73K321L
CCITT V.23, V.21
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.
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. A 511
pseudo-random-bit pattern was used for each data
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.
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
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
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.
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
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.
21
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