LXT318 long-haul e1 transceiver line interface design guide application note january 2001 order number: 249152-001 as of january 15, 2001, this document replaces the level one document known as an036 .
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application note 3 LXT318 long-haul e1 transceiver line interface design guide contents 1.0 general description .................................................................................................. 5 1.1 transmit line interface.......................................................................................... 5 1.1.1 twisted-pair cable (120 ? ) ...................................................................... 5 1.1.2 coaxial cable (75 ? ) ................................................................................ 5 1.2 circuit modifications .............................................................................................. 5 1.3 receive line interface...........................................................................................6 1.3.1 twisted-pair cable (120 w) ..................................................................... 6 1.3.2 coaxial cable (75 w) ............................................................................... 6 figures 1 receive line interface diagram ............................................................................ 6
LXT318 long-haul e1 transceiver line interface design guide application note 5 1.0 general description this application note describes a standard line interface for the LXT318 transceiver in a long-haul e1 environment. the interface uses two pulse engineering transformers designed specifically for this transceiver. the design is simple and robust and will reduce in-house engineering and testing time. refer to the current LXT318 data sheet for complete information on the LXT318 transceiver. 1.1 transmit line interface the transmit line interface uses the following parts: 1. pulse engineering transformer, p/n pe68646 - 1:1.58:2 (one per transceiver) 2. 15 ? resistors (two per transceiver) connect them to the transmit tip and ring inputs (pins 13 and 16) of the transceiver as shown above. 1.1.1 twisted-pair cable (120 ? ) for a twisted-pair cable, use the full 1:2 step-up ratio provided at the end tap. this gives the required 120 ? impedance. z = ? x (step-up ratio) 2 = (l5 ? + l5 ? ) x 2 2 = 120 ? 1.1.2 coaxial cable (75 ? ) for a coaxial cable, connect the circuit at the transformer center tap. the 1:1.58 step-up ratio gives a value within 0.2% of the 75 ? impedance required. z = ? x (step-up ratio) 2 = (l5 ? + l5 ? ) x 1.58 2 = 74.89 ? 1.2 circuit modifications ? provides correct impedance match for both 75 ? coaxial and 120 ? twisted-pair cables meets 3 kv isolation requirement of many ptt ministries standard interface requires one-time design laboratory engineered and proven design requires minimal in-house testing readily available parts make building fast and easy
LXT318 long-haul e1 transceiver line interface design guide 6 application note 1.3 receive line interface the receive line interface uses the following parts: 1. pulse engineering transformer, p/n pe65839 - 1:1:1.26 (one per transceiver) 2. 120 ? resistors (one per transceiver) connect them to the receive tip and ring inputs (pins 19 and 20) of the transceiver as shown above. 1.3.1 twisted-pair cable (120 ? ) for a twisted-pair cable, use the 1:1 step-down ratio provided at the center tap. this gives the required 120 ? impedance 1.3.2 coaxial cable (75 ? ) for a coaxial cable, connect the circuit at the transformer center tap. the 1:1.26 step-down ratio gives a value within 1% of the 75 ? impedance required. figure 1. receive line interface diagram 1.0 19 LXT318 20 rtip rring 1.0 for 120 ? twp 1.26 for 75 ? coax p/n pe-65839 e1 receive input from line 120 ? trx z = ? = 120 ? = 120 ? (set-down ratio) 2 (1) 2 z = ? = 120 ? = 75.59 ? (set-down ratio) 2 (1.26) 2
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