LUCL9312AP-D AGERE [Agere Systems], LUCL9312AP-D Datasheet - Page 33

• Manufacturer Part Number: LUCL9312AP-D
• Description: Line Interface and Line Access Circuit Forward Battery SLIC and Ringing Relay for TR-57 Applications
• Manufacturer: AGERE [Agere Systems]

July 2001
ac Applications
ac Interface Network
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 L9312 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-
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.
Agere Systems Inc.
(continued)
(continued)
up to over 1000 . Thus, if
Forward Battery SLIC and Ringing Relay for TR-57 Applications
* Windows is a registered trademark of Microsoft Corporation.
† PSPICE is a registered trademark of MicroSim Corporation.
Thus, it appears the solution is to have a SLIC with a
low gain, especially in the receive direction. This will
allow the codec to operate near its maximum output
signal (to optimize S/N), without an external resistor
divider (to minimize cost).
Note also that some third-generation codecs require
the designer to provide an inherent resistive termina-
tion via external networks. The codec will then provide
gain shaping, as a function of frequency, to meet the
return loss requirements. Further stability issues may
add external components or excessive ground plane
requirements to the design.
To meet the unique requirements of both types of
codecs, the L9312 offers two receive gain choices.
These receive gains are mask programmable at the
factory and are offered as two different code variations.
For interface with a first-generation codec, the L9312 is
offered with a receive gain of 8. For interface with a
third-generation codec, the L9312 is offered with a
receive gain of 2. In either case, the transconductance
in the transmit direction, or the transmit gain, is 300 .
This selection of receive gain gives the designer the
flexibility to maximize performance and minimize exter-
nal components, regardless of the type of codec cho-
sen.
Design Tools
The following examples illustrate the design tech-
niques/equations followed to design the ac interface
with a first- or third-generation codec for both a resis-
tive and complex design. To aid the line circuit design,
Agere has available Windows *-based spreadsheets to
do the individual component calculations. Further,
Agere has available PSPICE
tion and verification. Consult your Agere Account Rep-
resentative to obtain these design tools.
First-Generation Codec ac Interface Network
Termination impedance may be specified as purely
resistive or complex, that is, some combination of
resistors and capacitors that causes the impedance to
vary with frequency. The design for a pure resistive ter-
mination, such as 600 , does not vary with frequency,
so it is somewhat more straightforward than a complex
termination design. For this reason, the case of a resis-
tive design and complex design will be shown sepa-
rately.
†
models for circuit simula-
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