NRND

TP3404

SNOS703 – DECEMBER 2004

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FUNCTIONAL DESCRIPTION

The QDASL contains 4 transceivers, each of which can interoperate with any of the TP340X family of single-

channel DASL transceivers. Each QDASL transceiver has its own independent line transmit and receive section,

timing recovery circuit, scrambler/descrambler and loop activation controller. Functions which are shared by the 4

transceivers include the Microwire control port and the digital interface with time-slot assignment.

BURST MODE OPERATION

For full-duplex operation over a single twisted-pair, burst mode timing is used, with the QDASL end of each line

acting as the loop timing master, and the DASL at the terminal being the timing slave (the QDASL transceivers

cannot operate in loop timing slave mode).

Each burst within a DASL line is initiated by the QDASL Master transmitting a start bit, for burst framing, followed

by the B1, B2 and D channel data from 2 consecutive 8 kHz frames, combined in the format shown in Figure 2.

During transmit bursts the receiver input for that channel is inhibited to avoid disturbing the adaptive circuits. The

slave's receiver is enabled at this time and it synchronizes to the start bit of the burst, which is always an

unscrambled “1” (of the opposite polarity to the last “1” sent in the previous burst). When the slave detects that

36 bits following the start bit have been received, it disables the received input, waits 6 line symbol periods to

match the other end settling guard time, and then begins to transmit its burst back towards the master, which by

this time has enabled its receiver input. The burst repetition rate is thus 4 kHz.

LINE TRANSMIT SECTIONS

Alternate Mark Inversion (AMI) line coding, in which binary “1”s are alternately transmitted as a positive pulse

then a negative pulse, is used on each DASL line because of its spectral efficiency and null DC energy content.

All transmitted bits, excluding the start bit, are scrambled by a 9-bit scrambler to provide good spectral spreading

Figure 2. Burst Mode Timing on the Line

Pulse shaping is obtained by means of a Digital to Analog Converter followed by a Continuous Smoothing Filter,

in order to limit RF energy and crosstalk while minimizing Inter-Symbol Interference (ISI). Figure 3 shows the

pulse shape at the Lo output, while a template for the typical power spectrum transmitted to the line with random

data is shown in Figure 4.

Each line-driver output, Lo0–Lo3, is designed to drive a transformer through a capacitor and termination resistor.

A 1:1 transformer, terminated in 100

Ω, results in signal amplitude of typically 1.3 Vpk on the line. Over-voltage

protection must be included in each interface circuit.

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