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The scoop on scopes
Engineers use scopes for everything from trouble shotting pc boards to measuring engine vibrations
By Julie Anne Schofield, Senior Editor -- Design News, October 23, 2001
Scope measures analog and digital circuits
Inexpensive 8-bit microcontrollers are replacing 4-bit chips in thousands of consumer, industrial, automotive, and communications products. The in-crease in these mixed-signal (analog/digital) applications puts pressure on design engineers to find the most effective and efficient way to test these systems. Requirements include probing many more points on each chip, dealing with digital signals of widely different frequencies, and capturing waveform data for long periods of time, while preserving the ability to resolve small time intervals.
Hewlett-Packard has responded with the HP 54645D mixed-signal oscilloscope. This single instrument is an easy-to-use alternative to separate oscilloscope and logic analyzer units--saving lab space, test time, and equipment dollars.
The rugged, portable unit has two 100-MHz, 200-megasample/sec, 1-Mbyte oscilloscope channels; 16 logic channels; and scope controls and features. Combined with powerful triggering capabilities, the tester lets engineers view and measure the relationship of both analog and digital circuits on a single display screen.
Key to the scope is HP's MegaZoom technology, which uses a 5-processor architecture in the signal path between the scope and its display. The architecture results in a scope with deep memory; high signal-update rates; a user interface with familiar, analog-like knobs; and fast, responsive post-acquisition waveform pan and zoom.
Base price: $4,995. A $295 option is HP BenchLink/scope Windows-based software for transferring data and screen images from the scope to a PC. Users can also store scope setups for later recall.
Handheld scope packs benchtop performance
New from Tektronix is the 200-MHz TekScope--the THS730A, which combines an oscilloscope with a digital multimeter. The handheld unit can sample data at 1 gigasample/second in real time--true bench-level performance--and features the company's IsolatedChannelTM Architecture. Tek engineers developed the architecture to safely test and measure various combinations of power voltage. The dual channels/dual digitizers isolate the power sources, yet keep the timing relationship intact.
Developed for design and test engineers responsible for electronic circuit design and debut, as well as for field troubleshooting, the THS730A offers high-speed measurement and triggering capabilities for quick timing error detection. Other features include an easy-to-use graphical user interface, advanced triggering functions, glitch capture, and dB and dBm measurements. Battery-powered, the unit trickle charges in only nine hours.
Prices start at $2,995, including two 200-MHz probes.
Industrial troubleshooter in mini package
Smaller and less expensive than the Tektronix offering, Fluke's ScopeMeter 123 combines a 20-MHz scope, digital multimeter, plus a paperless recorder for less than $1,000. The unit is geared for por-table test applications, such as industrial machinery service, plant maintenance, electrical troubleshooting, electronics maintenance, and process control. Even those with little scope experience will find the tester easy to use.
At 9×4×2 inches, Fluke touts the ScopeMeter 123 as the smallest and lightest oscilloscope, multimeter, and recorder combination in the world. It runs on a 5-hour rechargeable nickel cadmium battery pack, requires no grounding, and includes a bracket that lets users mount the unit on a wall or top of a door.
A Windows-like user interface and intuitive keypad with large color-coded keys make it easy for even occasional oscilloscope users to get up to speed quickly. A simple selection menu guides setup and operation. One press of the menu button provides instant access to 26 commonly used measurements, including volts, time, frequency, temperature, resistance, duty cycle, phase, and capacitance.
Users need connect only one test lead to access both oscilloscope and multimeter functions on each of the two channels, eliminating the need to switch probes and leads when switching functions.
Design partners for the ScopeMeter 123 include Philips Semiconductor, which helped design two analog ASICs (application-specific integrated circuits) to handle complex analog and high-speed digital functions, and Motorola, whose FlexCore technology led to a single digital ASIC for acquisition, peak detection, constant signal analysis, trigger control, display control, communication, real-time clock, and processing. Both components contribute to the tool's compact form factor and low price.
Scope hits 'giga' heights
LeCroy's latest DSOs--the LC574 Series--have "giga" banner specs: four input channels with 1-GHz bandwidth, 1-gigasample/sec sampling per channel, and up to 4 gigasamples/sec when using a single channel. Uses include such high-end applications as designing and testing computers, hard-disk drives, computer peripherals, telecom equipment, ultrasound and MRI instruments, and aerospace systems. Prices start at $26,490.
Other notable new features: maximum vertical sensitivity of 5 mV/division with amplifier zoom to 40 µV/division, an internal high-speed printer, floppy-disk drive, histogram analysis package, 10-inch color CRT, and 64 Mbytes of processing memory on the LC574AL model. A popular option is the PCMCIA type III slot with 170-Mbyte hard-disk-drive card.
The scopes' long record lengths--up to 2 Mbytes per channel or 8 Mbytes on a single channel--lets engineers capture complex signals with high accuracy. They also improve the scopes' ability to zoom in on key details of long signals. A wide range of triggering features capture signal aberrations, and the Analog Persistence display mode enables users to look at multiple signals by toggling between opaque/transparent displays and different-color waveforms.
Powered by a 96-MHz PowerPC microprocessor, the LC series provides an advanced set of signal diagnostic, troubleshooting, and documentation tools. They include: measurement of 42 signal parameters; worst-case analysis (maximum, minimum, average, and standard deviation) of those parameters; an FFT (fast Fourier transform) package that can resolve 4 million time domain samples into the frequency domain; and a math package with integration, differentiation, square root, absolute value, exponential, log, and a set of six selectable filters.
Cyber contacts
Visit the following Web sites to get the latest scope information from the companies featured in this article:
Fluke Corp., http://www.fluke.com
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LeCroy Corp., http://www.lecroy.com
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Hewlett-Packard Co., http://www.hp.com
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National Instruments, http://www.natinst.com
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Tektronix Inc., http://www.tek.com/Measurement
How DSOs work
A basic digital storage oscilloscope (DSO) has two or four channels. Signal probes attach to the input BNC connectors, which lead the vertical system's amplifiers and attenuators. These components match the signal to the rest of the circuit.
Next, the analog-to-digital converter (ADC) in the acquisition system samples the signal at discrete points in time. The horizontal system's sample clock determines how often the ADC takes a sample. This frequency is called the sample rate and is measured in samples per second.
Each time it takes a sample, the ADC converts the signal's voltage to 8- to 12-bit digital values called sample points. These values then go to data-acquisition memory, which stores them as waveform points. Together, the waveform points make up one waveform record. The trigger system determines the record's start and stop points.
The record points go to the digital display system, which sends them to the display. Most scopes also let you do some post-acquisition processing, such as FFTs. In this case, the data go from the memory to the processing circuitry, back to the memory, and then to the display.
Boards let PCs double as scopes
Virtual instrumentation technology lets engineers turn PCs into all kinds of test equipment, including DSOs. National Instruments' DAQScope boards and PCMCIA card, along with VirtualBench-Scope software, convert PCI and ISA bus-based PCs and laptops into 20-MHz scopes. Board prices range from $995 to $1,195, including software.
Why use a PC as a scope? Many engineers need to do more than make a measurement--they need to make decisions based on the data they collect. Traditionally, they use a stand-alone scope to acquire the data, then send the data via cabling to a PC, where they can use such tools as Word, Windows, Excel, or LabView to analyze the data.
Using a plug-in scope board puts the instrument inside the computer, eliminating the cabling and large stand-alone piece of equipment. "If you look at a traditional scope, what you have in it is memory, a bus for sending data, a small display, and some acquisition components," notes Ed McConnell, DAQ product manager for National Instruments. "Well, a computer has a bus, memory, a large display--the only thing it doesn't have is the scope hardware for acquiring and digitizing a signal, and that's what we bring to the computer."
Another reason an engineer might choose DAQ Scope over a stand-alone scope is that PC technology lets the user network instruments or create a web page and send data over the Internet.
The VirtualBench scope interface looks almost identical to those of traditional scopes. Instead of having physical knobs you turn with a finger, you have virtual knobs you turn with the mouse. A touch screen is an intuitive mouse replacement.
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