73K224L
V.22bis, V.22, V.21, Bell 212A, 103
Single-Chip Modem
DATA SHEET
CRYSTAL OSCILLATOR
MODEM PERFORMANCE CHARACTERISTICS
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 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 Hayes SmartModem™ 2400 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.
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 0.22 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. The ISET resistor and capacitor should be
mounted near the ISET pin, away from digital signals.
High speed digital circuits tend to generate a
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 modem will exhibit better
BER-performance test curves receiving in the low
band than in the high band.
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.
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.
Page: 27 of 31
© 2005, 2008 TERIDIAN Semiconductor Corporation
Rev 7.1
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