73M2921

Advanced Single

Chip Modem

Page 36 of 41

TDK Semiconductor

February 99 Rev M

DESIGN CONSIDERATIONS

TDK Semiconductor’s single chip modem solutions include all the basic modem functions. This makes these

devices adaptable to a variety of applications, and as easy to control as conventional digital bus peripherals.

Unlike digital logic circuitry, modem designs must contend with precise frequency tolerances and verify low level

analog signals, to ensure acceptable performance. Using good analog circuit design practices will generally

result in a sound design. The crystal oscillator should be held to a 50ppm tolerance. Following are additional

recommendations that should be taken into consideration when starting new designs. Additional information is

available in the 73M2921 Design Guide.

LAYOUT CONSIDERATIONS

Good analog/digital design rules must be used to control system noise in order to obtain high performance in

modem designs. The more digital circuitry present on the PC board, the more attention to noise control is needed.

The 73M2921 should be considered a high performance analog device. A 10

µF electrolytic capacitor in parallel

with a 0.1

µF Ceramic capacitor should be placed between VPD and VND as well as between VPA and VNA. A

0.1

µF ceramic capacitor should be placed between VREF and VNA as well as VBG and VNA. Liberal use of

ground planes and large traces on power are also highly recommended. 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

the modem should be located next to each other near where the telephone line connection is accessed. To avoid

problems, power supplies and ground traces should be routed separately to the analog and digital portions on the

board. Digital signals should not be routed near low level analog or high impedance analog traces.

MODEM PERFORMANCE CHARACTERISTICS

The curves presented here define modem IC performance under a variety of line conditions typical of those

encountered over public service telephone lines.

BER vs. SNR (see Figure 9)

This test represents the ability of the modem to operate over noisy lines with a minimum amount of data transfer

errors. Since some noise is generated in the best dial up lines, the modem must operate with the lowest signal to

noise ratio (SNR) 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 (answer mode) than in the high band (originate

mode).

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 possible is desirable. The minimum Bell specification calls for 36dB of dynamic

range. The SNR is held constant at the indicated values as the Receive level is lowered from very a very high to

a very low signal level. The width of the bowl of these curves, taken at the BER point is the measure of the

dynamic range.