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LANSDALE Semiconductor, Inc.

ML145442, ML145443

GENERAL DESCRIPTION

The ML145442 and ML145443 are full–duplex low–speed

modems. They provide a 300–baud FSK signal for bidirectional

data transmission over the telephone network. They can be operated

in one of four basic configurations as determined by the state of

MODE (pin 13) and LB (pin 2).The normal (non–loopback) and

self test (loopback) modes in both answer and originate modes will

be discussed.

For an originate or channel 1 mode, a logic high level is placed

on MODE (pin 13) and a logic low level is placed on LB (pin 2). In

this mode, transmit data is input on TxD, where it is converted to a

FSK signal and routed through a low–band band–pass filter. The

filtered output signal is then buffered by the Tx op–amp line driver,

which is capable of driving –9 dBm onto a 600

Ω line. The receive

signal is connected through a hybrid duplexer circuit on pins 15 and

16, RxA2 and RxA1. The signal then passes through the anti–alias-

ing filter, the sample–and–hold circuit, is switched into the

high–band band–pass filter, and then switched into the AC amplifi-

er circuit. The output of the ac amplifier circuit is routed to the

demodulator circuit and demodulated. The resulting digital data is

then output through RxD (pin 5). The carrier detect circuit receives

its signal from the output of the AC amplifier circuit and goes low

when the incoming signal is detected (see Figure 4a).

In the answer or channel 2 mode, a logic low level is placed on

MODE (pin 13) and on LB (pin 2). In this mode, the data follows

the same path except the FSK signal is routed to the high–band

band–pass filter and the sample–and–hold signal is routed through

the low–band band–pass filter (see Figure 4b).

In the analog loopback originate or channel 1 mode, a logic high

level is placed on MODE (pin 13) and on LB (pin 2). This mode is

used for a self check of the modulator, demodulator, and low–band

pass–band filter circuit. The modulator side is configured exactly

like the originate mode above except the line driver output (TxA,

pin 17) is switched to the negative input of the hybrid op–amp. The

RxA2 input pin is open in this mode and the non–inverting input of

the hybrid circuit is connected to VAG. The sample–and–hold out-

put bypasses the filter so that the demodulator receives the modu-

lated Tx data (see Figure 4c). This test checks all internal device

components except the high–band band–pass filter, which can be

checked in the answer or channel 2 mode test.

In the analog loopback or channel 2 mode, a logic low level is

placed on MODE (pin 13) and a logic high level on LB (pin 2).

This mode is used for a self check of the modulator, demodulator,

and high–band pass–band filter circuit. This configuration is exact-

ly like the originate loopback mode above, except the signal is rout-

ed through the high–band pass–band filter (see Figure 4d).

Issue b