T7256 Agere Systems, T7256 Datasheet - Page 35

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

Lucent Technologies Inc.
February 1998
Questions and Answers
U-Interface
Q12: I am planning on using a Raychem PTC (p/n
A12: The NT1 impedance limits looking into tip/ring are
TR600-150) on the U-interface of the T7234 as
shown in Figure 11. The device is rated at 6 —
12 . I am concerned about the loose tolerance
on the PTC resistance. Will I be able to pass the
return loss requirements in ANSI T1.601 Section
7.1?
derived from the T1.601 return loss requirements
(Figure 9 in T1.601). At the narrowest point in the
templates, the permissible range is between
111
reduce the impedance margin somewhat, but
should still be acceptable.
10000
1000
100
to 165 . The tolerance on the PTC will
10
1
(continued)
1.0
1.4
2.0
2.8
(continued)
Figure 15. Transceiver Impedance Limits
4.0
5.6
T7234 Single-Chip NT1 (SCNT1) Euro-LITE Transceiver
7.9
FREQUENCY (kHz)
UPPER BOUND > 165
LOWER BOUND < 110.4
11.2
15.8
Figure 15 is derived from the return loss template
in ANSI T1.601. Return loss is a measure of the
match between two impedances on either side of
a junction point. The following equation is an
expression of return loss in terms of the complex
impedances of the two halves of the circuit Z
When the impedances are not matched, the junc-
tion becomes a reflection point. For a perfectly
matched load, the return loss is infinite, whereas
for an open or short circuit, the return loss is zero.
The return loss expresses the ratio of incident to
reflected signal power and should consequently
be fairly high.
22.4
31.6
RL (dB) = 20 log
44.7
63.1
89.1 125.9 177.8 251.2
Z
------------------ -
Z
1
1
+
–
Z
Z
2
2
5-4056 (C)
1
, Z
31
2.