L9215A/G
Short-Loop Sine Wave Ringing SLIC
Data Sheet
September 2001
Codec Types
Periodic Pulse Metering (PPM)
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.
Periodic pulse metering (PPM), also referred to as tele-
tax (TTX), is input to the PPMIN input of the L9215.
Upon application of appropriate logic control, this sig-
nal is presented to the tip/ring subscriber loop. The
state of the L9215 should be changed while applying
PPM signals during the quiet interval of the PPM
cadence. The L9215 assumes that a shaped PPM sig-
nal is applied to the PPMIN 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 stages, differential analog out-
put 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.
PPM input signals may be a maximum 1.25 V at
PPMIN. The gain from PPMIN to tip/ring is 6 dB. Thus,
for 1.0 Vrms at tip and ring, apply a 0.50 Vrms signal at
PPMIN. The PPM signal should be ac coupled to
PPMIN through a 10 nF capacitor.
When applied to tip and ring, the PPM signal will also
be returned through the SLIC and will appear at the
SLIC VITR output. The concern is that this high-voltage
signal can overload an internal SLIC amplifier or the
codec input and cause distortion of the (desired) ac
signal. Because the L9215 is intended for short dc
loops, the assumption is that low meter pulse signals
are sufficient. The maximum allowed PPM current at
the 200 Ω ac meter pulse load to avoid saturation of the
device’s internal AAC amplifier is 3 mArms. This signal
level is sufficient to provide a minimum 200 mVrms to
the 200 Ω PPM load under maximum specified dc loop
conditions. Above 3 mArms PPM current, external
meter pulse rejection may be required. If on-hook
transmission of PPM is required, sufficient overhead to
accommodate on-hook transmission must be pro-
grammed by the user at the OVH input.
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.
Further ac parameters are fixed by the external R/C
network so software control of ac parameters is diffi-
cult.
Third-Generation Codecs
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 features), T8533/4
(5 V only, quad with echo cancellation), and the
T8531/32 (5 V only 16 channel).
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 L9215 and the
codec will vary depending on the codec selected. With
a first-generation codec, the interface between the
L9215 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
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Agere Systems Inc.
AGERE [ AGERE SYSTEMS ]
