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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)： | 95/116頁 |
| 文件大?。?/td> | 1056K |
| 代理商： | T7256A |
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Data Sheet
January 1998
T7256 Single-Chip NT1 (SCNT1) Transceiver
Lucent Technologies Inc.
91
Questions and Answers
(continued)
S/T-Interface
(continued)
A36:
(continued)
T7256 Transmit to T7903/T7250C Receive
a) Transmitter Load:
The T7256 S/T line interface transformer has
a turns ratio of 2.5, and the transmitter drives
a line-side load of 50
. Reflecting this imped-
ance to the device side of the transformer
results in 312.5
(50
x N
2
). This resis-
tance, combined with the 242
total resis-
tance of the device-side resistors, results in a
total of 554.5
that the transmitter typically
drives. So, to optimize the transmitter part of
the circuit based on the load the transmitter
expects to drive, the transmitter should see a
total resistance of approximately 554.5
.
b) Receiver Levels:
T7903: The S/T line interface transformer has
a turns ratio of 2.0. The receiver expects to
see nominal pulse levels of 750 mV x 2.0 =
1.5 V. The T7256 transmitter circuit is a cur-
rent source of 6.0 mA. To generate a voltage
of 1.5 V with 6.0 mA requires a resistance of
1.5/0.006 = 250
.
T7250C: The S/T line interface transformer
has a turns ratio of 2.5. The receiver expects
to see nominal pulse levels of 750 mV x 2.5 =
1.875 V. The T7256 transmitter circuit is a cur-
rent source of 6.0 mA. To generate a voltage
of 1.875 V with 6.0 mA requires a resistance
of 1.875/0.006 = 312.5
.
c) Resistor Selection:
In this section, the term receiver implies not
only the receive section on the chip, but also
the external 10 k
resistors connected to the
receiver. These resistors remain unchanged
from the standard line interface circuit in order
to maintain the same total receiver imped-
ance.
T7903: Ideally, the T7256 transmitter should
be driving into 554.5
, and the T7903
receiver wants to see the levels that would
result if the transmitter drove 6 mA through
250
. So, the total transmit path resistance
should be divided into three resistors. The first
is the resistor across which the receiver is
connected and should be approximately
250
so that the receiver sees the correct
levels. A standard 249
value is adequate for
this case. The remainder of the 554.5
should be divided equally between two other
series resistors in the transmit path, and
(554.5
– 249
)/2 is 152.7
, so 150
is
chosen for the two other series resistors as
illustrated in Figure 37.
T7250C: Ideally, the T7256 transmitter should
be driving into 554.5
, and the T7250C
receiver wants to see the levels which would
result if the transmitter drove 6 mA through
312.5
. So, the total transmit path resistance
should be divided into three resistors. The first
is the resistor across which the receiver is
connected and should be approximately
312.5
so that the receiver sees the correct
levels. A standard 309
value is adequate for
this case. The remainder of the 554.5
should be divided equally between two other
series resistors in the transmit path, and
(554.5
– 309
)/2 is 122.6
, so 121
is
chosen for the two other series resistors as
illustrated in Figure 38.
d) Receiver Bias:
The receiver bias is not an issue for the same
reasons discussed in the T7903/T7250C
Transmit to T7256 Receive section.
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