The Challenge of Testing ADSL Modems

One of the major test challenges facing today’s PCB manufacturers is the need to achieve high throughput of very dense boards populated with increasing numbers of mixed-signal devices. Test techniques that sufficed only a few years ago no longer can do the job.

For example, in telecommunications, manufacturers and service providers seek new ways to supply high-speed data transmission to homes and businesses, even as product designs challenge the capability of traditional test solutions to keep pace. Market-generated demand also exerts enormous pressure on manufacturers to ship large numbers of reliable products at reasonable prices, burdening the production process even more.

Consider Asymmetric Digital Subscriber Line (ADSL) technology, the most likely short-term successor to analog and Integrated Services Digital Network (ISDN) modems for data transfer. ADSL provides an impressive 6 to 8 Mb/s downstream—sufficient for full-motion video and other data-intensive applications—over the existing worldwide network of 700 million copper telephone lines.

The upstream speed of only 640 kb/s, still five times as fast as ISDN, provides plenty of capacity for two-way teleconferencing, high-speed Internet surfing, and local area network access. The only major infrastructure change necessary to initiate an ADSL connection is a special modem at each end of the local loop to modulate and demodulate the data.

Faster Than It Looks

For more than 60 years, the voice band of a copper twisted-pair local telephone line for plain old telephone service (POTS) has remained in the range of 300 Hz to 3.4 kHz, with a maximum signal-to-noise ratio of 30 dB. Because conventional modems mimic voice transmissions, they do not carry data elements, known as symbols, any faster. If a carrier signal contains a single bit of information—a phase shift from 0° to 180° signifying a shift from 0 to 1, for example—data transmission cannot exceed 3.4 kb/s.

However, clever modulation schemes permit encoding more than one data bit per symbol. For example, a signal that can attain four states phase-shifted by 90° represents 2 bits, effectively doubling transmission speed to 6.8 kb/s. Similarly, sending 4 bits per symbol requires eight states 45° out of phase, increasing the effective speed to 13.6 kb/s.

An extension of these methods, quadrature-amplitude modulation, encodes the data by varying both the phase and the amplitude of the carriers. Using such techniques, conventional analog-modem data rates have skyrocketed from the 300- to 1,200-b/s range common only a few years ago to today’s top speeds of up to 56 kb/s.

To reach megabit-per-second speeds without compromising signal reliability, ADSL approaches the problem differently. Although the voice band of a telephone line is confined to a bandwidth of 3.4 kHz, the entire line provides more than 1 MHz. A low-pass filter at the line-card input in the telephone company’s central office (CO) produces the voice band.

Conventional modems must operate in the voice band because the system routes the signal through the public switching telephone network (PSTN). ADSL modems bypass the PSTN completely. An ADSL modem in the home connects to a matching one in the CO. The data traffic then is sent through the CO’s ADSL modem to the data network. By connecting the remote ADSL modem to the CO modem, the entire bandwidth of the twisted-pair wire is used.

Unlike ISDN, ADSL is a pass-band system operating between 30 kHz and 1.1 MHz. This architecture leaves room at the low end of the frequency band for conventional voice transmission. Passive filters called POTS splitters separate the voice band from the ADSL band, so both types of signals can coexist on a single line.

ADSL Modem Design


Figure 1 shows a typical ADSL architecture, including the ADSL Transceiver Unit-Central Office (ATU-C) and the user-premise (ATU-R) modems. Downstream, the network sends data

to the CO modem (ATU-C) through an ATM (cell-based) or Ethernet (packet-based) interface. At the CO, the signal goes through a digital-to-analog converter (DAC) and a transmission filter to remove unwanted frequencies, especially any signal elements below 30 kHz and in the upstream band.

Parametric Test

Reference T1.413

(issue 2)


Manufacturing Faults Found


Test Mode Recommended or Required


Return Loss (30 kHz to 1,100 kHz, four frequencies)



Termination faults (output amplifiers and filters)

Quiet Mode

Longitudinal Balance (20 kHz to 1,100 kHz, four frequencies)

10.3.2 (12.3.2)

Transformer and protection circuit parasitic ground impedances

Quiet Mode

Transmitter Dynamic Range (noise/distortion floor)

6.11.2 7.11.2 (6.13.2, 7.13.2)

DAC, filters, AGC, output amplifiers, and transformer

REVERB 1 (initialization signal without cyclic prefix)

Transmitter Spectral Response, Transmit Power Spectral Density and Aggregate Power

6.12 6.13.1 7.12 7.13.1 (6.14, 7.14)

Passband flatness of the DAC, filters, AGC, output amplifiers, filtering of out-of-band spurious signals, and power cutback/boost functionality


Multitone Power Ratio (transmitter)

(6.13.2, 7.13.2)

Verifies operation of DAC, filters, AGC, output amplifiers, and transformer

REVERB 1 and additional test mode with missing tones

Multitone Power Ratio (receiver)

(6.13.2, 7.13.2)

AGC, receiver filter, and receiver linearity

REVERB 1 on test modem or test system instrumentation Access to the signal

Receiver Spectral Response, Receiver Sensitivity


Receiver filter, AGC, and hybrid

REVERB 1 on test modem or test system instrumentation Access to the signal

Steady-State Noise (idle channel noise)


(12.2.2, 6.14.3, 7.14.3)

Verify POTS splitter and ADSL signal power spectral density prevent interference with voice POTS, finds noise coupling faults via parasitic elements


Copyright 1998 Nelson Publishing Inc.

October 1998

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