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| 型號(hào)： | T7234 |
| 英文描述： | Compliance with the New ETSI PSD Requirement |
| 中文描述： | 符合新的ETSI PSD的要求 |
| 文件頁(yè)數(shù)： | 84/116頁(yè) |
| 文件大小： | 1056K |
| 代理商： | T7234 |
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Data Sheet
January 1998
T7256 Single-Chip NT1 (SCNT1) Transceiver
80
Lucent Technologies Inc.
Questions and Answers
Introduction
This section is intended to answer questions that may
arise when using the T7256 Single-Chip NT1 Trans-
ceiver.
The questions and answers are divided into three cate-
gories: U-interface, S/T-interface, and miscellaneous.
U-Interface
Q1:
Is the line interface for the T7256 the same as for
the T7264
A1:
Yes. The U-interface section on these chips is
identical, so their line interfaces are also identical.
Q2:
Why is a higher transformer magnetizing induc-
tance used (as compared to other vendors)
A2:
It has been determined that a higher inductance
provides better linearity. Furthermore, it has been
found that a higher inductance at the far end pro-
vides better receiver performance at the near end
and better probability of start-up at long loop
lengths.
Q3:
Can the T7256 be used with a transformer that
has a magnetizing inductance of 20 mH
A3:
The echo canceler and tail canceler are opti-
mized for a transformer inductance of approxi-
mately 80 mH and will not work with lower
inductance transformers.
Q4:
Are the Lucent Technologies U-interface trans-
formers available as surface-mount components
A4:
Not at this time.
Q5:
Are there any future plans to make a smaller
height 2-wire transformer
A5:
Due to the rigid design specifications for the
transformer, vendors have found it difficult to
make the transformer any smaller. We are con-
tinuing to work with transformer vendors to see if
we can come up with a smaller solution.
Q6:
The line interface components’ specifications
require 16.9
resistors on the line side of the
transformer when using the 2754H2. For our
application, we would like to change this value.
Can the U-interface line-side circuit be rede-
signed to change the value of the line-side resis-
tors
A6:
Yes. For example, the line-side resistances can
be reflected back to the device side of the trans-
former so that, instead of having 16.9
on each
side of the transformer, there are no resistors on
the line side of the transformer and 24.4
resis-
tors on the device side (16.9
+ 16.9
/N
2
,
where N is the turns ratio of the transformer).
Note that the reflected resistances should be kept
separate from the device-side 16.9
resistors,
and located between VR1 and T1 in Figure 20.
This is necessary because the on-chip hybrid
network (pins HP, HN) is optimized for 16.9
of
resistance between it and the LOP/LON pins.
Q7:
Table 29, T7256 Reference Schematic Parts List,
states the 0.1
μ
F capacitor that is used with the
LH1465 (C15) must have an insulation resistance
of >2 G
. Why
A7:
This capacitor is used to set the gate/source volt-
age for the main transistor in the device. The
charging currents for this capacitor are on the
order of microamps. Since the currents are so
small, it is important to keep the capacitor leak-
age to a minimum.
Q8:
The dc blocking capacitor (C16 in Figure 20)
specified is 1.0
μ
F. Can it be increased to at least
2
μ
F
This value can be increased to 2
μ
F without an
effect on performance. However, for an NT1 to be
compliant with T1.601-1992 Section 7.5.2.3, the
dc blocking capacitor must be 1.0
μ
F
±
10%.
Q9:
Why is the voltage rating on 1
μ
F dc blocking
capacitor (C16 in Figure 20) so high (250 V)
A8:
A9:
In Appendix B of T1.601, the last section states
that consideration should be given to the handling
of three additional environmental conditions. The
third condition listed is maximum accidental ring-
ing voltages of up to –200.5 V peak whose
cadence has a 33% duty cycle over a 6 s period.
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