Figure 1: E1/T1 systems feature separate Tx and Rx cables with the ability to switch each to a redundant, standby electronics board.

In a typical E1/T1 system, separate coaxial or twisted-pair telecommunications cables connect to the receiver and transmitter boards. In case of a failure, the system maintains operation by quickly switching to a standby protection board.

Figure 2: A protection stage, transformer stage, and switching stage are common to each E1/T1 line.

Upon looking at the transmitter or receiver path in greater detail, the signal line first enters the primary-protection stage, which includes voltage-limiting devices such as transient-voltage suppressors (TVS) or gas-discharge tubes. The second stage is a transformer, which provides isolation, impedance matching, and any signal-level adjustment necessary to meet the mask shape specified for E1/T1 transmission.

Figure 3: The transformer in an E1/T1 line helps to form the output pulses per E1/T1 specifications.

Beyond the transformer are line drivers and receivers followed by the digital electronics necessary for data communications. These components operate at 5 V and below and cannot tolerate any overvoltage. Schottky diodes are placed between these components and the second stage with connections arranged to clamp any excessive voltage to the positive or negative supply rail.

With respect to reliability, the weak link in these signal paths is the transmission boards. The first stage is generally located close to the Tx and Rx connectors to eliminate excess voltage immediately and prevent its coupling to other parts of the system. The second transformer stage is highly reliable. Therefore, the switches must be placed between the electronics board and the transformer. They can be electromechanical relays or solid-state analog switches.

Relays have been in use for years, providing contacts that connect to the main board in one position and to the protection board in the other. Disadvantages include the amount of space on the board, because some boards have up to 24 protection lines, and power dissipation. Power required by a single relay is not great, but when multiplied by the N lines in a large telecom system, it becomes a substantial amount to generate and dissipate.

The second option, analog switches, is relatively new. Devices such as the MAX314 and MAX4606 offer the low on-resistance and low-parasitic capacitance necessary to manage E1/T1 data rates without a significant insertion loss. Placed after the transformer, where the signal is already clamped to the bus voltage, they need not withstand hundreds of volts. They are controlled by a simple logic-level signal and draw almost zero current. The MTBF of an analog switch, because it is a semiconductor with no mechanical parts, is comparable with or better than any electromechanical relay.

To work, an analog switch requires a polarization supply voltage larger than the absolute maximum rating of the signal it handles. For E1/T1 signals, ±5-V supplies are sufficient. Supply current for the MAX314 is in the 1-µA range, so its negative supply rail is easily generated with a simple charge-pump converter such as the MAX871.

The MAX871 comes in a SOT23 and requires an external ceramic capacitor to derive -5 V from +5 V. These dual rails are essential for switch operation, so a redundant supply is necessary in case of failure. For the -5-V rail, two MAX871s decoupled by an output diode are sufficient.

The prospect for reducing electromechanical-relay dimensions is not great, but the future of analog switches is promising. Maxim has already reduced the channel resistance in its MAX4661 and the footprint in the MAX4624—a SPDT device in a SOT23 package.

Maxim analog switches already withstand higher voltages than those specified for E1/T1 systems. The MAX314's working voltage of up to ±20 V also suits it for protection boards in xDSL transmission systems, which are similar to E1/T1 systems.