T7256 Agere Systems, T7256 Datasheet - Page 34

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

T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
Questions and Answers
U-Interface
A9: (continued)
Q10: What is the purpose of the 3300 pF capacitors
A10: The capacitors are for common-mode noise
30
This statement could be interpreted to mean that
a protector such as VR2 in Figure 11 should not
trip if subjected to a voltage of that amplitude.
This interpretation sets a lower limit on VR2’s
breakover rating. Since capacitor C16 will be
exposed to the same voltage as VR2, its voltage
rating must be greater than the maximum break-
over rating of VR2. This sets an upper limit on the
protector breakover voltage. The result is a need
for a capacitor typically rated at about 250 V.
However, an argument can be made that it
doesn’t matter whether VR2 trips under this con-
dition, since it is a fault condition anyway, and a
tripped protector won’t do any damage to a cen-
tral office ringer.
The only other similar requirement, then, is found
in Footnote 8, referenced in Section 7.5.3 of ANSI
T1.601. The footnote implies that the maximum
voltage that an NT will see during metallic testing
is 90 V. The breakover voltage VR2 must be large
enough not to trip during the application of the
test voltage mentioned in the footnote. This
means that a protector with a minimum breakover
voltage of 90 V can be used that would permit a
capacitor of lower voltage rating (e.g., 150 V) to
be used. This is the approach we currently favor,
although Figure 11 illustrates the more conserva-
tive approach.
(C13 and C14) in Figure 11 in the data sheet?
rejection. The ANSI T1.601 specification contains
no requirements 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 T7234 may be desen-
sitized such that tone detection is not guaranteed
on long loops. The 3300 pF was selected to pro-
vide attenuation of this common-mode noise so
(continued)
(continued)
Q11: Are there any recommended common-mode fil-
A11: The only common-mode filtering parts we have
* Corcom is a registered trademark of Corcom, Inc.
that tone detector sensitivity is not adversely
affected. Since the 3300 pF capacitor was
selected based only on guidelines, it is not man-
datory, but it is recommended in applications
which may be susceptible to high levels of com-
mon-mode noise. The final decision depends on
the specific application.
As for the size of the capacitors, lab tests indicate
the following:
1. The performance of the system suffers no
2. The return loss at 25 kHz increases with
3. The capacitor value has no effect on longitudi-
4. A large unbalance in the capacitor values did
tering parts for the U-interface? I suspect that our
product may have emissions problems, and I
want to include a provision for common-mode fil-
tering on the U-interface.
any data on are two common-mode chokes from
Pulse Engineering (
intended to help protect against external com-
mon-mode noise. The part numbers are
PE-68654 (12.5 mH) and PE-68635 (4.7 mH),
and in lab experiments, no noticeable degrada-
tion in transmission performance was observed.
These chokes are typically effective in the fre-
quency range 100 kHz—1 MHz.
As far as emissions are concerned, we don’t have
a lot of data. We have seen some success with
the use of RJ-45 connectors that have integral
ferrite beads such as those from Corcom*, Inc.,
[phone]. These provide some flexibility in
that they have the same footprint as some stan-
dard RJ-45 connectors.
degradation until the values are increased to
about 0.1 F.
increasing capacitor value.
nal balance.
not affect return loss, longitudinal balance, or
performance.
619-674-8100
Lucent Technologies Inc.
February 1998
) that are