MT88E45
2130 Hz and 2750 Hz
CAS/DT-AS Characteristics
Duration
Reject Duration
Signal to Noise Ratio
Speech
Speech
Bellcore
a
(Off-hook only)
75 to 85 ms
ETSI
b
(Off-hook)
75 to 85 ms
Data Sheet
BT
c
(Off-hook = ‘Loop State’)
(On-hook = ‘Idle State’)
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 3 V and 5 V nominal Vdd, with a
±
10% Vdd variation.
For 3 V operation, the Hybrid op-amp gain should be reduced from the 5 V 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 55 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).
9
Zarlink Semiconductor Inc.
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
