LUCL9311AP-D AGERE [Agere Systems], LUCL9311AP-D Datasheet - Page 41

• Manufacturer Part Number: LUCL9311AP-D
• Description: Line Interface and Line Access Circuit Full-Feature SLIC with High Longitudinal Balance, Ringing Relay,and GR-909 Test Access
• Manufacturer: AGERE [Agere Systems]

July 2001
ac Applications
ac Interface Network
The ac interface network between the L9311 and the
codec will vary depending on the codec selected. With
a first-generation codec, the interface between the
L9311 and codec actually sets the ac parameters. With
a third-generation codec, all ac parameters are set dig-
itally, internal to the codec; thus, the interface between
the L9311 and this type of codec is designed to avoid
overload at the codec input in the transmit direction,
and to optimize signal to noise ratio (S/N) in the receive
direction.
Because the design requirements are very different
with a first- or third-generation codec, the L9311 is
offered with two different receive gains. Each receive
gain was chosen to optimize, in terms of external com-
ponents required, the ac interface between the L9311
and codec.
With a first-generation codec, the termination imped-
ance is set by providing gain shaping through a feed-
back network from the SLIC VITR output to the SLIC
RCVN/RCVP inputs. The L9311 provides a transcon-
ductance from T/R to VITR in the transmit direction and
a single ended to differential gain in the receive direc-
tion, from either RCVN or RCVP to T/R. Assuming a
short from VITR to RCVN or RCVP, the maximum
impedance that is seen looking into the SLIC is the
product of the SLIC transconductance times the SLIC
receive gain, plus the protection resistors. The various
specified termination impedance can range over the
voiceband as low as 300
the SLIC gains are too low, it will be impossible to syn-
Agere Systems Inc.
(continued)
up to over 1000 . Thus, if
thesize the higher termination impedances. Further, the
termination that is achieved will be far less than what is
calculated by assuming a short for SLIC output to SLIC
input. In the receive direction, in order to control echo,
the gain is typically a loss, which requires a loss net-
work at the SLIC RCVN/RCVP inputs, which will
reduce the amount of gain that is available for termina-
tion impedance. For this reason, a high-gain SLIC is
required with a first-generation codec.
With a third-generation codec, the line card designer
has different concerns. To design the ac interface, the
designer must first decide upon all termination imped-
ance, hybrid balances, and TLP requirements that the
line card must meet. In the transmit direction, the only
concern is that the SLIC does not provide a signal that
is too large and overloads the codec input. Thus, for
the highest TLP that is being designed to, given the
SLIC gain, the designer, as a function of voiceband fre-
quency, must ensure the codec is not overloaded. With
a given TLP and a given SLIC gain, if the signal will
cause a codec overload, the designer must insert some
sort of loss, typically a resistor divider, between the
SLIC output and codec input.
In the receive direction, the issue is to optimize the
S/N. Again, the designer must consider all the consid-
ered TLPs. The idea, for all desired TLPs, is to run the
codec at or as close as possible to its maximum output
signal, to optimize the S/N. Remember, noise floor is
constant, so the larger the signal from the codec, the
better the S/N. The problem is if the codec is feeding a
high-gain SLIC, either an external resistor divider is
needed to knock the gain down to meet the TLP
requirements, or the codec is not operated near maxi-
mum signal levels, thus compromising the S/N.
Ringing Relay, and GR-909 Test Access
41