All manufacturers value a test environment that minimizes waste and delay by connecting or bridging inefficient and isolated in-circuit and functional board test islands. A test island is incompatible, in both hardware and software, with other test installations.
In today’s manufacturing environment of shrinking product life cycles, steep ramps to volume, and complex technology packed onto denser boards, barriers of incompatibility can severely limit test strategy options. Manufacturers must be able to respond quickly and efficiently to rapidly changing process and functional test requirements and not be constrained by the consequences of past hardware or software decisions. This is especially true where new changing technologies are inserted into manufacturing lines and challenge existing test processes to accommodate them.
VXI is the foundation of the modern ATE solution. It can accommodate today’s requirements and provide the building blocks for a bridge to tomorrow’s technology while providing the common core for a wide range of test-system solutions.
VXI-Based Hardware
VXI instrumentation alone is not enough. A connections building block must be able to switch hybrid analog and digital signals across a wide number of test-system channels and at the same time provide excellent signal integrity for high-performance instrumentation applications. A hardware bridge connecting islands of test must allow addition or removal of OEM instruments as production requirements dictate.
The instrumentation can include timers/counters, arbitrary waveform generators, and high-speed formatted digital I/O. Further extensions allow implementations of application-specific solutions, such as Asymmetrical Digital Subscriber Line (ADSL) test.
Testing for Tomorrow’s ADSL Technology
ADSL is one of several emerging technologies that offer a low-cost path for high-speed internet access by using the existing copper lines running from the local telephone central office to the client’s home or business. A VXI-based ADSL test platform is able to provide in-circuit, functional-board and system-level test for ADSL modems, ADSL line cards, and DSL Access Multiplexer (DSLAM) units (Figure 1). As ADSL technology matures, a VXI architecture provides test scalability and the versatility needed to adapt to the unforeseen directions in which the technology may evolve.
Two types of functional tests usually are performed. Parametric testing validates certain data or interface specifications and faults (Figure 2). Operational testing validates normal operating capabilities and faults.
In ADSL modems, functional test error sources arise from errors in the digital sections, the DSP and analog-to-digital (A/D) and digital-to-analog (D/A) conversion sections, and the analog/interface sections. Multiple test strategies may be implemented during the ADSL product life cycle including structural test, ADSL conformance test, ADSL transmission test, and ADSL performance test.
Modem Training
In ADSL modems, digital processes detect the signal coding of the analog stream while operating well below performance maximums. The fundamental limitations of discrete multitone (DMT) ADSL technology are those imposed by the analog side processing. Analog side processing is the step by which a telephone signal is converted into digital bits.
The analog circuitry in question comprises the analog I/O section of both modems (send and receive) and the network electrical properties of the telephone line connecting them. Modem training is the process by which two modems link and establish a data channel. For example, when you dial a modem from your computer and you hear a whirl of activity coming from your speaker, you are listening to the process of modem training.
DMT ADSL modems naturally compensate for defects and impairments in the analog domain via training, reducing bit rates, eliminating DMT bins that are nonfunctional, and using built-in error correction. However, training also masks similar symptoms caused by improper analog board assembly and analog integration errors.
BERT
The challenge facing ADSL manufacturers is measuring the actual delivered performance of the unit under test (UUT). Conventional testing of communications channel quality employs Bit Error Rate Testing (BERT), the measure of the ratio of defective bits passed to total bits representing the quality of the network.
ADSL presents two problems for BERT testing. First, the analog sources of defective ADSL bins are masked during training, making BERT a measure of the effective error rate. Second, BERT uses a trained data pipe consisting of two modems and a physical link and, as a result, is not a measure of the specific modem under test. Finally, with DMT ADSL’s many bins, BERT is unable to resolve bit error to a specific bin. Consequently, conventional testing via BERT is inadequate to detect subtle functional operational errors for DMT ADSL unless the errors are sufficiently gross as to substantially affect the basic data rate.
Performance Testing for ADSL
VXI’s standards-based interface allows ATE manufacturers to develop more specialized instrumentation targeted at application specific test. For example, ADSL manufacturers who want to ensure the performance of their product during production can verify maximum data rates using signal-to-noise ratio (SNR) measurements with the Missing Tone Test technique. Based on exclusive semiconductor test technology, a missing tone test can be implemented as a VXI hardware/software solution. The missing tone test assures correlation of a rapid functional test result with industry BERT test specifications and is unique in ADSL application-specific test.
Missing Tone Test
A missing tone test used at the circuit board level represents the fastest and most accurate means to establish the proper operation of DMT ADSL modems. DMT ADSL modem encoding schemes make it theoretically possible to transmit data up to 15 Mb/s over standard gauge twisted-pair telephone lines. More practical estimates by current DMT ADSL hardware vendors range from 6 to 8 Mb/s.
A missing tone test is not affected by modem training results, bit rate reductions, marginally performing DMT bins, or digital data correction. It provides the most accurate view of the performance of the modem because it is a direct measurement of the relevant parameter controlling bit error rate, rather than an inferred measurement. A VXI-based ADSL functional test system can execute the missing tone test concurrently across multiple ADSL circuits, in seconds, making it ideal to test up to several hundred boards per hour.
The missing tone test is based on the measurement of noise in a missing tone (one of the tones in the DMT spectrum is turned off while all others remain on) relative to the signal levels of the tones that are present (Figure 3).
The key limitation of the modem comprising 200+ individual single-carrier modems spread out over 1.1 MHz, is nonlinearity or distortion in the analog front end (AFE). The AFE circuitry section is made up of amplifiers, A/D, D/A, receivers and other circuitry that drive and receive the analog signals. Nonlinearity and distortion in the circuit directly correlate to decreased performance and increased BERT of the DMT ADSL modem.
With simple tones, such as sine waves, nonlinearity shows up at discrete frequencies in the form of harmonics. In the case of ADSL with its spectrally rich bandwidth, nonlinearity and distortion in the AFE have the effect of producing additional noise floor and thus increased BER, this time spread out over the entire 1.1-MHz spectrum.
Nonlinearity and distortion have the effect of reducing the signal-to-noise ratio in each of the 200+ modem bands. It is expected that nonlinearity (and not signal- based noise) is the performance limiting factor for DMT ADSL signal transmission.
The missing tone test intentionally creates a null in the DMT spectrum by turning off one of the active tones while leaving all others active. The notch represented by the missing tone exposes the residual noise caused by the distortion and nonlinearity of the AFE. This notch then facilitates a direct measurement of the noise floor of the AFE, caused by its intermodulation characteristics, which is the limiting performance factor.
Therefore, the relative amplitude of the distortion-created tone in the notch, compared to the signal levels of on tones, is the key measurement of actual modem performance and thus data rate, and is made directly during the missing tone test. The missing tone test saves manufacturers time by eliminating the conventional challenges that modem training and BER present.
A VXI Bridge for ADSL
The VXI format has advantages for ADSL manufacturers who want to use the missing tone test. First, it is an ideal technique—fast, accurate, repeatable, and easily expressed as a VXI-based solution. Secondly, the VXI format and related software provides the ADSL manufacturer with a powerful solution for debug, development, and test maintenance. Thirdly, the VXI format is ideal to implement concurrent missing tone tests—for parallel testing—providing for very high test throughput and testing of multiple lines.
Isolated board test islands and specialized-function testers usually do not adapt to product design changes efficiently. ADSL manufacturers are challenged with shorter product life cycles and purchasing test equipment for a technology that is still evolving.
A hardware backplane consisting of VXI instrumentation and related software allows for a scalable application-specific test solution to be put where it is needed and when it is needed in the production line. Innovative performance testing such as the missing tone test is just one example of the test flexibility available to ADSL manufacturers when using a VXI-based open architecture. VXI plays an integral role in connecting islands of board test and bridging manufacturers to testing tomorrow’s technology.
About the Author
John Arena is the commercial functional test product manager for the Teradyne Assembly Division. He received a B.S. in EE from Rensselaer Polytechnic and an M.B.A. from Boston University. Mr. Arena has been employed by Teradyne since 1979. Teradyne, 600 Riverpark Dr., North Reading, MA 01864, (978) 370-1000.
Copyright 1999 Nelson Publishing Inc.
September 1999