TDK 73M2901/5V manual Design Considerations, Modem Performance Characteristics

73M2901/5V Advanced Single Chip Modem

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.

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.

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 in the application, the more attention to noise control is needed.

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. It is recommended that 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.

The 73M2901/5V should be considered a high performance analog device. A 10μF electrolytic

capacitor in parallel with a 0.1μF Ceramic capacitor should be placed between VPD and VND as well as

between VPA and VNA. A 0.1μF ceramic capacitor should be placed between VREF and VNA as well as VBG and VNA. Use of ground planes and large traces on power is recommended.

The 73M2901/5V is the first of a series of parts with different and/or additional features. In order to insure full lay out compatibility for all the series, it is recommended to implement three additional resistors in the schematics as shown in the recommended schematics arrangement (R11, R12 and R13).

TELEPHONE LINE INTERFACE

Transmit levels at the line are dependent on the interface used between the pins and the line. In order to save having to provide external op-amps to drive the line coupling transformer, the analog outputs (TXAP and TXAN) have the capability to be used as the hybrid drivers for connecting to the

transformer directly (with the required impedance matching series resistor). Used in this configuration, there is loss associated in both the receive path and transmit path.

The line interface circuit shown on the following page represents the basic components and values3 for interfacing the TDK 73M2901/5V analog pins to the telephone line.

MODEM PERFORMANCE

CHARACTERISTICS

The curves presented in this data sheet define modem IC performance under a variety of line conditions typical of those encountered over public service telephone lines.

BER VS. SNR

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 is desirable. 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.

3TDK73M2901 Demo boards use the line interface shown on the

following page. Other designs may have different requirements and thus will require different component values or a different configuration. With the shown configuration, there is approximately an 8dB loss in the transmit path, and approximately a 5dB loss in the receive path.

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