73k322l ETC-unknow, 73k322l Datasheet - Page 26

• Manufacturer Part Number: 73k322l
• Description: Ccitt V.23, V.22, V.21 Single-chip Modem
• Manufacturer: ETC-unknow

CCITT V.23, V.22, V.21
Single-Chip Modem
DESIGN CONSIDERATIONS
TDK Semiconductor Corporation’s 1-chip modem
products include all basic modem functions. This
makes these devices adaptable for use in a variety
of applications, and 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.
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 (antiresonant) 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 load
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
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 impedance 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. To avoid problems, power
supply and ground traces should be routed
separately to the analog and digital functions on the
Using good analog circuit design
easy
to control
as
26
board, and digital signals should not be routed near
low level or high impedance analog traces.
analog and digital grounds should only connect at
one point near the K-Series device ground pin to
avoid ground loops. The K-Series modem IC’s
should have both high frequency and low frequency
bypassing as close to the package as possible.
MODEM PERFORMANCE
CHARACTERISTICS
The curves presented here define modem IC
performance under a variety of line conditions while
inducing disturbances that are typical of those
encountered during data transmission on public
service telephone lines. Test data was taken using
an AEA Electronics’ “Autotest I” modem test set and
line simulator, operating under computer control. All
tests were run full-duplex, using a Concord Data
Systems 224 as the reference modem.
pseudo-random-bit pattern was used for each data
point.
signal-to-noise (S/N) ratios reflect total power
measurements
measurement specification. The individual tests are
defined as follows.
BER vs. S/N
This test measures the ability of the modem to
operate over noisy lines with a minimum of data-
transfer errors. Since some noise is generated in
the best of dial-up lines, the modem must operate
with the lowest S/N ratio possible. Better modem
performance is indicated by test curves that are
closest to the BER axis. A narrow spread between
curves representing the four line parameters
indicates minimal variation in performance while
operating over a range of aberrant operating
conditions. Typically, a DPSK modem will exhibit
better BER-performance test curves receiving in the
low band than in the high band.
BER vs. Receive Level
This test measures the dynamic range of the modem.
Because signal levels vary widely over dial-up lines,
the widest possible dynamic range is desirable. The
minimum Bell specification calls for 36 dB of dynamic
range. S/N ratios are held constant at the indicated
values while the receive level is lowered from a very
high to very low signal levels. The width of the “bowl”
of these curves, taken at the BER point, is the
measure of dynamic range.
Noise was C-message weighted and all
similar
to
the
CCITT
A 511
V.56
The