參數(shù)資料
型號： T7256A
英文描述： Compliance with the New ETSI PSD Requirement
中文描述： 符合新的ETSI PSD的要求
文件頁數(shù)： 87/116頁
文件大?。?/td> 1056K
代理商： T7256A
Data Sheet
January 1998
T7256 Single-Chip NT1 (SCNT1) Transceiver
Lucent Technologies Inc.
83
Questions and Answers
(continued)
U-Interface
(continued)
A12:
(continued)
It is desirable to express the return loss in terms of im-
pedance bounds, since an impedance measurement is
relatively simple to make. From the above equation, up-
per and lower bounds on impedance magnitude can be
derived as follows:
Z
O
= return loss reference impedance = 135
Z
U
= upper impedance curve
Z
L
= lower impedance curve
Upper bound (Z
U
>
Z
O
)
:
RL (dB) = 20 log
Lower bound (Z
L
< Z
O
):
RL (dB) = 20 log
Note that the higher the minimum return loss require-
ment, the tighter the impedance limits will be around
Z
O
, and vice versa.
So, for the upper bound, solve for Z
U
:
For the lower bound, solve for Z
L
:
Plotting the above equations (using 135 for Zo and Fig-
ure 16 in T1.601 for the RL values) results in the graph
shown in Figure 32, which shows the return loss
expressed in terms of impedance upper and lower
bounds.
Q13:
Why must secondary protection, such as a SGS-
Thomson SM6T6V8CA protection diode, be
used
A13:
The purpose of the diode is to protect against
metallic surges below the breakdown level of the
primary protector.
Such metallic surges can be coupled through the
transformer and could cause device damage if
the currents are high. The protector does not pro-
vide absolute protection for the device, but it
works in conjunction with the built-in protection
on the device leads.
The breakdown voltage level for secondary pro-
tection devices must be chosen to be above the
normal working voltage of the signal and typically
below the breakdown voltage level of the next
stage of protection. The SM6T6V8CA has a mini-
mum breakdown voltage level of 6.4 V and a
maximum breakdown voltage of 7.1 V.
The chip pins that the SM6T6V8CA protects are
pins 36 (HP), 31 (HN), 32 (LOP), and 35 (LON).
The 16.9
resistors will help to protect pins 32
and 35, but pins 31 and 36 will be directly
exposed to the voltage across the SM6T6V8CA.
The on-chip protection on these pins consists of
output diodes and a pair of polysilicon resistors.
These pins have been thoroughly tested to
ensure that a 7.1 V level will not damage them;
therefore, no third level of protection is needed
between the SM6T6V8CA and the HP and HN
pins.
The SM6T6V8CA has a maximum reverse surge
voltage level of 10.5 V at 57 A. Sustained cur-
rents this large on the device side of the trans-
former are not a concern in this application.
Thus, there should never be more than 7.1 V
across the SM6T6V8CA, except for possibly an
ESD or lightning hit. In these cases, the T7256 is
able to withstand at least
±
1000 V (human-body
model) on its pins.
Z
Z
U
U
Z
O
–
--------------Z
Z
O
Z
U
Z
L
Z
L
+
–
--------------------
Z
U
Z
O
10
20
1
+
10
20
1
–
----------------------
Z
O
1
-------------------------
10
-20
+
1
10
-20
–
=
=
Z
U
Z
O
10
20
1
10
20
1
+
------------------------
Z
O
1
--------------------------
10
--20
1
10
-20
+
=
=
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