73K222BL
V.22, V.21, Bell 212A, 103 Single-Chip
Modem with Integrated Hybrid
DATA SHEET
DESIGN CONSIDERATIONS
APPLICATIONS INFORMATION (continued)
TERIDIAN Semiconductor's 1-chip modem products
include all basic modem functions. This makes these
DIRECT ACCESS ARRANGEMENT (DAA)
The DAA (Direct Access Arrangement) required for
the 73K222BL consists of an impedance matching
resistor, telecom coupling transformer, and ring
detection and fault protection circuitry.
devices adaptable for use in
a
variety of
applications, and as easy to control as conventional
digital bus peripherals.
Unlike digital logic circuitry, modem designs must
properly contend with precise frequency tolerances
and very low level analog signals, to ensure
acceptable performance. Using good analog circuit
design practices will generally result in a sound
design. Following are additional recommendations,
which should be taken into consideration when
starting new designs.
The transformer specifications must comply with the
impedance of the country in which the modem is
being operated. Transformers designed specifically
for use with the telephone network should be used.
These may present a DC load to the network
themselves (a “wet” transformer) or they may require
AC coupling with a DC load provided by additional
devices (a “dry” transformer). A dry transformer will
generally provide higher performance and smaller
size than a wet transformer. A wet transformer
allows a simpler design, but must not saturate with
the worst case DC current passing through it or
distortion and poor performance will result.
CRYSTAL OSCILLATOR
The K-Series crystal oscillator requires a parallel
mode (antiresonant) crystal, which operates at
11.0592 MHz. It is important that this frequency be
maintained to within ±0.01% accuracy.
The protection circuitry typically consists of a transient
suppression device and current limiter to protect the
user and the telephone network from hazardous
voltages that can be present under fault conditions.
The transient suppressor may be a MOV (metal oxide
varistor), Sidactor (Teccor Electronics Inc.), spark gap
device , or avalanche diode. Some devices clamp the
transient to their specified break down voltage and
others go into low impedance crowbar state. The
latter require that the fault current to cease before
they can return to their inactive state.
In order for a parallel mode crystal to operate
correctly and to specification, it must have a
capacitor connected to the junction of each of the
crystal and internal inverter connections, terminated
to ground. The values of these capacitors depend
primarily on the crystal’s characteristics, and to a
lesser degree on the internal inverter circuit. The
values used affect the accuracy and start up
characteristics of the oscillator.
LAYOUT CONSIDERATIONS
Current limiting devices can consist of a resistor,
Raychem PolySwitch resettable fuse, or slow blow
fuse that can withstand the transient tests without
permanent damage or replacement.
Good analog/digital design rules must be used to
control system noise in order to obtain highest
performance in modem designs. The more digital
circuitry present on the PC board, the more this
attention to noise control is needed. The modem
should be treated as a high performance analog
device. A 22 µF electrolytic capacitor in parallel with a
0.1 µF ceramic capacitor between VDD and GND is
recommended. Liberal use of ground planes and larger
traces on power and ground are also highly favored.
High speed digital circuits tend to generate a significant
amount of EMI (Electro-Magnetic Interference), which
must be minimized in order to meet regulatory agency
limitations. To accomplish this, high speed digital
devices should be locally bypassed, and the telephone
line interface and K-Series device should be located
close to each other near the area of the board where
the phone line connection is accessed.
Ring detection circuitry is not required by the FCC,
but may be required by the application. The ring
detector usually consists of an optoisolator,
capacitor, and resistor to present the proper AC load
to the network to meet the REN (Ring Equivalency
Number) regulations of FCC Part 68. The K-Series
Design Manual contains detailed information on the
design of a ring detect circuits as well as the other
topics concerning the DAA.
Page: 22 of 26
© 2005, 2008 TERIDIAN Semiconductor Corporation
Rev 7.2
TERIDIAN [ TERIDIAN SEMICONDUCTOR CORPORATION ]
