Testing and process monitoring for a multifunction synthesizer - with reference to the HP 8904A Multifunction Synthesizer - includes related article on reliability of the HP 8904A - technical

Hewlett-Packard Journal, Feb, 1989 by David J. Schwartz, Alan L. McCormick

Testing and Process Monitoring for a Multifunction Synthesizer

THE TEST STRATEGY FOR THE HP 8904A required a twofold solution. The first step was to understand the instrument and its performance characteristics. Because of its state-of-the-art design, there are no reliable, fast, and automated means of directly measuring some of its critical parameters. Also, its versatility makes it impossible to measure even a significant fraction of the waveforms it is capable of producing. Understanding the instrument at this level made it possible for us to find a concise set of parameters that can be measured accurately and quickly to verify that the unit under test is working correctly, and allowed us to make small design changes that enhanced testability.

The second step of the solution was to develop a test strategy for the HP 8904A that emphasized process control, rather than intense end-of-the-line testing. By testing critical parts and modules and then verifying the assembly process, the performance of the unit under test is assured quickly and inexpensively. Fault isolation and repair are also easier because problems are identified earlier in the production process.

The production test strategy for the HP 8904A takes advantage of the instrument's straightforward block diagram and the capabilities of the HP 3065 Automated Board Test System, on which all testing is done. The output board and the digital board determine the performance of the HP 8904A. The interface between these two is clearly defined and simple to characterize. The HP 3065 gave us built-in process control routines and freedom from making numerous connections during the test process. Fig. 1 shows a block diagram of the production flow for the HP 8904A.

Two primary functions in the HP 8904A need to be tested: the generation of correct digital data and the conversion of this digital data to an analog waveform. The first function is performed entirely on the digital board. The second is done entirely on the output board. After each of these is verified, what remains is to ensure that they are connected correctly.

DWSIC and Digital Board Test

Testing of the DWSIC is done in two phases. The primary testing is done at HP's Colorado Integrated Circuit Division where the IC is tested at the functional block level. The second phase of testing is an incoming inspection, which is a simple go/no-go test intended to catch catastrophic part failures. Incoming inspection also provides us with a basis for monitoring our process to ensure that we are not damaging parts in handling them. For this test the device under test is installed in an HP 8904A test bed and instructed to produce a variety of waveforms chosen to exercize as much of the chip as possible within a reasonable amount of test time. The resulting digital data is captured by a logic analyzer and compared with stored good data.

In the remainder of the digital board testing, logic and microprocessor components are used well within their performance limits. The board's performance is verified by conventional HP 3065 testing, the HP 8904A power-up self-check, and the ability of the board to pass the final check at the end of the line.

Output Board Test

Testing the analog output board begins with conventional HP 3065 in-circuit testing. Following in-circuit testing, a special test fixture equipped with coaxial probes is used on the HP 3065 to calibrate and verify the board under test. The tests are designed to verify the ability of the output board to convert any arbitrary stream of digital data into an analog waveform. Thus, one set of tests verifies the ability of the output board to reproduce all of the myriad waveforms that the DWSIC can generate. The output board test accomplishes this by measuring five primary characteristics: level accuracy, flatness as a function of frequency, amplitude linearity, noise and spurs, and delay distortion. From these five characteristics, the ability of the output board to reproduce a waveform can be predicted.

The biggest difficulty in making these measurements was to get the necessary accuracy. The test's measurement uncertainty had to be only a few hundredths of a percent out to 100 kHz for us to verify the amplitude flatness. At the same time, the test had to be fast enough and fully automated to meet our cost goals. This was achieved by making two-port transfer function measurements of amplitude and delay flatness with a network analyzer, using a high-speed reference DAC as a source driving the node between the HP 8904A's DAC and track-and-hold gate. The measurement is calibrated against a reference two-port network and a short length of 50-ohm coaxial cable, and gets its absolute reference from one-port voltage measurements. A separate two-port measurement is made to align the anti-aliasing filter. Two analyzers are used, the HP 3562A for frequencies below 100 kHz and the HP 3577A for those above 100 kHz. The test setup for the output board test is shown in Fig. 2.