
40
Lucent Technologies Inc.
Data Sheet
April 1998
T7264 U-Interface 2B1Q Transceiver
Appendix A. Questions and Answers
Introduction
The questions and answers are divided into three cate-
gories: U-interface, K2 interface, and miscellaneous.
For detailed application information, also refer to the
application notes Implementation of an ANSI Standard
ISDN NT1 Using the Lucent T7262A/63, T7252A,
T7270, and an 8-Bit Microcontroller and Performance
Factors in Line-Powered 2B1Q Applications.
U-Interface
Q1
:
Is the line interface for the T7264 the same as the
T7262A/63
A1
:
The interface is different in that the T7262A/63
uses a Lucent 2754G transformer (2.5:1 turns
ratio), and the T7264 uses a Lucent 2754H trans-
former (1.5:1 turns ratio).
Other changes to the line interface include
device-side resistors of 16.9
±
1% and line-side
resistors of 16.9
±
7%. Part of the line-side
resistance will likely be a PTC in those applica-
tions requiring power cross protection.
The T7264 device-side protection is unchanged
(521A diodes), and the dc blocking capacitor is
unchanged (Illinois Capacitor* 1.0
μ
F
±
5%).
Line-side protection must be tailored to individual
system needs.
Q2
:
Why is a higher transformer magnetizing
inductance 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 T7264 be used with a transformer that
has a magnetizing inductance of 20 mH
A3
:
The echo canceler and tail canceler are
optimized for a transformer inductance of approx-
imately 80 mH and will not work with a value this
low.
Q4
:
Are the Lucent U-interface transformers 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. We would like to change this value
for our application. Can the U-interface line-side
circuit be redesigned to change the value of the
line-side resistors
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). However,
there may be some performance penalty in this
case since the on-chip hybrid network is opti-
mized for 16.9
of resistance on the device side
of the transformer.
* Illinois Capacitor is a registered trademark of Illinois Capacitor Inc.