linear frequency axis; pole-zero plots and coefficient export. Due to its flexibility, Signal Wizard is particularly well suited to the real-time processing of audio bandwidth signals. Applications include not only audio signal processing, but sensor signal conditioning, signal analysis, and vibration analysis in industrial, education and research situations in electrical, electronic and other physical sciences. Other applications include noise cancellation, geological sensor signal conditioning, musical instrument research, human ear transfer function emulation, etc. A version has been made which contains the acoustic characteristics of a Stradivarius violin, in order to “improve” musical instrument performance for traveling performers.

You may already be experiencing the valuable technique of adaptive filtering in noise and echo cancellation and signal prediction situations without even realizing it, but Signal Wizard lets anybody play! ■

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Fig.3 Signal Wizard Hardware Control Panel

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By Jonathan L. TuckerKeithley Instruments, Inc.

Electrostatic discharge (ESD) can damage sensitive electronic components, leading to failures, reduced reliability and increased rework costs, or premature component failures in equipment in the field. That potential for damage makes characterizing charge accurately crucial for electronics manufacturers. Ensuring the accuracy of these measurements requires choosing the right instrumentation, being aware of possible sources of error, and following good measurement practices consistently.

Sources of Error

PART

1

Good Measurement Practices Essential to Characterizing Charge Accurately

What Is Charge?

As every student of introductory physics should recall, charge is the time integral of current.

Potential Sources of Error When Measuring Charge

Although electrometers offer high accuracy, a variety of error sources must be taken into account to avoid degrading charge measurement integrity. These error sources include input offset current, voltage burden, generated currents, and low source impedance.

Input Offset Current

Input offset currents are background currents present in the measurement instrument when no signal current is applied. With an electrometer, the input offset current is very low. However, at low charge levels, even this small current may be a significant error factor. Over long periods, the instrument will integrate the offset current, which will be seen as a long-term drift in the charge measurement. Typical offset current is four femto-amps, which will cause a change in the charge measurement of four femto-coulombs per second. If the offset current is known, it’s possible to compensate for this error simply by subtracting the charge drift due to offset current from the actual reading. However, determining the offset current of the entire system is likely to be significantly more complicated.

Q =

∫ tf

ti

Idt

Charge is often measured on a quantity of particles, on a surface, or on a component such as a capacitor. It is sometimes measured on a continuous basis, such as when using a coulombmeter to measure very low current. An electrometer makes an ideal coulombmeter because it has very low input offset current and high input resistance. The coulombmeter function of the electrometer measures charge by integrating the input current. An integrating capacitor is used in the feedback loop of the input stage. An electrometer’s columbmeter circuit is illustrated in Figure 1.

Voltage Burden

The voltage burden of an ammeter is the voltage drop across the input terminals. The voltage burden of a feedback coulombmeter is generally quite low (less than 100 microvolts). However, if the instantaneous peak current is more than 10 micro-amps, the voltage burden can exceed this level momentarily. In an overload condition, the voltage burden can reach many volts, depending on the input value. If the source voltage is at least 10 millivolts, the typical electrometer in the coulombs mode will integrate the current accurately. However, if the source voltage is much lower, the voltage burden may become a problem, and the input stage noise will be amplified so much that making accurate measurements is impossible.

Generated Currents

Generated currents from the input cable or induced currents due to insufficient shielding can cause errors in charge measurements, especially with charge levels of 100 pico-coulombs or less. To minimize generated currents, always use low noise cable and electrostatically shield all connections and the DUT.

Source Impedance

The magnitude of the source impedance can affect the noise performance of the feedback coulombmeter. Figure 2 illustrates a generalized feedback circuit connected to a source impedance.

Figure 1: Electrometers use a feedback circuit to measure charge, as shown here. The input capaci-tance of this configuration is ACF. Therefore, large effective values of input capacitance can be obtained using reasonably sized capacitors for CF.

In a coulombmeter, the feedback impedance is a capacitor. The noise gain of the coulombmeter can be calculated from this equation:

Output Noise = Input Noise × (1 + ZF/ZS)

where:

• ZS is the source impedance• ZF is the feedback impedance of the

coulombmeter• Input Noise is the noise of the input stage of

the electrometer

In general, as ZF becomes larger, the noise gain in-creases. The documentation or specifications for the particular electrometer being used will gener-ally provide the value of its feedback impedance.

Part 2 of this post will address additional important charge measurement considerations.

About the Author

Jonathan Tucker is a Senior Marketer and Product Manager for Keithley Instruments in Cleveland, Ohio, which is part of the Tektronix test and meas-urement portfolio. He is responsible for business development of Keithley’s research and educa-tion business with emphasis in the areas of nano-technology, semiconductor, energy, printable/organic electronics, and electrochem. He is also product manager for Keithley’s sensitive meas-urement instruments. He joined Keithley Instru-ments in 1987 and has held numerous positions, including test engineer, applications engineer, applications manager, and product marketer. ■

Typical offset current is four femto-amps,

which will cause a change in the charge

measurement of four femto-coulombs per

second. Figure 2: Generalized feedback circuit connected to a source impedance.

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COVER INTERVIEW

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Moving Towards

Modular Agilent Technologies is a leading test and measurement company, offering one of the broadest ranges of innovative measurement solutions in the industry. Since spinning off from Hewlett-Packard almost 15 years ago, Agilent technologies has established a global presence as more customers in the industry recognize its commitment to quality. In recent years, the company has made efforts to develop reliable modular test solutions that offer new and innovative capabilities for system developers. To get a better understanding of the modular solutions that Agilent offers, we spoke with Mario Narduzzi, Marketing Manager of Agilent's Modular Solutions, about the key benefits of the product line and how working closely with customers helps add new beneficial features to the technology.

Agilent’s unique solutions for evolving test and measurement challenges.

Mario Narduzzi Marketing Manager of Modular Solutions at Agilent Technologies

COVER INTERVIEW

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Modern Test & Measure

Can you tell us about your current role as Marketing Manager of Software and Modular Solutions of Agilent Technologies?

I became the Marketing Manager of Software and Modular Solutions this past spring, after having spent 7 years working in China. The Modular Solutions organization is growing and expanding and I was brought into this position to both build upon that growth and help the organization transition from a purely product-focused strategy to more of a solutions-oriented portfolio strategy.

About 2 or 3 years ago, Agilent launched a big modular program and initially came out with around 50 modular products. It had become very clear at the time that offering a choice in format is really a huge benefit to customers, allowing them to create the test system that best meets their particular

needs - be it smaller footprint, which is just one of the benefits of the modular format, or portability, as provided by handheld instruments, for example. As the spectrum of format offerings has expanded over the years, we now also offer software consistency across a variety of platforms and form factors. This dramatically increases the customers’ ability to customize their test solutions and meet their engineering challenges from R&D all the way through manufacturing across a variety of applications, including wireless communications and aerospace defense.

What types of customers are very well suited for modular systems?

One of the most obvious customer benefits offered by a modular system is the smaller footprint in production. For example, customers who are typically racking and stacking equipment in large racks would like to conserve space and modularity

allows them to do that. Beyond size, modular solutions offer a speed advantage due to backplane optimization, so customers who require speed are particularly well suited for modular solutions. Another clear customer benefit offered by modular solutions is flexibility, which allows for multichannel measurements, synchronization and special triggering. Modular systems eliminate the need to rack and stack equipment which must then be connected together. Modular solutions have a certain architecture that allows you to mix and match different products with different capabilities encompassed by software that brings the whole solution together.

Are there any limitations with going modular against bench-top systems?

The limitations of modular stem from their inherent flexibility. In general, whenever you have a highly flexible solution, ease of use suffers. In modular systems you have to re-integrate disaggregated modules back into solutions. Some vendors leave this as an “exercise for the user,” but we have provided a much higher level of integration to address this. If a user wants to make a measurement they can do so quickly, leveraging the software applications they’re already familiar with, such as the 89600 VSA software or industry standard X-Series measurement applications. We also allow users to interface to these applications from their own software applications, in whatever language they prefer, or to bench-top instruments they may be using. The result is that our customers are telling us they can get up and running in a matter of minutes instead of days or weeks. They also have confidence in their measurements because they are using the same measurement IP that they are familiar with from our bench-top instrument products.

Another limitation may simply be familiarity and comfort with a specific form factor. For example, some customers enjoy diagnosing problems using the manual user interface and front panel controls on bench-top instruments. Of course, this can also be accomplished with a modular system by using a laptop or a monitor. My vision is that in the future, the younger generations, especially those who are now graduating from college, will be quite comfortable working with a modular system controlled by a tablet or smart phone.

“Beyond size, modular solutions offer a speed

advantage due to backplane optimization,

so customers who require speed are

particularly well suited for modular solutions.”

“ My vision is that in the future, the younger generations, especially those who are now graduating from college, will be quite comfortable working with a modular system controlled by a tablet or smart phone.”

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As far as looking at the cost-effectiveness of a modular system goes, how does it compare to the traditional bench-top system?

I think they are similar. Essentially what we are doing with the modular form factor is disaggregating or breaking apart the current architecture. There are aspects of modular products and solutions that may be considered expensive, such as the cost of putting things together in multichannel applications, which allows you to take advantage of on-board processing within the module, enabling greater speed. While on-board processing in an FPGA within a module can be costly, modularity allows the user to create a system with just enough performance to meet their specific needs. Because of this, I would say that modular systems and bench-tops are similar in terms of cost, especially for the higher

solution upgrades, not to mention the flow of additional software connectivity, we are regularly bringing new capabilities to our customers. The frequency of these introductions varies greatly however, because the design challenges for a digital I/O module versus a microwave module are quite different and, as such, their design cycles are also quite different. Decades of experience working with customers engaged in RF and microwave tests have taught us that it is extremely valuable to customers to have the ability to use license keys to upgrade their products rather than having to swap out existing products, and we have applied this learning to our modular product and solution introduction strategy as well.

Additionally, from a supportability standpoint, in terms of providing better performance, we have thought about how customers would need to calibrate modules. The e-calibration feature in the modular systems addresses customers’ calibration needs by providing e-calibrated modules that are pre-calibrated for a set of particular instruments through a computer.

What kind of feedback do you get from customers either with these newly added features, or with special requests that customers might have in the development of newer modules?

We have been getting very positive feedback, such as, during the announcement of the release of the VSA (vector signal analyzer) and VSG (vector signal generator) products back in August, the first among a series of products in the field of RF test. Customers were very interested to learn about measurement science consistency across box instruments and modular products. One customer had previously owned one of our bench-top spectrum analyzers and wanted to make a similar measurement with our modular VSA, but were concerned about whether or not they would get the same result. This is both a critical and common concern because bench-top equipment may be used in R&D to validate the design, whereas a modular product may be used in manufacturing. Customers need to verify that the production engineering team will get the same result as the design engineering team. And our answer is “Yes!” It is very exciting to be able

to confidently communicate that message to our customers and we are continuing to get a lot of positive feedback on that subject. When customers either move to modular solutions from bench-top or integrate modular solutions with bench-top, they will benefit from a wide range of readily-available known and trusted software with which they are already familiar, including the Agilent X-Series measurement applications and the 89600 VSA software.

You have quite a lot of management courses up your sleeve. Which among these courses do you think has helped you with your role now as Marketing Manager?

This is really a great question. When I reflect on leadership development, there are a few areas I am really passionate about. One of them would probably be the challenge of maintaining balance as a leader. A balanced leader in my mind is one who can both lead and coach, while focusing on the creation of strategy and defining operational elements.The other part is clearly communicating to the organization so that everyone understands the journey we are on. Over my many years in this business, I have studied quite a bit and read a lot of books about effective leadership. I really find this topic to be very fascinating and I know it is important in my role as a leader.

“A balanced leader in my mind is one

who can both lead and coach,

while focusing on the creation of strategy and

defining operational elements.”

performance applications such as the RF and microwave products. Now, if we are talking about digital applications and lower frequency products, I would argue that a modular solution is less expensive because you don’t need to purchase more performance and capabilities than you need- knowing that in the future you can expand and grow your system as needed.

I think the big message about modular, though, is its ability to increase throughput, which allows customers to enjoy a much lower overall cost of test.

How often are new modules released and existing modules updated for better performance?

Between new product and solution introductions and existing product and

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I have studied widely in the art of strategy development. For me, strategy is all about choosing what you will and will not do to drive business results in the chosen direction. Having a vision and strategy is really important, but the question that you are inevitably left with is how you execute that strategy. I call this “operationalizing a strategy” so that you can deliver the desired business results.

There were a few courses I took that really stood out. One was a course taught by a Harvard professor on driving corporate performance. It covered ideas on how to methodically build a structural model to take your strategy to different levels and help an organization decide what to do to deliver on those objectives outlined in the strategy.

Another course I remember clearly was a country manager program I participated in, while in China. I went to China not just to help Agilent drive the business results or build a marketing organization there, but also to experience the business culture in China. I met with other Chinese business leaders to understand how they ran their business as well as the implications and impacts of being either a state-owned company or a

multi-national company. Every conversation I had with international business leaders, as well as Chinese business leaders, gave me a better understanding of what their challenges really were.

What is the Culture like in Agilent?

Agilent is a great company with a culture focused around business results, achieving objectives, and empowering the people to achieve goals. It goes way back beyond when I first joined the company, 32 years ago. I guess the fact that I am still with Agilent-HP would tell you how much I love the culture here. The environment is one built on a solid foundation of trust. Working together, collaboratively, everyone knows that we are all in this together. If we are successful as both individuals and organizations, then we can help the company become successful as well. I have talked with many of the younger engineers, and it is inspiring to me to hear how the experienced people in the company are very generous with their time, often offering to sit down and take the time to teach the younger generation what they need to know to succeed in this industry. I guess that the bottom line is that I love being able to work with so many experienced and talented people and I have a deep admiration and appreciation for the generous culture. It is this kind of growth and culture that continues to keep me energized at Agilent.

Anything we should look forward with Agilent Modular Solutions?

Our goal is to provide the very best design and measurement capability to our customers. In delivering to this, I expect that our modular instruments and bench-top instruments will co-exist as we continue to innovate our measurement technology. Solutions have to match the user’s environment. Whether their needs are best served by bench-top, hand-held, or modular form factors, our software will continue to ensure consistent results no matter what the customers’ product lifecycle phase may be. In the future, I expect this to be the case across our whole portfolio of measurements from basic voltage to high speed digital and microwave. And there are some exciting developments coming with some new solutions and approaches that will fundamentally improve our customers’ product life cycle effectiveness. ■

“ Agilent is a great company with a culture focused around business results, achieving objectives, and empowering the people to achieve goals.”

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Low Cost PortableLogic AnalyzersTake On New Debug Tasks

By Brad Frieden, Agilent Technologies

Today’s digital designs involving FPGAs, System on Chip (SOC) ICs, and memory are increasing both in complexity and speed. Designers require tools that provide a way to trigger on a “symptom” and then look back in time with high speed timing capture to find the root cause of failure. There needs to be a way to distinguish between functional and timing related problems associated with a large number of signals as is typical in many of today’s embedded designs.

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The same is true for tracking high-speed data buses with synchronous “state mode” capture and having a high-speed trigger sequencer to find specific system conditions. It’s also important to be able to trace out DDR2 and DDR3 memory address and control buses, apply decoders, compliance tools and performance analysis tools. Finally, having both single-ended and true differential probing is needed for the evolving high speed differential interfaces in order to accurately capture signals of interest.

All of these digital debug capabilities are now possible with a new generation of general purpose logic analyzers that offer performance and capabilities that were previously only available in higher priced modular logic analyzer systems.

Trigger on a symptom and look back in time for the root cause with greater detail

A very common scenario occurs when a design team turns on their digital prototype only to discover that it is not operating as it was designed. Trying to discover the actual problem usually requires a view of the target activity in enough detail to start isolating the root cause of failure. For more complex systems, in which an oscilloscope or even a Mixed Signal Oscilloscope is not sufficient to get a quick system-level view, a logic analyzer is often preferred as the first step in gaining an overview of the functional behavior, so that further debug and validation can be applied

to areas that seem suspect. Early on in the debug process it is difficult to know how to trigger the analyzer, but a timeout signal or flag is a good starting point. The analyzer can detect an error in the target by triggering on an error condition such as a timeout signal or a flag that is pulled high.

Consider, for example, a design implemented in a Xilinx Zynq 7000 series SoC in which a basic communication system is not moving data to another device as it should. A basic state machine should drive a process in which data is written out to another device and an acknowledge bit should be received back from that device indicating that the data was received and now more data can be sent. That back-and-forth process should continue until all the data is sent.

In this example, FPGA I/O signals are being probed with flying lead probes connected onto 0.1-inch spaced header pins. The logic analyzer is set up with a bus label called “state” for the five-bit “one-hot” state machine, another label for the acknowledge signal called “ACK”, and a label for the timeout signal called “timeout.” This bus and signal set-up is shown in Figure 1.

The acquisition mode is set to Timing Capture with a 5 GHz sampling speed and 4M sample memory depth in a menu under the “Sampling” tab. A trigger is set from the waveform window for the rising edge of the timeout signal since the timeout signal is suspected to go high due to the target failure. The logic analyzer Run button is pressed and the target system is initiated. The logic analyzer triggers on the rising edge of the timeout signal, and the resultant trace is seen in Figure 2.

Notice that, other than the timeout signal rising edge and the 250 MHz running clock, all the other signals are a flat line or locked up. The state machine value of “1F” is an invalid state and not much more information can be gleaned. But since the timing capture had a 5 GHz, 200ps sampling with high resolution, the designer can right-click on the screen and select “Zoom Out Full” to see what has been captured in the deep memory buffer of the logic analyzer. This is shown in Figure 3.

Now, by looking back in deep memory capture, activity can be seen around 500 μs prior to the timeout flag. This is where the root cause of failure might be found. A box can

be drawn around the area of activity to zoom in and look closer as shown in Figure 4. This zoomed view enables the designer to see that the state machine had a burst of activity, an acknowledge bit was returned, a second burst occurred, and another acknowledge bit was returned before it is obvious that something went significantly wrong. A third state machine burst occurred but no acknowledge bit was returned and the state machine then stopped running altogether.

Of great interest at this point is looking more closely at that third burst of the state machine—when there is no responding acknowledge bit—to closely examine the timing and logic of this circuit. With the ability to zoom in to the third burst, some very interesting developments can be seen as shown in Figure 5.

The general functionality of the state machine looks correct—from the IDLE state, to the START state, to the ADDR state, to the DATA state and finally to the WAITING FOR ACK state. Zooming in with deep memory it is now possible to look at the timing between the clock and the state machine data bits. A first marker can be snapped to the rising edge of the clock associated with the START state, a second marker snapped to the point in time where the START state has stabilized. With the 5 GHz timing capture, it is possible to discern that there is a serious timing problem because the rising edge of clock is very close to the point in time when the state machine bits have settled to the START state, leaving little set-up time between clock and data, as shown in the Delta time measurement between marker one and two of only 600 ps.

The clock edge should be centered on the START state machine data valid region. Notice there is an intermediate value on the state machine bus between the IDLE and START state, and it is possible that the invalid state value is getting clocked into the other device due to the minimal setup time. Such an occurrence could explain why no acknowledge bit ever comes back from the other device and why the design malfunctions. This allows further investigation into the specific cause of the clock-to-data timing error. The result in this case was that the designer left out the Xilinx User Constraint File (UCF) which defined the clock-to-data timing. The quick overview provided by the logic analyzer, with the added benefit of

Figure 1: Bus and channel setup to capture the state machine, acknowledge bit, and timeout bit.

Figure 2: Capture with trigger on rising edge of the timeout flag finds that the state machine is locked up.

Figure 3: Signals shown with “Zoom Out Full” feature combined with deep memory reveals activity 500 usec before the trigger point.

Trying to discover the actual problem usually requires a view of the target activity in enough detail to start isolating the root cause of failure.

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being able to zoom in on specific areas allowed the designer to quickly find the root cause of error. Transitional timing extends the total target time that can be observed

Often designers need to see a certain amount of target activity in order to determine if proper operation is happening. An example of this might be the observation of a complete video frame. A modern day design can easily require memory depth of 8 M samples to verify a video frame.

An important timing capture mode in the logic analyzer is called “transitional timing.” In this mode acquired samples are only placed into memory if there is a change in any of the bits acquired. This results in extremely efficient use of the memory in the logic analyzer, especially with a target exhibiting bursty behavior. In this mode, memory use is optimized, thus allowing the capture of a longer period of target operation.

Take, for example, the previous situation where the state machine burst of activity occurred 500 µs prior to the trigger point. With the transitional timing mode, even a trace as shallow as 1M samples with 5 GHz timing speed could capture this activity and still allow high-resolution timing evaluation, as shown in Figure 6. Notice that the samples (shown as tick marks on the lower time scale) are only saved when there is a transition (two samples per transition in 5 GHz half-channel mode).

Summary

Now that general purpose portable logic analyzers offer up to 5 GHz timing capture combined with deep memory, it is possible to trigger on the main symptom of a problem, such as a timeout flag, and look back into deep memory capture in search of the root cause of failure. Added capabilities such as a high-speed trigger sequencer and differential probing also extend the range of applications that can be addressed with portable logic analyzers.

About the Author

Brad Frieden is a Product Planner / Product Manager for the Oscilloscopes and Protocol Division of Agilent and has been focused on general purpose logic analyzers and related applications. He has been with HP/Agilent for 29 years and been involved in a variety of marketing roles in areas including fiber optic test, pulse generators, oscilloscopes, and logic analyzers. He received his BSEE from Texas Tech in 1981 and his MSEE from The University of Texas at Austin in 1991. ■

Figure 4: Missing acknowledge signal found after third state machine burst.

Figure 5: Clock rising edges are not in the center of data thus likely violating setup and hold times.

Figure 6: Transitional timing extends target capture time.

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