73K302L Datasheet, PDF (25/29 Pages) TDK Electronics – Single-Chip Modem

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.
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 board, and digital signals should not be routed near
low level or high impedance analog traces. The 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.
.
73K302L
Bell 212A, 103, 202
Single-Chip Modem
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. A 511 pseudo-random-bit
pattern was used for each data point. Noise was C-
message weighted and all signal-to-noise (S/N) ratios
reflect total power measurements similar to the CCITT
V.56 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.
25