ADSL is a modem technology that makes full use of the unused resource capacity on the ordinary telephone twisted pair. It uses asymmetric transmission, and the downlink speed from the central office (CO) to the remote (RT) can reach up to 4 times the uplink speed. . This asymmetric performance is ideal for market-oriented consumer broadband applications such as video and Internet access, where the downstream data rate must be very high and the upstream data from the user to the central office (CO) Generally less. This usage model also applies to business communications from company servers to employees, partners, and customers. Unlike analog modems, ADSL modems do not enter the Public Switched Telephone Network (PSTN), and use advanced modulation techniques that transmit signal frequencies and data rates much higher than analog modems. ADSL supports downlink rates up to 8 Mbps and uplink rates up to 832 kbps. 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 client and the central office is less than 12,000 feet, the rate of ADSL can be maintained at 8 Mbps, and when the distance is increased to 18,000 feet, the rate can only reach 1.5 Mbps. ADSL uses the familiar frequency division multiplexing (FDM) approach to provide broadband services while supporting traditional "normal telephone service" (POTS) networks. The FDM method adopted by ADSL is mainly discrete multitone (DMT) modulation. The DMT modulation method divides the spectrum of approximately 1.1 MHz into 256 equally spaced subchannels or tones, each of which accounts for 5.3125 KHz. Each channel operates in a separate channel mode in the DMT spectrum and uses a quadrature amplitude modulation (QAM) modulation method to encode digital information. In addition to data transmission, these channels can be used for independent network management or performance testing. Lower frequency channels are not used to transmit signals and are typically reserved for protection bandwidth to avoid interference with legacy POTS equipment at the low end of the spectrum. In the frequency band immediately adjacent to and above these guard channel frequencies, a small number of channels for uplink data transmission are allocated, and the remaining higher frequency channels are used for downlink data transmission. Like other modem technologies, such as V.32 and V.34 modems, ADSL modems also require echo cancellation techniques to resolve the overlap of upstream and downstream channels. To provide both telephone and data services, a low-pass filter or splitter must be relied upon to achieve separation. Test Methods In order to improve the price/performance ratio of ADSL, manufacturers need to provide equipment that can extend the distance between the central office and the customer end, thus reducing the number of terminal points and reducing the cost of laying the fan-shaped subscriber line. In addition, the coverage performance of ADSL equipment is one of the most competitive factors. Longer telephone lines can cause up to 90 dB of attenuation for the high-end band signals used by ADSL. As a result, semiconductor manufacturers typically use analog-to-digital converters (ADCs) and digital-to-analog converters (DACs) with higher dynamic range and lower noise specifications in the design of ADSL devices. In ADSL modems, the noise and linearity of the analog front end (AFE) is key to the ability of an ADSL modem to achieve an ideal data rate over a long cable. Unsatisfactory noise and linear design margins tend to increase the challenge of testing because manufacturers need to provide ADSL test instruments with wider dynamic range and higher accuracy, in which the test cost is less than or at least equal to the previous generation of low-performance ADSL. The cost of testing the equipment. Manufacturers can use the tone test method to determine the pure dynamic range, standard distortion, and noise floor level of ADSL. This direct test method is sufficient for quickly identifying various defects. The tone test method is effective for the signal-to-noise ratio (SNR) of the test equipment. Although the linearity of ADSL converters is strict, SNR is still an important device parameter necessary to ensure proper operation of ADSL. The tone test also measures the overall harmonic distortion (THD) and distortion-free dynamic range (SFDR) of the device. These basic dynamic linear tests require only a small amount of additional processing after calculating the SNR, so this test does not cost much test time overhead. In other words, the test time requirement for a tone test is very modest relative to its efficiency. In addition, many industry-leading test systems provide pre-made routines to facilitate the development of these tone tests. Since the tone test method can test multiple key parameters with a single set of data, most of the defective devices do not pass the tone test. Although static linear testing is a traditional part of the ADC specification, it is not well suited for device evaluation for ADSL devices. The high slew rate of an ADSL ADC is usually offset by high resolution. At this time, a large amount of data needs to be captured, which takes up a lot of DSP operation time. Another factor to consider is the high frequency signal used in ADSL equipment. The static linear test is very different from the dynamic response of the device. Comparing the various tests of such devices, it can be found that the most practical effect is still the basic loopback test. The engineer connects the transmitter to the receiver and then checks if the encoded data sent by the output is correctly decoded at the input. While this highly practical test method does not provide the information necessary to isolate a faulty device in a complex design, it performs very quickly and at the lowest cost. IMD test Although traditional static linear measurement methods such as integral nonlinearity (INL) and differential nonlinearity (DNL) are very important, they are not sufficient to characterize the performance of AFE. For ADSL devices, the THD specification does not provide enough information about how DUT nonlinearity affects the input signal. In multi-tone ADSL, the dynamic linear range of the entire bandwidth determines the capacity of the modem. In a nonlinear system, many frequency components that do not exist in the input signal appear in the output signal, the most common of which are harmonics. Harmonic signals can be easily estimated using the monophonic sine wave test method. The intermodulation component is another frequency component that can seriously affect signal fidelity.
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