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NW6005 Datenblatt(Datasheet) 6 Page - Integrated Device Technology

Teile-Nr. NW6005
Beschreibung  ENHANCED TYPE II CALLER ID DECODER
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Hersteller  IDT [Integrated Device Technology]
Homepage  http://www.idt.com
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6
INDUSTRIAL TEMPERATURE RANGE
NW6005 ENHANCED TYPE II CALLER ID DECODER
DUAL TONE ALERT SIGNAL DETECTION
The Dual Tone Alert Signal is used only in off-hook signalling in
Bellcore system and ETSI system, but in BT system it is used in both
on and off-hook signalling. The low and high tone frequencies of three
different systems are as follows:
When the device selects DT-AS detection, the bi-purpose output
pin DR/STD is STD. STD goes low when DT-AS has been detected
and return high after DT-AS has ended.
The incoming Alert Signal goes through anti-alias filter and then is
separated into high band and low band by two bandpass filters. The
tone detection algorithm examines the filter outputs to validate the
arrival of the DT-AS. The EST pin becomes active when both tones
are detected. The EST is only the preliminary indication, it must be
qualified by the “guard time” as required by Bellcore, BT and ETSI (a
minimum duration for valid signals). STD is the guard time qualified
DT-AS detection output, it indicates the correct detection.
Fig. 5 shows the operation of the guard time circuit and Fig. 6
shows the waveform of the EST, ST/GT and STD pins. The total
recognition time is t
REC = tDP + tGP, where tDP is the tone present detection
time and t
GP is the tone present guard time. The total absent time is tABS
= t
DA + tGA, where tDA is the tone absent detection time and tGA is the tone
absent guard time. The guard time is the RC time constant for the
capacitor charge to VCC or discharge to GND. To get the unequal
present and absent guard time, a diode can be connected as shown
in Fig. 7 to provide different RC time constant (varying resistance
value) during charging and discharging.
Figure 7. Guard Time Circuits with Unequal Present and
Absent Times
tGP > tGA
tGP=R1CIn(VCC/(VCC-VTGT ))
tGA=RPCIn((VCC-Vd(RP/R2))/
(VTGT-Vd(RP/R2)))
RP=R1R2/(R1+R2)
Vd=diode forward voltage
tGP < tGA
tGP=RPCIn((VCC-Vd(RP/R2))/
(VCC-VTGT -Vd(RP/R2)))
tGA=R1CIn(VCC/VTGT )
RP=R1R2/(R1+R2)
Vd=diode forward voltage
BT
Bellcore & ETSI
Low Tone
Frequency
2130 Hz ± 1.1% (on-hook)
2130 Hz
± 0.6% (off-hook)
2130 Hz
± 0.5%
High Tone
Frequency
2750 Hz
± 1.1% (on-hook)
2750 Hz
± 0.6% (off-hook)
2750 Hz ± 0.5%
Figure 5. Guard Time Circuit of Dual Tone Alert Signal
Detection
Alerting Signal
ON
ON
ON
t
DP
t
DA
t
GA
t
GP
V
TGT
V
TGT
t
REC
t
ABS
DT-AS
EST
ST/GT
STD
Q1
Switch
Q2
Switch
Figure 6. Guard Time Waveform
R1
R2
C
VCC
ST/GT
EST
R1
R2
C
VCC
ST/GT
EST
NW6005
Dual tone detected
VCC
VCC
C3
R5
Q1
Q2
P
N
Comparator
VTGT
ST/GT
EST
STD




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