73k224bl ETC-unknow, 73k224bl Datasheet - Page 29

• Manufacturer Part Number: 73k224bl
• Description: Single-chip Modem Integrated Hybrid
• Manufacturer: ETC-unknow

APPLICATIONS INFORMATION
DIRECT ACCESS ARRANGEMENT (DAA)
The DAA (Direct Access Arrangement) required
for the 73K224BL consists of an impedance
matching resistor, telecom coupling transformer,
and ring detection and fault protection circuitry.
The transformer specifications must comply with
the impedance of the country in which the modem
is
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
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.
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 suppresser may be
a MOV (metal oxide varistor), Sidactor
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 cease before they can return
to their inactive state.
Current limiting devices can consist of a resistor,
Raychem PolySwitch
blow fuse that can withstand the transient tests
without permanent damage or replacement.
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.
being
by
operated.
additional
resettable fuse, or slow
Transformers
devices
Single-Chip Modem w/ Integrated Hybrid
(continued)
(a
designed
(Teccor
“dry”
29
V.22bis/V.22/V.21/Bell 212A/103
DESIGN CONSIDERATIONS
Semiconductor's
include all basic modem functions. This makes
these devices adaptable for use in a variety of
applications,
conventional digital bus peripherals.
Unlike digital logic circuitry, modem designs must
properly
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.
CRYSTAL OSCILLATOR
The K-Series crystal oscillator requires a parallel
mode (anti-resonant) crystal which operates at
11.0592 MHz. It is important that this frequency be
maintained to within ±0.01% accuracy.
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
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
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
and
contend
and
internal
one-chip
as
with
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precise
73K224BL
modem
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