DSOs Track Elusive High-Speed Waveforms

Jan. 1, 2004
The latest generation of digital oscilloscopes offers lightning-fast sampling rates and sophisticated timing and display systems to analyze hard-to-capture electronic events.

Oscilloscopes are not often considered the main measurement tools of the RF/microwave engineer. However, with the increasing use of complex modulation formats in commercial communications, and "exotic" signal formats in military systems, the digital storage oscilloscope (DSO) is now a vital test instrument throughout the high-frequency industry. Understanding some of the key performance specifications can help simplify the task of finding the right DSO for the job.

Digital oscilloscopes have all but replaced analog models where performance, flexibility, and functionality are critical, although older analog units are still available from some sources, such as on-line auction firms such as Ebay.com and rental/leasing companies such as Electro Rent Corp. (www.electrorent.com). Experienced RF engineers may recall setting up a Polaroid instant camera on a hood in front of an analog oscilloscope, trying to freeze the image of a fast-moving waveform. Since a DSO captures a signal by means of sampling through an analog-to-digital converter (ADC), any captured signal information exists in digital form and can be saved and recalled at a user's convenience. Many modern DSOs include analog-like display capabilities, such as a persistence mode, to simplify operation for users familiar with older analog oscilloscopes.

DSOs can be compared in terms of various key performance parameters, including those related to the instrument's display, time base, and acquisition/triggering capabilities. Display specifications usually include the nominal analog input bandwidth, the typical rise time of a measurement channel, the number of channels, the sensitivity (usually in millivolts per display division), the maximum input voltage, the vertical resolution (in bits), DC gain accuracy (as a percentage of the full-scale reading), and offset range and accuracy. The time base or horizontal system, which is tied to the quality of the internal reference source (usually a stabilized crystal oscillator), is usually characterized in terms of the time/division range (usually from ps/division to s/division), the clock accuracy (in PPM), and the jitter noise floor. The acquisition system, which handles how input signals are sampled and stored in memory, is usually characterized in terms of the single-shot sample rate per channel (the capability of capturing a single random event), the interleaved sample rate (in which the sampling power of ADCs from two channels are combined to effectively double the single-shot rate), and the random interleaved sample rate (which provides high sample rates for capturing more predictable, repetitive events).

Although oscilloscopes have traditionally been classified as either high-performance or general-purpose units, the gap between the two types continues to narrow. The Infiniium series of DSOs from Agilent Technologies (www.agilent.com) includes not only general-purpose (with bandwidths from 600 to 1000 MHz) and high-performance models (bandwidths from 2250 to 6000 MHz), but also portable instruments (from 60 to 500 MHz), models for mixed-signal use (from 60 to 1000 MHz), and even instruments that combine a digital communications analyzer (DCA), oscilloscope, and time-domain reflectometer within a single mainframe (with bandwidths from 3 to 80 GHz). The Infiniium instruments typically provide four measurement channels with 12-b vertical resolution and less than 1-ps root-mean-square (RMS) time base jitter. The DCA models, which provide specialized communications testing with data presented on eye diagrams and constellation displays, can be configured with a wide range of electrical and electro-optical modules to meet the needs of wired, wireless, and optical communications systems.

The WaveRunner 6000 series DSOs from LeCroy Corp. (Chestnut Ridge, NY) are sold as general-purpose scopes, but provide man of the features and performance levels associated with high-performance instruments. For example, all models include 8.4-in. color touch-sensitive screens, along with at least 1 million points of memory per channel, and 5 PPM time base stability; options provide 2, 4, 8, or 12 Mpoints of memory per channel. Models include the 500-MHz WaveRunner 6050, the 1-GHz WaveRunner 6100 (Fig. 1), and the 2-GHz WaveRunner 6200. These typically four-channel oscilloscopes feature a 5-GSamples/s ADC on each channel. The ADCs from two channels can be interleaved to create a single 10 GSamples/s channel (resulting in a total of two measurement channels instead of four). The WaveRunner instruments include an analog persistence feature to create an analog-like display that can ease the analysis of some measurement data (such as video signals). For more high-performance applications, the company also offers the WaveMaster 8000A Series with single-shot sampling rates to 20 GSamples/s and repetitive interleaved sampling to 200 GSamples/s at bandwidths to 6 GHz and the 1-to-3-GHz midrange WavePro Series oscilloscopes.

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Another company that is migrating high performance levels to more "general-purpose" applications is Tektronix (www.tektronix.com), with its advanced Digital Phosphor Oscilloscope (DPO) technology in which capture rates exceeding 400,000 waveforms per second are used to instantaneously collect and analyze data about high-speed signals and provide meaningful two- and three-dimensional displays of test results. The company's TDS7000B series of DPOs include a triggering system based on silicon-germanium (SiGe) components and trigger jitter as low as 1 s RMS. The flagship model TDS7704B DPO features a 7-GHz input analog bandwidth and 43-ps rise time along with a 20 GSamples/s single-shot sampling rate well suited for analysis of 4.25-Gb/s serial data (Fig. 2). As with Agilent Technologies, Tektronix also offers a comprehensive line of communications signal analyzers (CSAs) with signal processing and display capabilities designed to provide meaningful displays of captured communications signals, and the new RSA Series real-time spectrum analyzers which are designed to capture and digitize signal bandwidths as wide as 10 MHz at frequencies to 8 GHz. The RSA instruments also offer a DSO-like time-domain mode based on the use of Fast Fourier Transform (FFT) capability.

While the instruments mentioned so far represent traditional, stand-alone units, several oscilloscope suppliers leverage personal-computer (PC) technology for their products, including Acqiris Technology (www.acqiris.com), Gage Applied Sciences (www.gage-applied.com), and Pico Technology (www.picotech.com). For example, the model DP240 PCI digitizer card from Acqiris provides the characteristics of a DSO for two measurement channels, a 1-GHz bandwidth, and a 2 GSamples/s sampling rate. The company's DC271 CompactPCI digitizer offers four channels, a 1-GHz analog bandwidth, sampling rates from 1 to 4 GSamples/s, and as much as 32 Mpoints acquisition memory. The CompuScope 82G PCI digitizer card from Gage is now available with optional 1-GHz bandwidth and 1 GSamples/s sampling rate simultaneously on two channels. The card can be equipped with as much as 16 MB of acquisition memory.

Several suppliers offer analog and digital oscilloscopes designed for portability, generally with bandwidths of 100 MHz or less. These include the three-channel, 100-MHz, 100-MSamples/s model LS8106A oscilloscope from Leader Instruments (www.leaderusa.com) and the 100-MHz D7510 Series DSOs from HC Protek (www.hcprotek.com). Perhaps the most compact and portable digital oscilloscopes currently available are the ScopeMeter instruments from Fluke (www.fluke.com), which also include digital-multimeter (DMM) capability. For example, the battery-powered, handheld model 199C features a 200-MHz input bandwidth, 2.5 GSamples/s sampling rate, and 1200 points of acquisition memory, with captured information shown on a color screen with digital and variable-persistence display modes.