TUTORIALS 784

Solid-state analog switches can now replace the traditional electromechanical relays in E1 and T1 telecommunication systems.

The E1 (European) and T1 (North American) telephony standards govern high-rate voice and data transmissions either directly to the end customer (access) or between central offices (transport). Because a failure in either case is unacceptable, these systems include a local battery backup, redundant power supplies, and 1+1 or 1+N redundancy for the signal-processing boards (one standby for every working board, or one standby for every N identical working boards).

In a typical E1-T1 system, separate coaxial or twisted-pair telecommunications cables connect to the receiver and transmitter board (see Figure 1). In case of a failure, the system maintains operation by quickly switching to a standby "protection board".

Figure 1. E1-T1 systems feature separate Tx and Rx cables, with capability on each for switching to a redundant, standby electronics board.

The diagram in Figure 2 shows 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).

Figure 2. Common to each E1-T1 line is a protection stage, a transformer stage, and a switching stage.

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 5V and below, and cannot tolerate any overvoltage. Schottky diodes are therefore placed between these components and the second stage, with connections arranged to clamp any excessive voltage to the positive or negative supply rail.

In terms of 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, so 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 space on the board (some boards have up to 24 protection lines or more), 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 to dissipate.

The second option (analog switches) is relatively new. Devices like 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 drawing almost zero current. Furthermore, the MTBF of an analog switch (a semiconductor with no mechanical parts) is comparable to 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,therefore, ±5V supplies are sufficient. Supply current for the MAX314 is in the 1µA range, so its negative supply rail is easily generated with a simple charge-pump converter like the MAX871. The MAX871 comes in a SOT23 package and requires only one external ceramic capacitor to derive –5V from +5V. These dual rails are essential for switch operation, so a redundant supply is necessary in case of failure. For the –5V rail, two MAX871s decoupled by an output diode is 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 (MAX4661) and the footprint (MAX4624, a SPDT device in a SOT23 package). Maxim analog switches withstand higher voltages than those specified for E1-T1 systems. The MAX314 working voltage (up to ±20V) also makes it attractive for use with protection boards in XDSL transmission systems, which are similar to the E1-T1 system described above.

 

A209, August 2001