T7256 Agere Systems, T7256 Datasheet - Page 33

• Manufacturer Part Number: T7256
• Description: (T7234 - T7256) Compliance
• Manufacturer: Agere Systems

Lucent Technologies Inc.
February 1998
Questions and Answers
Introduction
This section is intended to answer questions that may
arise when using the T7234 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 T7234 the same as for
A1: Yes. The U-interface section on these chips is
Q2: Why is a higher transformer magnetizing induc-
A2: It has been determined that a higher inductance
Q3: Can the T7234 be used with a transformer that
A3: The echo canceler and tail canceler are opti-
Q4: Are the Lucent Technologies U-interface trans-
A4: Not at this time.
Q5: Are there any future plans to make a smaller
A5: Due to the rigid design specifications for the
the T7264?
identical, so their line interfaces are also identical.
tance used (as compared to other vendors)?
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.
has a magnetizing inductance of 20 mH?
mized for a transformer inductance of approxi-
mately 80 mH and will not work with lower
inductance transformers.
formers available as surface-mount components?
height 2-wire transformer?
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.
T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
Q6: The line interface components’ specifications
A6: Yes. For example, the line-side resistances can
Q7: Table 5, T7234 Reference Schematic Parts List,
A7: This capacitor is used to set the gate/source volt-
Q8: The dc blocking capacitor (C16 in Figure 11)
A8: This value can be increased to 2 F without an
Q9: Why is the voltage rating on 1 F dc blocking
A9: In Appendix B of T1.601, the last section states
require 16.9
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?
be reflected back to the device side of the trans-
former so that, instead of having 16.9
side of the transformer, there are no resistors on
the line side of the transformer and 24.4
tors on the device side (16.9
where N is the turns ratio of the transformer).
Note that the reflected resistances should be kept
separate from the device-side 16.9
and located between VR1 and T1 in Figure 11.
This is necessary because the on-chip hybrid
network (pins HP, HN) is optimized for 16.9
resistance between it and the LOP/LON pins.
states the 0.1 F capacitor that is used with the
LH1465 (C15) must have an insulation resistance
of >2 G . Why?
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.
specified is 1.0 F. Can it be increased to at least
2 F?
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 F
capacitor (C16 in Figure 11) so high (250 V)?
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.
resistors on the line side of the
+ 16.9 /N
resistors,
10%.
on each
2
resis-
,
of
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