TS5070 Datasheet, PDF (13/32 Pages) STMicroelectronics – PROGRAMMABLE CODEC/FILTER COMBO 2ND GENERATION

TS5070 - TS5071
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 VF_XI, 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 Z_L’ then the echo path trans-
fer function from VFRO to VFXI is :
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 terminationimpedancein 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 Z_L 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 signalanalysisor 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 :
Register 1: select/de-select Hybrid Balance Filter;
invert/non-invert cancellation signal;
select/de-select Hybal2 filter section;
attenua tor setting.
Register 2: select/de-select Hybal1 filter;
set Hybal1 to Bi-Quad or 1st order;
program pole and zero frequency.
Table 10: Hybrid Balance Register 2 Byte 2 in-
structions
Bit Number
7 654 3210
Function
0 0 0 0 0 0 0 0 By Pass Hybal 1
Filter
X X X X X X X X Pole/zero Setting
Register 3 : programpole frequency in Hybal2 filter;
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).
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