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參數(shù)資料
| 型號： | T7264 |
| 廠商： | Lineage Power |
| 英文描述： | U-Interface 2B1Q Transceiver(U接口 2B1Q收發(fā)器) |
| 中文描述： | U型接口2B1Q收發(fā)器（ü接口2B1Q收發(fā)器） |
| 文件頁數(shù)： | 41/54頁 |
| 文件大?。?/td> | 876K |
| 代理商： | T7264 |
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Lucent Technologies Inc.
41
Data Sheet
April 1998
T7264 U-Interface 2B1Q Transceiver
Appendix A. Questions and Answers
(continued)
Q7
:
Why are the line-side resistors (16.9
) rated at
7% tolerance in Figure 4 of the data sheet
A7
:
This tolerance was determined based on
analysis of the output voltage tolerance of the
T7262A/63 solution as compared to the T7264
solution, taking into account the tolerances of the
transformer, resistors, etc. It is not a standard
resistor tolerance, such as 5%, because as much
flexibility as possible was desired. The 16.9
resistor is often a combination of a resistor and a
positive temperature coefficient (PTC) surge-
protection resistor, and PTCs generally have
wider tolerances.
Some 2B1Q manufacturers try to match the
PTCs in such a way that their variations from the
nominal value are in the same direction. How-
ever, it is important to use as tight a tolerance as
possible for the following reasons:
I
The 16.9
resistor is also part of the termina-
tion impedance looking into Tip/Ring. ANSI
specifies this impedance at 135
nominal, and
the impedance template over the specified
frequencies is derived from the return loss
requirement (T1.601, Sections 7.1, 7.2, and
Figure 19). A tighter tolerance on the 16.9
resistors allows more margin on the return loss
template.
I
If resistors are unmatched, longitudinal balance
(described in T1.601, Section 7.3) may be
affected. Designers can experimentally deter-
mine the maximum tolerance allowed in their
system by varying the 16.9
resistors slightly
and observing the effect on the parameters
mentioned above.
Q8
:
The dc blocking capacitor specified is 1
μ
F. Can it
be increased to at least 2
μ
F
A8
:
Yes. This value can be increased without any det-
rimental effect.
Q9
:
The application diagram in the data sheet shows
the loopback relay on the line side of the trans-
former. Why is it not shown on the device side
A9
:
The relay can be located on the device side
between the 521A surge protector and the trans-
former, and the local loopback will function prop-
erly. It is shown on the line side because in many
applications the relay is a DPDT type and is also
used for metallic access to some line testing
equipment (at an LT, for example).
Q10
: If the relay is on the line side, what type should it
be
A10
: Relay type is application-dependent. An example
of a relay which would work in most cases is an
Aromat* DS Series 2 A relay.
Q11
: Clarify the meaning of the note concerning the
3000 pF capacitors in the U-line interface figure
(Figure 4).
A11
: The capacitors are not absolutely required. The
ANSI T1.601 specification contains no require-
ments on longitudinal noise immunity. Therefore,
these capacitors are not required in order to meet
the specification. However, there are guidelines
in IEC 801-6 which suggest a noise immunity of
up to 10 Vrms between 150 kHz and 250 MHz. At
these levels, the 10 kHz tone detector in the
T7264 may be desensitized such that tone detec-
tion is not guaranteed on long loops. The
3000 pF was selected to provide attenuation of
this common-mode noise so that tone detector
sensitivity is not adversely affected. Since the
3000 pF capacitor was selected based only on
guidelines, it is not mandatory, but it is recom-
mended in applications which may be susceptible
to high levels of common-mode noise. The final
decision depends on the specific application.
* Aromat is a registered trademark of Aromat Corporation.
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