Data Sheet
2130 Hz and 2750 Hz
CAS/DT-AS Characteristics
Frequency Tolerance
Signal Level (per tone)
Reject Level (per tone)
Maximum Twist (V
2130Hz
/V
2750Hz
)
Duration
Reject Duration
Signal to Noise Ratio
Speech
Speech
Bellcore
a
(Off-hook only)
+/-0.5%
-14 to -32 dBm
d
-45 dBm
+/-6 dB
75 to 85 ms
+/-6 dB
75 to 85 ms
ETSI
b
(Off-hook)
+/-0.5%
-9.78 to -32.78 dBm
(-12 to -35 dBV
e
)
MT88E45B
BT
c
(Off-hook = ‘Loop State’)
(On-hook = ‘Idle State’)
Off-hook: +/-0.6%
On-hook: +/-1.1%
+0.22 to -37.78 dBm
(-2 to -40 dBV)
On-hook: -43.78 dBm
(-46 dBV)
+/-7 dB
Off-hook: 80 to 85 ms
On-hook: 88 to 110 ms
Off-hook: <=70 ms
On-hook: <=20 ms
Off-hook: Speech
On-hook: >= 20 dB
(300-3400 Hz)
0 dB
-3.5 dB
Hybrid Op-amp (GS2) Gain
Vdd = 5V +/- 10%
Hybrid Op-amp (GS2) Gain
Vdd = 3V +/- 10%
0 to -5 dB
-3.5 to -8.5 dB
0 to -5 dB
-3.5 to -8.5 dB
a. SR-TSV-002476, Issue 1 Dec 1992
b. ETS 300 778-2 Jan 98. The DT-AS plus FSK variant of ETSI on-hook signalling described in ETS 300 778-1 is not supported
because on-hook DT-AS uses the GS1 op-amp. With the GS1 gain in Table 4, the DT-AS minimum level will be below the
MT88E45B’s minimum accept level.
c. SIN227 Issue 3 Nov 97, SIN242 Issue 2 Nov 96
d. dBm - Decibels above or below a reference power of 1 mW into 600 ohms. 0 dBm = 0.7746 Vrms.
e. dBV - Decibels above or below a reference voltage of 1 Vrms. 0 dBV = 1 Vrms
Table 3 - CAS/DT-AS Characteristics
Table 3 shows the Hybrid op-amp (GS2) gain for
operation at 3V and 5V nominal Vdd, with a
±
10%
Vdd variation. For 3V operation, the Hybrid op-amp
gain should be reduced from the 5V setting to
maintain the CAS reject level and to maintain the
talkoff immunity: the CAS threshold is directly
proportional to Vdd, when Vdd is reduced the
threshold becomes lower, hence lower level CAS are
accepted. If the gain is not reduced, the MT88E45B
will be more talkoff prone. In Table 3, the GS2 gain is
shown as a range. By adopting the lower gain, talkoff
immunity can be improved.
When CAS detection is selected, the dual purpose
output pin DR/STD is STD. STD goes low when CAS
has been detected, and returns high after CAS has
ended.
CAS Guard Time
The guard time circuit shown in Figure 5 implements
a timing algorithm which determines whether the
signal is a CAS. Proper selection of the guard time(s)
is key to good speech immunity. The first indication
that there might be a CAS is when EST goes high.
EST high indicates that both tones are present. EST
low indicates that one or both tones is not present.
STD low indicates that CAS has been detected.
When STD returns high it indicates that CAS has
ended.
The timing algorithm consists of 2 components: a
tone present guard time (t
GP
) and a tone absent
guard time (t
GA
). t
GP
sets the minimum accept
duration for CAS. That is, both tones must be
detected continuously for t
GP
for STD to go low to
indicate that CAS has been detected. For STD to
return high to indicate that CAS has ended, one or
both tones must have disappeared for t
GA
. The
purpose of t
GA
is to bridge over momentary EST
dropouts once EST has met the minimum tone
duration so as to decrease the likelihood of a long
talkoff being broken up into several talkoffs. Usually
t
GA
is set very short or removed altogether because
there is another way to deal with the problem (by
ignoring further detections for 2 seconds after every
detection).
7
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