L9214A/G
Low-Cost Ringing SLIC
Preliminary Data Sheet
October 2001
Third-Generation Codecs
Periodic Pulse Metering (PPM)
This class of devices includes all ac parameters set
digitally under microprocessor control. Depending on
the device, it may or may not have data control latches.
Additional functionality sometimes offered includes
tone plant generation and reception, PPM generation,
test algorithms, and echo cancellation. Again, this type
of codec may be 3.3 V, 5 V only, or ±5 V operation, sin-
gle-, quad-, or 16-channel, and µ-law/A-law or 16-bit
linear coding selectable. Examples of this type of
codec are the Agere T8535/6 (5 V only, quad, standard
features), T8537/8 (3.3 V only, quad, standard fea-
tures), T8533/4 (5 V only, quad with echo cancellation),
and the T8531/32 (5 V only, eight- or 16-channel).
Periodic pulse metering (PPM), also referred to as tele-
tax (TTX), is applied to the audio input of the L9214.
When in the active state, this signal is presented to the
tip/ring subscriber loop along with the audio signal. The
L9214 assumes that a shaped PPM signal is applied to
the audio input.
ac Applications
ac Parameters
There are four key ac design parameters. Termination
impedance is the impedance looking into the 2-wire
port of the line card. It is set to match the impedance of
the telephone loop in order to minimize echo return to
the telephone set. Transmit gain is measured from the
2-wire port to the PCM highway, while receive gain is
done from the PCM highway to the transmit port.
Transmit and receive gains may be specified in terms
of an actual gain, or in terms of a transmission level
point (TLP), that is the actual ac transmission level in
dBm. Finally, the hybrid balance network cancels the
unwanted amount of the receive signal that appears at
the transmit port.
ac Interface Network
The ac interface network between the L9214 and the
codec will vary depending on the codec selected. With
a first-generation codec, the interface between the
L9214 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 L9214 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.
Codec Types
Because the design requirements are very different
with a first- or third-generation codec, the L9214 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 L9214
and codec.
At this point in the design, the codec needs to be
selected. The interface network between the SLIC and
codec can then be designed. Below is a brief codec
feature summary.
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 L9214 provides a transcon-
ductance from T/R to VITR in the transmit direction and
a single-ended to differential gain from either RCVN or
RCVP to T/R in the receive direction. Assuming a short
from VITR to RCVN or RCVP, the maximum imped-
ance 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 speci-
fied termination impedance can range over the voice-
band as low as 300 Ω up to over 1000 Ω. Thus, if the
SLIC gains are too low, it will be impossible to synthe-
size 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.
First-Generation Codecs
These perform the basic filtering, A/D (transmit), D/A
(receive), and µ-law/A-law companding. They all have
an op amp in front of the A/D converter for transmit
gain setting and hybrid balance (cancellation at the
summing node). Depending on the type, some have
differential analog input and output stages, +5 V only or
±5 V operation, and µ-law/A-law selectability. These
are available in single and quad designs. This type of
codec requires continuous time analog filtering via
external resistor/capacitor networks to set the ac
design parameters. An example of this type of codec is
the Agere T7504 quad 5 V only codec.
This type of codec tends to be the most economical in
terms of piece part price, but tends to require more
external components than a third-generation codec.
The ac parameters are fixed by the external R/C net-
work so software control of ac parameters is difficult.
Agere Systems Inc.
29
AGERE [ AGERE SYSTEMS ]
