REFERENCE

VOLTAGE

CARRIER

DETECTION

Rx BAND-PASS

S.C. FILTER

Tx BAND-PASS

S.C. FILTER

CORRELATOR

POST-DEMO

S.C. FILTER

FSK DEMODULATOR

27

5

13

12

11

10

7

6

9

8

FSK

MODULATOR

17

16

18

19

24

20

22

23

I.F. BAND-PASS

S.C. FILTER

3

26

4

21

2

14

28

15

1

25

RAI

RxFO

CD

ATO

PAFB

PABC

PABC

RxD

IFO

XTAL1

XTAL2

MCLK

WD

RSTO

Rx/Tx

TxD

DEMI

TxIFI

TEST LOGIC

TEST1

TEST2

TEST3

TEST4

ST7537HS1

BLOCK DIAGRAM

TRANSMIT SECTION

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

is held at 0 longer than 1 second, then the 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 microsecond period

before being set to 0.

The Transmit Data (TxD) enter asynchronously the

FSK modulator with a nominal intra-message data

rate of 2400 bps.

The basic transmit frequencies are :

- f(TxD=0) = 133.05kHz

- f(TxD=1) = 131.85kHz

These frequencies are synthesized from a

11.0592MHz crystal oscillator; their precision is the

same as the crystal one’s (100ppm).

The modulated signal 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 final stage of the Tx path consists of an opera-

tional amplifier which needs a feed-back signal

(PAFB) from the power amplifier as shown on

Application Schematic Diagram.

In Tx mode the Receive Data (RxD) signal is set to 1.

RECEIVE SECTION

The receive section is active when Rx/Tx = 1.

The Rx signal is applied on RAI and filtered by a

band-pass switched capacitor filter (Rx band-pass)

centered on the carrier frequency and whose band-

width is around 12kHz.

The Rx filter output is amplified by a 20dB gain

stage which provides symetrical limitations for

large voltage. The resulting signal is down-con-

verted by a mixer which receives a local oscillator

synthesized by the FSK modulator block. Finally an

intermediate frequency band-pass filter (IF band-

pass) whose central frequency is 5.4kHz improves

the signal to noise ratio before entering the FSK

demodulator.

The coupling of the intermediate frequency filter

output (IFO) to the FSK demodulator input (DEMI)

is made by an external capacitor C5 (100nF

±10%,

10V) which cancels the Rx path offset voltage.

The RxD output delivers the demodulated signal if

the carrier detect (CD) signal is low and is set to

high level when CD = 1.

The RxD output can delivers the demodulated

signal whatever the level of CD (0 or 1) if Rx/Tx = 1

and TxD = 0 (see Figure 1).

ST7537HS1

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