[Design View / Design Solution]
Prevent ADSL Modems From Getting Zapped
Familiarity with the standards, guidelines, and devices strengthens your ability to protect an ADSL modem's telephone and data lines from overvoltage surges.
Major regulation authorities have set specific standards or test recommendations for qualifying telecom equipment, including central-office and customer-premises ADSL modems. Such standards depend on the type of equipment, its location in the network, and the network's geographical region. Yet a parallel element, particularly one with "crowbar" characteristics, supplies the necessary protection against lightning strikes. A crowbar device can short-circuit the twin-pair telephone lines to dissipate the overvoltage with minimal heat.
Protection standards also address overvoltages, which occur when an ac power line makes electrical contact with the telephone lines. To block such overcurrents, the protection circuit typically combines the parallel crowbar device with a voltage-clamping device, and possibly a positive-temperature-coefficient (PTC) resistor, in series with each telephone line.
This article details the various standards with which designers must become familiar. The more familiar they are, the better armed they are to protect an ADSL modem's telephone and data lines from overvoltage surges. Four tables list the main protection standards associated with specific telecom equipment, both in the U.S. and in Europe and Asia.
Also described are the various devices used in protection schemes, such as the aforementioned crowbar and clamping devices that help suppress damaging overvoltages.
HIGHLIGHTS:
Protecting The Line Interface
To cover the entire ADSL modem system, protective circuitry starts at the line interface and is usually placed between the telephone lines and splitter. There are three key parameters when selecting the right protection components: standoff voltage, surge current, and capacitance.
Data-Line Interface
When it comes to the data line, the three most common interfaces are the Universal Serial Bus (USB), Ethernet, and the Peripheral Component Interface (PCI). USB and Ethernet connect to external equipment; PCI connects to internal plug-in boards.
Sidebar: The ABCs Of ADSL Modems
By using advanced modulation techniques, ADSL service can move digital data at up to 8 Mbits/s continuously over conventional phone lines while allowing voice communication.
Tables: Protection Standards
The main protection standards used in the U.S. and Europe/Asia are broken down for telecom in general, and specifically for central-office and customer-premises equipment, and Ethernet. Lightning and power contact are highlighted.
Because they connect to ordinary phone lines, asymmetrical-digital-subscriber-line (ADSL) modems are subject to electrical hazards like lightning, power-line crossings, and electrostatic discharge (ESD). As a result, they must be adequately protected against damaging overvoltages and overcurrents.
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