For decades, the analog oscilloscope has been the least costly way of making the AC and timing measurements essential for verifying performance of most electronic products.
But it lacks certain features needed to support the changing needs of manufacturing. Test professionals are turning to a new class of digital oscilloscope that addresses manufacturing test requirements—ease of use, versatility, repeatability and low cost—and also offers compactness, plug&play printer outputs and stored setups.
The analog scope has performed admirably in manufacturing for decades. Its bandwidth, usually in the 20- to 60-MHz range, suffices for all but the most critical measurements. It provides acceptable test times on routine production tests. The addition of on-screen cursors and CRT readouts has made available a degree of automation, although these features add cost and still require a skilled operator’s eye.
But the needs of the manufacturing test environment are changing. Increasing functionality in the end product can mean more potential trouble spots, buried more deeply in high-density circuit boards and ASICs. At the same time, factory capacity—throughput—is being called upon to balance the ever-growing costs of materials, equipment, floor space and labor.
Manufacturers now want fast, foolproof production measurements with a minimum of operator intervention. The analog scope lacks the automation features needed to support the predicted growth in capacity.
The technique of using a grease-penciled mark or a piece of masking tape on the scope graticule as a measurement template (a common substitute for automated cursors) has been superseded by faster and more accurate methods. As process control structures become more prevalent in the factory, instruments will be required to implement consistent procedures, and to produce written documentation of test results.
Digital Oscilloscopes Take Aim
Breakthroughs in digitizing technology, such as digital real-time oversampling, have brought full-bandwidth, multichannel acquisition to production-level scopes. And for the first time, digital scopes are competing with their analog counterparts in price.
Digital instruments are an enabling technology for the more demanding high-throughput test procedures taking shape today. Even though the measurements they make might be the same as those performed in the past by analog scopes, the digital tools produce more consistent results.
The consistency is due to two features:
The scope’s acquisition settings are stored within the instrument. Test engineers can set up the measurement once, store it and then have confidence that the procedure will be carried out the same way indefinitely. Less skilled personnel can be assigned to the test station without risking the integrity of the quality control process.
Stored waveforms can be used as templates. Here again, test or QC engineers can acquire a “golden” reference waveform and store it onboard. Production-line operators can evaluate test results relative to the stored reference, rather than making a subjective judgment. Built-in cursors expedite common voltage and time measurements.
A further benefit of the digital platform is its ability to print waveforms and test results on ordinary office printing equipment.
Of course, none of this is useful unless the digital scopes can meet the basic accuracy and bandwidth needs of the application. The latest digital scopes for production test offer sample rates at least 10 times their bandwidth. This provides faithful acquisition of the fast edges and complex waveforms common in today’s video, communications and computer products.
A Production-Line Application
A typical consumer TV manufacturing line illustrates the productivity benefits that digital scopes can provide in production test applications. This example is based on observations at a U.S. manufacturing site for name-brand color TV/VCR combination units.
The factory integrates preassembled components, mounts them into a cabinet, and tests and packages the product for shipment. Many of the components are pre-tested at the original manufacturing site, so the tests described here provide final verification as the product is integrated. The production line is a series of assembly and test stations along a moving conveyor.
Some test stations use oscilloscopes, others use specialized video instruments and still others deploy computer-automated systems. At each test station, a written procedure outlines the measurement step and the waveforms that relate to it, ensuring consistency from operator to operator.
The first series of test stations (Figure 1) validates VCR assemblies before installation into the chassis. While the VCR runs a pre-recorded test tape, the digital oscilloscopes monitor audio amplitude, X-Adjust and pulse generator (PG) Shift settings.
The measurements affect the VCR’s ability to play back a clear, stable picture with adequate audio volume. Contact with the UUT is via a one-touch fixture that positions the UUT and drives the scope probes downward onto the circuit board test points.
PG Shift Adjustment
In the PG Shift adjustment procedure, the scope acquires a modulated signal envelope from the VCR’s playback head circuits. Figure 2 shows the simplified tape path across the VCR heads. “A” and “B” are the video heads. The rotating drum causes A and B to read alternating interleaved tracks of information stored in a helical pattern on the tape.
The output of the A head should be exactly 180° out of phase with the B head; otherwise picture quality deteriorates. A PG control track on the tape identifies the position of the VCR head drum as it rotates.
The effects of a misadjusted PG signal are easily detected on the scope screen. As shown in Figure 3A, a gap appears in what should be a continuous envelope. This gap is mirrored by an actual gap in the video picture—an unacceptable degradation of image quality. Figure 3B shows the corrected waveform envelope, which exhibits small peaks where head switching occurs.
If the UUT’s PG Shift waveform cannot be brought within range, the digital scope prints a hard copy of the failing waveform, which is then sent along with the unit to a repair station.
Next, a series of assembly steps integrates the VCR with the TV chassis, CRT and cabinet. The digital oscilloscope checks the screen voltage, a quantitative measurement that again uses a template to ensure that the signal meets amplitude tolerances.
The high voltage test is carried out with an HV attenuating probe applied to the CRT anode. A DMM captures the reading. After an “aging” step (to expose premature component failures) the unit goes through computer-controlled video purity and convergence tests.
A digital oscilloscope is used for subcontrast and subtint measurements after the convergence test. The settings are essential to a good video image, and the observed waveform must fall within a narrow amplitude tolerance.
In this case, the signal peaks must conform closely to those of the reference. A misadjustment here would render the television’s front-panel contrast and tint controls ineffective. Figure 4 shows the waveform at the subtint (also known as tint centering) test point, and how a stored waveform can be used to set up this critical parameter.
Scope Offers Choice
The digital scope offers a choice of solutions for measurements like subtint. Less-skilled production personnel can rely on cursor lines, set in advance by a test engineer, to define the limits of the waveform amplitude envelope. The operator simply trims the amplitude until it is within the cursor boundaries.
Likewise, the template method shown in Figure 4 is well-suited to production use. Using a stored reference waveform like this permits foolproof amplitude adjustment and reveals any phase problems.
If quantitative measurements—the signal’s actual peak-to-peak amplitude in volts—are needed, either cursor-based measurements or fully automated, continuously sampled readings can be used.
In any of these situations, a technician can freeze the display for a closer look at the instantaneous amplitude if problems arise.
The Last Steps
The last production measurement steps rely on a combination of video instruments and subjective judgments. After a white balance test and automatic brightness limiter (ABL) adjustments, the unit is sent to the ultimate judge of image and sound quality—the human eye and ear. The final QC steps on the production line involve looking and listening rather than quantitative measurements.
Throughout the process, failed units are delivered to a repair station, where a technician uses a digital scope to troubleshoot problems. This scope’s high bandwidth and resolution help pinpoint signals outside normal margins.
In the finished goods warehouse, randomly selected units from stock are subjected to a full teardown and inspection. Using the same stored setup routines and waveforms as those used in production, technicians with digital oscilloscopes confirm the measurements and settings in the finished product.
Conclusion
Analog scopes are being challenged by a new generation of digital oscilloscopes that are competitively priced yet far more capable than the analog tools. These digital scopes will deliver the increased throughput and productivity that today’s production line demands.
About the Author
Jack Johnson is Product Marketing Manager for Tektronix. He has a B.S. degree in chemical engineering from Washington State University and an M.B.A. degree from the University of Washington. Tektronix, P.O. Box 500, Beaverton, OR 97077-0001, (800) 479-4490.
Copyright 1996 Nelson Publishing Inc.
September 1996