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73K321L

CCITT V.23, V.21 Single-Chip Modem

TDK Corporation 

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.

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.

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.

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