Electronic Design
Scopes And Arbs Rule The Roost In Test For 2011

Scopes And Arbs Rule The Roost In Test For 2011

In the world of test and measurement, a product or technology occasionally appears that gives one pause, something that makes you quietly say “wow.” Such is the case with Tektronix’s MDO4000 series scopes, which speak to the need of today’s designers for instruments that span disparate signal types and domains.

Likewise, Agilent’s M8190A arbitrary waveform generator delivers a pristine output that lends itself to development of digital radar systems. And, Keithley Instruments’ 4225-PMU current-voltage (I-V) module for its 4200-SCS semiconductor characterization system exemplifies elegance in modular format. These three products are this year’s best-in-class in test and measurement.

The “Wow” Factor

Designers often find themselves with the need to acquire and analyze analog, digital, and RF signals when troubleshooting wireless system designs. This typically requires access to both a scope and a spectrum analyzer. In fact, a recent survey by Tektronix of its customers found that 64% of scope users also frequently use a spectrum analyzer.

The MDO4000’s “wow” factor is that it is both a scope and a spectrum analyzer, all in a package that is just 5.8 inches deep (see “A Hands-On Look At Tek’s MDO4000 Mixed-Domain Scope” at www.electronicdesign.com). Claimed by Tektronix as the world’s first mixed-domain oscilloscope, the MDO4000 sports the ability to capture time-correlated analog, digital, and RF signals, allowing users to see how the RF spectrum changes over time (Fig. 1).

The instrument’s built-in spectrum analyzer offers a 3-GHz or 6-GHz RF port, which addresses the vast majority of wireless test needs that embedded system designers would encounter. It augments that with a capture bandwidth of at least 1 GHz at all center frequencies, which is considerably broader than typical spectrum analyzers.

With the scope’s ability to display all signals in a time-correlated fashion, designers can see all of their analog, digital, and RF signals accurately correlated with each other. This is a boon to the troubleshooting of embedded systems. The scopes offer up to 21 channels (four analog, 16 digital, one RF) with analog bandwidths of 500 MHz or 1 GHz.

For spectral analysis, the instrument provides both automated markers for quick, easy identification of signal peaks and manual markers for measurement of non-peak areas of interest. A spectrogram display provides visualization of slowly changing RF phenomena.

The oscilloscope itself, based on the MSO4000B series, offers a maximum waveform capture rate of 50,000 waveforms/s. There are more than 125 trigger combinations, including the ability to trigger on serial packet content. The scope’s MagnaVu high-speed digital acquisition delivers 60.6-ps resolution. Low-capacitance passive probing offers twice the bandwidth (1 GHz) and half the loading (4 pF) of typical passive probes.

An Arb For The Ages

Meanwhile, it seems that everything is going digital, and radar systems are no exception. Designers of radar equipment increasingly rely on transmit/receive modules with high dynamic range for the detection of extremely low-level signals. The coming generation of these modules will contain digital I/Q or digital IF capabilities as well as very high-speed digital buses. This in turn is driving a need for updated test equipment that supports the move from analog to digital radar systems.

With its M8190A arbitrary waveform generator, Agilent delivers spurious-free dynamic range of up to 80 dBc at 14-bit resolution (Fig. 2). The instrument offers switchable resolution, as the digital-to-analog converter (DAC) operates at 8 Gsamples/s at 14-bit resolution and at 12 Gsamples/s at 12-bit resolution. Thus, users can decide if they need higher bandwidth or higher resolution for a given measurement.

In traditional I/Q modulation schemes used to generate HF signals, a pair of DACs generates the baseband signal for subsequent modulation up to RF. This generally works well but the output may be plagued by spurious signals. These glitches do not affect the final settled value of the signal, but they do corrupt the spectral content of the output signal. The risk is misinterpreting these glitches as analog output.

Modulation adjustment tames the spurs, but this tends not to hold over time and temperature. In the M8190A, Agilent’s proprietary DACs deliver intermodulation that is down about 70 dB. You can use the output directly if it’s high enough in frequency for your measurement, or it can be mixed up to RF.

The secret to removing the DAC output glitches lies in the use of a resampling switch on each of the 26 current sources that make up the DAC. Each of these switches handles only one value of current, on or off, making the process extremely linear. The values are then added in the resistor termination to achieve the final output.

Resolution and bandwidth are important, but for realistic test scenarios, an arbitrary waveform generator needs a lot of memory for long play times. The M8190A features up to 2 Gsamples of waveform memory, yielding 1/6 seconds of play time at the highest sampling rate.

That memory is put to its best possible use through sequencing. The memory is divisible into as many as 256,000 segments, each of which can be looped up to 4 billion times. Additionally, three levels of sequencing are available. The advanced mode permits the setup of very complex waveforms. There is also the option of real-time memory access.

The M8190A includes three user-selectable amplifiers, each of which is optimized for different signal characteristics. The first is for I/Q signal generation, the second for high-bandwidth IF/RF applications at bandwidths up to 5 GHz, and the third for time-domain applications requiring low jitter and low overshoot.

Configurations for the instrument include a five-slot AXIe chassis, which fits up to two M8190A arbs with a system controller and external support module (ESM) for PCI Express connectivity. This configuration requires only the addition of a monitor to form a complete instrument. Alternately, a two-slot AXIe chassis can be fitted with a single arb and the ESM. This configuration would require a PC or a laptop with a PCI Express card.

When it comes to generating signal waveforms, users have various options including Matlab, LabVIEW, or home-brewed software created with Microsoft Visual Studio.

Modularity = Versatility

The beauty of modular instruments is their ability to be upgraded, expanded, or otherwise improved upon without having to invest in a completely new instrument. Keithley’s Model 4200-SCS semiconductor characterization system is just such an animal. The latest addition to the analyzer is the Model 4225-PMU, an ultra-fast current-voltage (I-V) module that adds ultra-fast voltage waveform generation and current/voltage measurement capabilities to the 4200-SCS’s arsenal.

Why do engineers characterizing devices and new materials and processes need ultra-fast I-V? It’s because three types of measurements are required to fully characterize devices, materials, and processes: precision dc I-V, ac impedance, and ultra-fast or transient I-V. In adding the 4225-PMU option to the 4200-SCS system, Keithley has rounded out the instrument’s capabilities. The option enables the characterization system to serve emerging applications from high-k dielectrics to silicon-on-insulator (SOI) in small-scale CMOS, flash memory, or phase-change memory.

Keithley’s goal for the 4200-SCS system was to make characterization easy through use of a GUI-based, point-and-click interface. In adding the ultra-fast I-V testing, the same goal applied. Further, Keithley hoped to make the 4225-PMU option look, act, and feel as easy to use as an ordinary high-resolution source-measure unit (SMU).

Achieving these goals required making the module’s measurement range very broad, with similarly broad dynamic range. It also required simplification of the interconnect. The result was extremely fast measurements with correspondingly high resolution. Current resolution ranges from 1 A to 1 pA. While the previous-generation 4200-SMU fast I-V module permitted measurement speeds as fast as 10 ms, the 4225-PMU can make some higher-current measurements as fast as 10 ns. It achieves all this while staying as close to the Johnson noise limit as possible.

Each 4225-PMU module provides two channels of integrated sourcing and measurement but takes up only one slot in the nine-slot mainframe chassis that houses the system. Each chassis can accommodate up to four modules for a maximum of eight ultra-fast source/measure channels. Each channel combines high-speed voltage outputs (with pulse widths ranging from 60 ns to dc) with simultaneous current and voltage measurements.

The module provides high-speed voltage pulsing with simultaneous current and voltage measurement, at acquisition rates of up to 200 Msamples/s with 14-bit analog-to-digital converters (ADCs), using two converters per channel (four per card). Users can choose from two voltage-source ranges (±10 V or ±40 V into 1 m?) and four current measurement ranges (800 mA, 200 mA, 10 mA, 100 µA).

Each 4225-PMU module can be equipped with up to two optional Model 4225-RPM remote amplifier/switches, which provide four additional low-current ranges. They also reduce cable capacitance effects and support automatic switching between the Model 4225-PMU and other SMUs installed in the chassis.

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