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LANSDALE Semiconductor, Inc.

ML145442, ML145443

RxA2, RxA1

Receive Carrier (Pins 15, 16)

The receive carrier is the FSK input to the demodulator through

the receive band–pass filter. RxA1 is the non–inverting input and

RxA2 is the inverting input of the receive hybrid (duplexer) oper-

ational amplifier.

LB

Analog Loopback (Pin 2)

When a high level is applied to this pin (SQT must be low), the

analog loopback test is enabled. The analog loopback test con-

nects the TxA pin to the RxA2 pin and the RxA1 to analog

ground. In loopback, the demodulator frequencies are switched to

the modulation frequencies for the selected mode. (See Tables 1

and 2 and Figures 4c and 4d.)

When LB is connected to analog ground (VAG), the modulator

generates an echo cancellation tone of 2100 Hz for ML145442

CCITT V.21 and 2225 Hz for ML145443 Bell 103 systems. For

normal operation, this pin should be at a logic low level (VSS).

The power–down mode is enabled when both LB and SQT are

connected to a logic high level (see Table 2).

MODE

Mode (Pin 13)

This input selects the pair of transmit and frequencies used dur-

ing modulation and demodulation. When a logic high level is

placed on this input, originate (Bell) or channel 1(CCITT) is

selected. When a low level is placed on this input, answer (Bell) or

channel 2 (CCITT) is selected. (See Tables 1 and 2 and Figure 4.)

CDT

Carrier Detect Timing (Pin 4)

A capacitor on this pin to VSS sets the amount of time the car-

rier must be present before CD goes low (see Legacy Applications

Information for the capacitor values).

CD

Carrier Detect Output (Pin 3)

This output is used to indicate when a carrier has been sensed

by the carrier detect circuit. This output goes to a logic low level

when a valid signal above the maximum threshold level (defined

by CDA, pin 7) is maintained on the input to the hybrid circuit

longer then the response (defined by CDT, pin 4). This pin is held

at the logic low level until the signal falls below the maximum

threshold level for longer than the turn off time. (See Legacy

Applications Information and Figure 5.)

CDA

Carrier Detect Adjust (Pin 7)

An external voltage may be applied to this pin to adjust the car-

rier detect threshold. The threshold hysteresis is internally fixed at

3 dB (see Legacy Applications Information).

Xout, Xin

Crystal Oscillator (Pins 8, 9)

A crystal reference oscillator is formed when a 3.579 MHz

crystal is connected between these two pins. Xout (pin 8) is the

output of the oscillator circuit, and Xin (pin 9) is the input to the

oscillator circuit. When using an external clock, apply the clock to

the Xin (pin 9) pin and leave Xout (pin 8) open. An internal 10

M

Ω resistor and internal capacitors, typically 10 pF on Xin and

16 pF on Xout, allow the crystal to be connected without any

other external components. Printed circuit board layout should

keep external stray capacitance to a minimum.

FB

Filter Bias (Pin 10)

This is the negative input to the AC amplifier. In normal opera-

tion, this pin is connected to analog ground through a 0.1µF

bypass capacitor in order to cancel the input offset voltage of the

limiter. It has a nominal input impedance of 16 k

Ω (see Figure 3).

SQT

Transmit Squelch (Pin 14)

When this input pin is at a logic high level, the modulator is

disabled. The line driver remains active if LB is at a logic low

level (see Table 2).

When both LB and SQT are connected to a logic high level (see

Table 2), the entire chip is in a power down state and all circuitry

except the crystal oscillator is disabled. Total power supply cur-

rent decreases from 10 mA (max) to 300 µA (max).

0.1 µF

FB

16 kΩ

10

490 kΩ

+

–

FROM

BAND–PASS

FILTER

TO

CARRIER DETECT CIRCUIT

AND DEMODULATOR

Figure 3. AC Amplifier Circuit

Table 2. Functional Table

MODE

Pin 13

SQT

Pin 14

LB

Pin 2

Operating Mode

1

0

0

Originate Mode

0

0

0

Answer Mode

X0

VAG (VDD/2)

Echo Tone

X

0

1

Analog Loopback

X

1

0

Squelch Mode

X1

VAG (VDD/2)

Squelch Mode

X

1

1

Power Down

Issue b