TS5071 datasheet(13/32 Pages) STMICROELECTRONICS

13 / 32 page

Figure 1 shows a simplified diagram of the local

echo path for a typical application with a trans-

former interface. The magnitude and phase of the

local echo signal, measured at VFXI, are a function

of the termination impedance ZT, the line trans-

former and the impedanceof the 2 W loop, ZL. If the

impedance reflected back into the transformer pri-

mary is expressed as ZL’ then the echo path trans-

fer function from VFROto VFXIis :

H(W) = ZL’ /(ZT +ZL’)

(1)

Figure 1: Simplified Diagram of Hybrid Balance Circuit

PROGRAMMING THE FILTER

On initial power-up the Hybrid Balance filter is dis-

abled. Before the hybrid balance filter can be pro-

grammed it is necessary to design the transformer

and terminationimpedance in orderto meet system

2 W input return loss specifications, which are nor-

mally measured against a fixed test impedance

(600 or 900

Ω in most countries). Only then can the

echo path be modeled and the hybrid balance filter

programmed. Hybrid balancing is also measured

against a fixed test impedance, specified by each

national Telecom administration to provide ade-

quate control of talker and listener echo over the

majority of their network connections. This test im-

pedance is ZL in figure 1. The echo signal and the

degree of transhybrid loss obtained by the pro-

grammable filter must be measured from the PCM

digital input DR0, to the PCM digital output DX0,

either by digital test signal analysis or by conversion

back to analog by a PCM CODEC/Filter.

Three registers must be programmed in COMBO

IIG to fully configure the Hybrid Balance Filter as

follows :

invert/non-invert cancellation signal;

select/de-select Hybal2 filter section;

attenuatorsetting.

set Hybal1 to Bi-Quad or 1st order;

program pole and zero frequency.

program zero frequencyin Hybal2 filter;

settings = Please refer to software

TS5077-2.

Standard filter design techniques may be used to

model the echo path (see equation (1)) and design

a matching hybrid balance filter configuration.Alter-

natively, the frequency response of the echo path

can be measured and the hybrid balance filter pro-

grammed to replicate it.

An Hybrid Balance filter design guide and soft-

ware optimization program are available under li-

cense from SGS-THOMSON Microelectronics (or-

der TS5077-2).

Bit Number

Function

210

By Pass Hybal 1

Filter

XXXXXX

Pole/zero Setting

Table 10: Hybrid Balance Register 2 Byte 2 in-

structions

TS5070 - TS5071

13/32

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