ST7537HS1 Datasheet, PDF (3/9 Pages) STMicroelectronics – HOME AUTOMATION MODEM

ST7537HS1
BLOCK DIAGRAM
AV _DD
2
DV _DD
14
AV _SS
28
DV _SS
15
V_CM
1
DV _CC
25
RAI 3
Rx BAND-PASS
S.C. FILTER
I.F. BAND-PASS
S.C. FILTER
26 IFO
RxFO 4
CD 21
CARRIER
DETECTION
ATO 7
PAFB 6
PABC 9
PABC 8
REFERENCE
VOLTAGE
Tx BAND-PASS
S.C. FILTER
17 XTAL2
16 XTAL1
18 MCLK
19 WD
24 RSTO
20 Rx/Tx
FSK
MODULATOR
22 TxD
5 TxIFI
RxD 23
TEST LOGIC
10
TEST1
11
TEST2
12
TEST3
13
TEST4
POST-DEMO
S.C. FILTER
CORRELATOR
FSK DEMODULATOR
27 DEMI
ST7537HS1
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).
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