AM79Q4457J ETC1 [List of Unclassifed Manufacturers], AM79Q4457J Datasheet - Page 33

• Manufacturer Part Number: AM79Q4457J
• Description: Quad Subscriber Line Audio Processing Circuit-Non-Programmable (QSLAC-NP) Devices
• Manufacturer: ETC1 [List of Unclassifed Manufacturers]

Calculation of Balance Network
Th e balan ce fu nctio n is implemen ted with the
QSLAC-NP device by connecting an external balance
network (Z
suming the uncancelled receive path signal, which ap-
pears in the SLIC’s transmit path, is out of phase with
the originating receive path (the QSLAC-NP device’s
V
for the needed cancellation. The I
summing node and is a virtual ac ground, simplifying
the implementation of this balance function. In many
cases, this balance network can be a single resistor,
and in other cases, depending on the balance imped-
ance and the transfer characteristics of the SLIC, it
may be necessary to add a capacitor. Complete defini-
tion of the balance network is dependent on the SLIC
and the balance impedance and, as such, cannot be
completely defined within this document. However, a
general method of calculation can be described as fol-
lows:
Figure 12 shows a simplified equivalent circuit of a
SLIC, along with the balance impedance and intercon-
necting networks to the QSLAC device. A SLIC circuit
can be represented by four gain parameters (G-param-
eters), where each of these blocks represents a com-
plex transfer function: G24 is the gain from the tip/ring
two-wire port toward the four-wire port; G42 is the un-
terminated gain from the four-wire port toward the two-
Note:
*G-parameter model includes the R
OUT
TIP/RING
) signal, this external network will provide a path
Z
TERM
BAL
) between the V
ZSL
G24
SLIC
OUT
G42
RX
IN
and I
G44
resistor inside SLIC.
terminal is a current
IN
terminals. As-
R
RX
Figure 12. Balance Network
*
SLAC Products
V
TX
R
TX
wire port; ZSL is the two-wire SLIC impedance; and
G44 is the gain that appears from the four-wire input to-
ward the four-wire output with the two-wire por t
shorted. Considering only the SLIC, plus the externally
connected balance impedance, Z
presented to the SLIC’s four-wire input will have a rep-
resentation at the V
SLIC’s G-parameters and the Z
(G44 loaded) can then be defined as the four-wire to
four-wire gain through the SLIC when loaded by Z
The R
translating the SLIC’s V
pears as a current into the QSLAC-NP device’s I
rent input and, as such, provides the scaling necessary
to define the transmit gain. If the G44L gain is then
known, the balance network can be calculated as fol-
lows:
G44L is a complex value, which implies that Z
also be complex. However, in many cases, satisfac-
tory balance over the voice band can be achieved by
using only one resistor. This configuration assumes
that R
greater than 10 ms. If that is not true, the balance net-
work should be moved to the QSLAC-NP device side
of the coupling capacitors and should also have a ca-
pacitor placed in series with the network.
Z
BAL
TX
TX
resistor establishes the transmit path gain by
C
TX
C
C
and R
TX
RX
Z
TX
BAL
four-wire output defined by the
RX
TX
=
Placement
output voltage so that it ap-
Optional
C
------------- -
G44L
R
Z
RX
TX 1
BAL
BAL
have time constants
TERM
termination. G44L
, any signal that is
QSLAC-NP
device
I
V
IN
BAL
OUT
IN
must
TERM
cur-
33
.