ADSL integrated circuit parameter test

ADSL is a modem technology that makes full use of the unused resource capacity on ordinary telephone twisted pair lines. It uses an asymmetric transmission method, and the downstream speed from the central office (CO) to the remote end (RT) can be up to 4 times the upstream speed. This asymmetric performance is ideal for market-oriented consumer broadband applications such as video and Internet access, because in these applications the downstream data rate must be very high, while the upstream data from the user to the central office (CO) is generally less. This usage model is also suitable for business communications from corporate servers to employees, partners and customers.

Unlike analog modems, ADSL modems do not access the public switched telephone network (PSTN) and use advanced modulation technology. The signal frequency and data rate they send are much higher than those of analog modems. ADSL supports a maximum downstream rate of 8Mbps and a maximum upstream rate of 832kbps. However, as the signal transmission distance increases, the data transmission rate using this technology will also drop rapidly. For example, when the distance between the user end and the central office is less than 12,000 feet, the ADSL rate can maintain 8Mbps. When the distance increases to 18,000 feet, its rate can only reach 1.5Mbps.

ADSL uses the familiar frequency division multiplexing (FDM) method to provide broadband services while supporting the traditional "plain old telephone service" (POTS) network. The FDM method used by ADSL is mainly discrete multi-tone (DMT) modulation. The DMT modulation method divides the approximately 1.1MHz spectrum into 256 equally spaced sub-channels or tones, each sub-channel occupies 5.3125KHz. Each channel in the DMT spectrum operates as an independent channel and uses quadrature amplitude modulation (QAM) modulation to encode digital information.

In addition to data transmission, these channels can also be used for independent network management or performance testing. The lower frequency channels are not used for signal transmission and are generally reserved as protection bandwidth to avoid interference with traditional POTS equipment at the lower end of the spectrum. In the frequency bands adjacent to and higher than these protection channel frequencies, a small number of channels are allocated for upstream data transmission, and the remaining higher frequency channels are used for downstream data transmission. Like other modem technologies such as V.32 and V.34 modems, ADSL modems also need to use echo cancellation technology to solve the overlap problem of upstream and downstream channels. To provide telephone and data services at the same time, low-pass filters or splitters must be used to achieve separation.

Test Method

In order to improve the cost-effectiveness of ADSL, manufacturers need to provide equipment that can extend the distance between the central office and the user end, so as to reduce the number of terminal points and thus reduce the cost of laying fan-shaped user lines. In addition, the coverage performance of ADSL equipment is the most competitive factor. Longer telephone lines can cause up to 90dB attenuation of the high-end frequency band signals used by ADSL. Therefore, semiconductor manufacturers usually use analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) with higher dynamic range and lower noise indicators in the design of ADSL equipment.

In ADSL modems, the noise and linearity performance of the analog front end (AFE) is the key to achieving the ideal data rate on longer cables. The tight noise and linearity design margins often increase the testing challenges, because manufacturers need to provide ADSL test instruments with wider dynamic range and higher accuracy, but the testing cost should be less than or at least equal to the testing cost of the previous generation of low-performance ADSL equipment.

Manufacturers can use the single-tone test method to determine the pure dynamic range, standard distortion and noise floor level of ADSL. This direct test method is sufficient to quickly find various defects. The single-tone test method is more effective in testing the signal-to-noise ratio (SNR) of the device. Although the linearity indicators of ADSL converters are relatively strict, SNR is still an important device parameter required to ensure the correct operation of ADSL. Single-tone testing can also measure the total harmonic distortion (THD) and distortion-free dynamic range (SFDR) of the device.

These basic dynamic linearity tests require only a small amount of additional processing after calculating the SNR, so this test does not take up much test time overhead. That is, the test time requirements of single-tone testing are very modest relative to its efficiency. In addition, many industry-leading test systems provide pre-made routines to facilitate the development of these single-tone tests. Because the single-tone test method can test multiple key parameters with a single set of data, most defective devices will fail the single-tone test.

[page] Although static linearity testing is a traditional part of ADC specifications, it is not suitable for evaluating ADSL devices. The high conversion rate of ADSL ADCs is usually offset by high resolution. This requires capturing a large amount of data, which takes up a lot of DSP computing time. Another factor to consider is the high frequency signals used in ADSL devices. Static linearity testing can be very different from the dynamic response of the device.

Comparing the various tests for these devices, the most practical is still the basic loopback test. Engineers connect a transmitter to a receiver and check whether the encoded data sent at the output can be correctly decoded at the input. Although this highly practical test method does not provide the information necessary to isolate a faulty device in a complex design, it is very fast and minimally expensive to perform.

IMD Test

While traditional static linearity measurements such as integral nonlinearity (INL) and differential nonlinearity (DNL) are important, they are not sufficient to characterize the performance of an AFE. For ADSL equipment, the THD specification does not provide enough information about how the DUT nonlinearity affects the input signal. In multi-tone ADSL, the dynamic linear range across the bandwidth determines the capacity of the modem.

In nonlinear systems, many frequency components that do not exist in the input signal will appear in the output signal. The most common frequency components are harmonics. Harmonic signals can be easily estimated using the single-tone sine wave test method. Intermodulation products are another type of frequency component that can seriously affect signal fidelity.

Intermodulation distortion (IMD) occurs when the input signal contains multiple tones. The generated IMD is mathematically related to the input tone frequency and spreads across the entire spectrum (see Figure 1). Generally speaking, the third-order component has the greatest impact on signal fidelity. The DMT modulation scheme used in ADSL equipment utilizes multiple orthogonal signal components with regular spacing, so not only the third-order IMD, but also the fifth-order and even the seventh-order IMD needs to be minimized.

Testing IMD requires two pure single tones to be input to the DUT. Careful consideration must be given to the choice of single tones, the sampling rate of the DUT, and the number of samples to ensure that the necessary information is correctly extracted from the spectrum without overlapping frequency components. This does not necessarily correctly characterize integer-based mathematical performance characteristics; therefore, heuristic schemes or forced integer searches are often used. Advanced test platforms such as the ASL3000 provide instrumentation suites with high sampling rates, high signal fidelity, and clock flexibility, all of which are necessary for an acceptable IMD test configuration.

Multi-tone test

Among the several tests of ADSL, multi-tone power ratio (MTPR) measurement is the most challenging test. This method is mainly used to test the signal power at a certain frequency point (slot). At this time, the device's stimulus signal should consist of all single tones except the corresponding measured frequency point (Figure 2). All single tones at normal spacing will generate a large amount of IMD components at the idle frequency point. Since ADSL technology relies on independent transmission through each frequency point, this test method can effectively evaluate the performance of the device under test. In a complete MTPR test, all 256 frequency points need to measure such residual power.

Test plan

Tests such as TOI and MTPR can be easily implemented using advanced test platforms. Some advanced mixed-signal platforms such as Credence ASL 3000 provide tools that can generate and observe the correct waveforms required for these tests. Engineers can use internal DSP functions to automatically perform FFT tests or find the amplitude and phase of the signal and write C code, and use certain tools to generate and observe high-precision waveforms related to these tests. These advanced test systems can also provide extended resolution and the accuracy required to reliably complete sensitivity measurements of complex ADSL components.

The dynamic performance characteristics of ADCs and DACs in many ADSL devices require improved bandwidth and resolution of generators and quantizers. For example, the ADSL standard stipulates that the signal level in the MTPR test is only 1V rms. When such a signal is extended to 255 bins, any residual signal level measured in the target frequency bin is very low, so 16-bit resolution is usually required to ensure the correctness of the test.

In the past, 12-bit ADCs and DACs (72dB dynamic range) were considered suitable for DSL components. Although 14-bit ADCs and DACs are considered the most practical in high-volume production, it is obviously risky to use 16-bit ADCs and DACs in a production environment with a lot of electronic noise. In order to complete these tests, the electrical environment of the tester requires very low noise, which should be enough to support a dynamic range of more than 96dB. In addition, engineers must be able to generate signals and sample DUT signals with very low jitter, because in such a dynamic range, jitter will directly convert into equivalent noise on the tested signal.

[page] Another benefit of the Credence ASL 3000 test platform is its ability to test the transmit portion of an ADSL device. In addition to its inherent sensitivity measurements, the ASL 3000 can also pass the transmit signal through a notch filter to further improve the dynamic range for a particular sensitivity measurement. Other key measurements such as distortion-free dynamic range can also be made in the same way. Another benefit of the test routines pre-installed in this test instrument is that the test procedure can be completed quickly.

The ASL 3000 test platform simplifies complex tests such as MTPR. ASL 3000 provides a DSP library with simplified test routines. In fact, this DSP library is the same as the library provided by Credence Quartet test system, which simplifies the migration between different platforms. In the past, it was very difficult to generate the tones with the dynamic range required for ADSL testing, but now the emergence of advanced waveform generator equipment has made it easier. The instrumentation used for multi-tone testing is more complex than other tests. Filters cannot be used in multi-tone testing, so engineers usually need to carefully balance the relationship between accuracy and test time.

IMD tests are generally more complex to set up than other tests. Fortunately, these IMD tests only add a small amount of additional test time, but the high-quality test results make it worthwhile. This type of test helps engineers better understand the nonlinearity of the device and better improve actual performance.

In fact, in a recent demonstration of Credence's testing methods, five components and other devices were provided to Credence engineers and all passed the test. When the engineers tested with the more advanced methods described above, they found that one of the components was actually defective. Generally speaking, discovering the defect later may not have much impact, but if the product component is sold through normal channels, it will affect the user's perception of brand quality.

Integrated equipment

The ASL 3000 test platform can test complex modulation signals used in ADSL devices. Its high performance and high test accuracy are very suitable for testing high-performance ADSL components, especially in high-throughput multi-point configurations. For complex mixed-signal devices with integrated analog front ends and a large number of digital pins, test engineers can usually choose test systems such as Quartet, SZ M3650 or Octet.

Quartet can meet the low jitter measurement requirements of mixed-signal devices with a large number of digital pins and analog front ends and requiring a large dynamic range. The SZ M3650 is more suitable for testing designs with more digital pins, because in this case, great flexibility is needed in providing test signals. There is also Octet, which supports high-throughput parallel testing of mixed-signal devices with both important digital content and embedded DSL elements. In addition to these productized test platforms, engineering verification systems such as IMS Gemini MS can also provide some special functions required to speed up chip debugging of complex ADSL devices.

The advanced technology used in the advanced mixed signal tester provides many dedicated resources to meet the complex single-tone, multi-tone and IMD test requirements required for multi-point testing of complex ADSL devices. Taking full advantage of these rich test methods, engineers can extract important information from faulty devices to better understand performance issues and factors affecting yield.

In other words, engineers can focus more on the special areas they care about, learn from each other's strengths and weaknesses, and enhance their development capabilities to provide ADSL products with higher performance and quality. By using a test system with good cost performance to perform relatively small amounts of complex tests, ADSL manufacturers can achieve maximum throughput performance and lower testing costs, thereby fully meeting the needs of the rapidly growing ADSL market.