SC9270D Ver la hoja de datos (PDF) - Silan Microelectronics
Número de pieza
componentes Descripción
Fabricante
SC9270D Datasheet PDF : 12 Pages
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Silan
Semiconductors
SC9270C/D
0
10
20
FREQUENCY
(dB)
30
40
50
XY
1kHz
AB C D
EF G
H
FREQUENCY
PRECISE
DIAL TONES
X=350Hz
Y=440Hz
DTMF TONES
A=697Hz
B=770Hz
C=852Hz
D=941Hz
E=1209Hz
F=1336Hz
G=1477Hz
H=1633Hz
Figure 5. Filter Response
2. DECODER SECTION
The decoder used digital counting techniques to determine the frequencies of the limited tones and to verify that they
correspond to standard DTMF frequencies. A complex averaging algorithm(protects) against tone simulation by
extraneous signals, such as voice, while providing tolerance to small frequency deviations and variations. This
averaging algorithm has been developed to ensure an optimum combination of immunity to “talk-off” and tolerance to
the presence of interfering signals (“third tones”) and noise. When the detector recognizes the simultaneous
presence of two valid tones (referred to as “signal condition” in some industry specifications), it raises the “early
steering” flag (ESt). Any subsequent loss of signal condition will cause ESt to fall.
3. STEERING CIRCUIT
Before registration of a decoded tone-pair, the receiver
checks for a valid signal duration (referred to as
“character- recognition-condition”). This check is per-
formed by an external RC time-constant driven by ESt. A
logic high on ESt causes VC (see Fig.4) to rise as the
capacitor discharges. Provided signal-condition is
maintained (ESt remains high) for the validation period
(tGTP), Vc reaches the threshold (VTSt) of the steering
logic to register the tone-pair, latching its corresponding
4-bit code (see Fig.3) into the output latch. At this point,
the GT output is activated and drives VC to VDD. GT
0.1µf
VDD
St/GT
ESt
R
StD
VDD
VDD
tGTA=(RC)ln(
)
VTST
VC
VDD
tGTA=(RC)ln(
)
VDD-VTST
Figure 6. Basic steering Circuit
HANGZHOU SILAN MICROELECTRONICS JOINT-STOCK CO.,LTD
Rev: 1.1 2001.04.27
9
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