FUNCTIONAL DESCRIPTION

1 - Transmit Section

The transmit mode is set when Rx/Tx = 0, if Rx/Tx

is held at 0 longer than 3s, thenthe device switches

automatically in the Rx mode. A new activation of

the Tx mode requires Rx/Tx to be returned to 1 for

a minimum 2

µs period before being set to 0.

The Transmit Data (TxD) is sampled on a positive

edge of CLR/T which delivers the transmit bit clock

when the transmit mode is selected. This data

enters a FSK modulator whose two basic frequen-

cies are selected by the Baud Rate Selection pin

(BRS) and the Channel Selection pin (CHS) ac-

cording to the Table 1.

CLR/T

TxD

DATA VALID

Figure 1 : Tx Data Input Timing

Table 1

BRS

CHS

Baud Rate

(Baud)

Tx Frequencies (kHz)

TxD=1 - TxD=0

0

0

600

81.75 - 82.35

0

1

600

67.2 - 67.8

1

0

1200

71.4 - 72.6

1

1

1200

85.95 - 87.15

These frequencies are synthesized from a

11.0592MHzcrystal oscillator ; theirprecision is the

same as the crystal one’s (100 ppm).

The modulatedsignal coming out of the FSK modu-

lator is filtered by a switched-capacitor band-pass

filter (Tx band-pass) in order to limit the output

spectrum and to reduce the level of harmonic com-

ponents.

The output stage of the Tx path consists of an

Automatic Level Control (ALC) system which keeps

the output signal (ATO) amplitude independant of

the lineimpedancevariations. ThisALC is a variable

gain system (with 32 discrete values) controlled by

an analog feed-back signal ALCI (see Figure 2).

The ALC gain range is 0dB to -26dB and gain

change is clocked at 7200Hz. Gain steps are of

magnitude 0.84dB typically.

A period of this clock is decomposed into a 34.7

µs

gain settling latency and a 104.2

µs peak detecting

time. The gain change is related to the result of a

peak detection obtained by making a direct com-

parison of ALCI maximum value (during detecting

Figure 2).

by 0.84dB,

by 0.84dB.

Amplitude

modification

due to an

external cause

High Gain

Low Gain

Correct Gain

Correct

Gain

VT2

VT1

ALCI SIGNAL ENVELOP

ALC CLOCK

Figure 2 : Automatic Level Control Timing Chart

ST7536

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