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參數(shù)資料
| 型號： | T7234A |
| 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)： | 96/116頁 |
| 文件大小： | 1056K |
| 代理商： | T7234A |
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Data Sheet
January 1998
T7256 Single-Chip NT1 (SCNT1) Transceiver
92
Lucent Technologies Inc.
Questions and Answers
(continued)
S/T-Interface
(continued)
A36:
(continued)
T7903/T7250C to T7256 Direct Connect with
External S/T-Interface Provided
First, we need to address the issue of the trans-
former turns ratio.
T7903: The T7903 uses a 2.0:1 transformer, and
the T7256 uses a 2.5:1 transformer. It is desirable
to be able to use a dual transformer, so we want
the transmit- and receive-side transformers to
have the same turns ratio. Also, it may be desir-
able to use a product with this arrangement as
just a TE (with an external NT1, i.e., no U-Inter-
face connected to the integrated NT1). Therefore,
we will select a 2.0:1 turns ratio transformer to
ensure T7903 pulses of sufficient amplitude on
the line side of the transformer and ensure that
an external transmitter won’t overdrive the T7903
receiver inputs.
T7250C: The T7250C and T7256 both use a
2.5:1 transformer, which simplifies the analysis
for this case.
T7903/T7250C Transmit to T7256 Receive
a) Transmitter Load:
If we use the same S/T transmitter line inter-
face circuit as in the normal (stand-alone TE)
case, the transmitter will see the load that it
expects to drive and is thus optimized in terms
of the load. The 100
terminations must be
user selected per the following table:
b) Receiver Levels:
The T7256 S/T line interface transformer has
a turns ratio of 2.5. The T7256 receiver thus
expects to see nominal pulse levels of 750 mV
x 2.5 = 1.875 V at the device side of the trans-
former.
T7903: The T7903 transmitter (or an external
TE on a 0-length loop) will drive 750 mV
pulses on the S/T line, and that voltage
reflected back to the device side of the trans-
former is 750 mV x 2.0 = 1.5 V. If the T7256
receiver is connected to the device side of the
transformer as shown in Figure 39, it will see
1.5 V instead of 1.875 V when a 750 mV
pulse is present on the line. Thus, there is an
inherent pulse attenuation in this scheme of
1.9 dB at the T7256 receiver.
We need to be sure that the receiver will have
adequate sensitivity to detect pulses from an
external TE that is some distance away. Refer-
ring to ITU I.430, this circuit can only be used
in a short passive bus (SPB) mode when
using the onboard NT1, because there is a
local TE (the T7903), so any external TE that
is also used will result in a passive bus config-
uration. ITU-T I.430 states that the maximum
attenuation in SPB configuration is 3.5 dB.
Combining this with the inherent 1.9 dB atten-
uation results in a total possible signal attenu-
ation of 5.4 dB. The receiver must have a
sensitivity of at least 7.5 dB per ITU-T I.430
Section 8.6.2.3, so 5.4 dB attenuation will
present no problem in this case.
T7250C: The T7250C transmitter (or an exter-
nal TE on a 0-length loop) will drive 750 mV
pulses on the S/T line, and that voltage
reflected back to the device side of the trans-
former is 750 mV x 2.5 = 1.875 V. If theT7256
receiver is connected to the device side of the
transformer as shown in Figure 40, it will see
the 1.875 V pulse level it expects when a
750 mV pulse is present on the line.
c) Receiver Bias:
In the T7903 to T7256 Direct Connect section
we showed that the receiver is biased by
about 0.67 dB from nominal due to the direct
connect of the T7903 to the T7256. Assuming
the receiver sensitivity decreases by this
much and combining this with the maximum
5.4 dB attenuation found in the previous sec-
tion results in a total of 6.07 dB of required
sensitivity, which is still within the 7.5 dB
requirement on the receiver.
Configuration
JMP1
JMP2
Integrated NT1 Used as NT1
(No External NT1 Connected)
No External TE
Connected
Unterminated
External
TE Connected
Terminated (100
)
External TE
Connected
Installed
Installed
Installed
Installed
Installed
Not
Installed
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