ST7536 데이터 시트보기 (PDF) - STMicroelectronics
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ST7536 Datasheet PDF : 9 Pages
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ST7536
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 2µs 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.
Figure 1 : Tx Data Input Timing
CLR/T
TxD
DATA VALID
Table 1
BRS
0
0
1
1
CHS
0
1
0
1
Baud Rate
(Baud)
600
600
1200
1200
Tx Frequencies (kHz)
TxD=1 - TxD=0
81.75 - 82.35
67.2 - 67.8
71.4 - 72.6
85.95 - 87.15
Figure 2 : Automatic Level Control Timing Chart
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) systemwhich keeps
the output signal (ATO) amplitude independant of
the lineimpedancevariations. ThisALC isa 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 34.7µs
gain settling latency and a 104.2µs 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
time) with two threshold voltages VT1 and VT2 (see
Figure 2).
- max (VALCI) < VT1 - The next gain is increased
by 0.84dB,
- VT1 ≤ max (VALCI) ≤ VT2 - No gain change,
- VT2 < max (VALCI) - The next gain is decreased
by 0.84dB.
Low Gain
VT2
VT1
Correct Gain
Amplitude
modification
due to an
external cause
High Gain
Correct
Gain
ALCI SIGNAL ENVELOP
ALC CLOCK
4/9
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