參數(shù)資料
型號: T7264
英文描述: T7264 U-Interface 2B1Q Transceiver
中文描述: T7264 U型接口2B1Q收發(fā)器
文件頁數(shù): 43/54頁
文件大?。?/td> 876K
代理商: T7264
Lucent Technologies Inc.
43
Data Sheet
April 1998
T7264 U-Interface 2B1Q Transceiver
Appendix A. Questions and Answers
(continued)
Q15
: ITU-T specification K.21 describes a lightning
surge test for NT1s (see Figure 1/K.21 and
Table 1/K.21, Test #1) in which both Tip and Ring
are connected to the source and a 1.5 kV voltage
surge is applied between this point and the
GND of the NT1. What are the protection consid-
erations for this test Are the HP and HN pins
susceptible to damage
A15
: The critical component in this test is the trans-
former since its breakdown voltage must be
greater than 1.5 kV. Assuming this is the case,
the only voltage that will make it through to the
secondary side of the transformer will be prima-
rily due to the interwinding capacitance of the
transformer coils.
This capacitance will look like an impedance to
the common-mode surge and will therefore limit
current on the device side of the transformer. The
device-side voltage will be clamped by the 521A
device. The maximum breakdown voltage of the
521A is 8 V. The 16.9
resistors will help protect
the LOP and LON pins on the T7264 from this
voltage. However, this voltage will be seen
directly on pins 36 and 31 (HP and HN) on the
T7264. The on-chip protection on these pins con-
sists of output diodes and a pair of polysilicon
resistors. These pins have been thoroughly
tested to ensure that an 8 V level will not damage
them; therefore, no third level of protection is
needed between the 521A and the HP and HN
pins.
Q16
: How is it possible to guarantee the
±
0.35 dB
power spectrum tolerance on the U-interface
A16
: By trimming to
±
0.1 dB; the rest of the variation is
for power supply, temperature, and aging.
Q17
: Is the
±
100 ppm free-run frequency (NT mode)
recommendation met in the T7264
A17
: In the free-run mode, the output frequency is
primarily dependent upon the crystal, not the sili-
con design. For low-cost crystals, initial toler-
ance, temperature, and aging effects may
account for two-thirds of this budget, and just a
couple of pF of variation in load capacitance will
use up the rest. Thus, the
±
100 ppm goal can be
met if the crystal parameters are well controlled.
See the Crystal Characteristics information in the
data sheet (Table 26).
Q18
: It has been noted in some other designs that the
crystal has a capacitor from each pin to ground.
Changing these capacitances allows the fre-
quency to be adjusted to compensate for board
parasitics. Can this be done with the T7264 crys-
tal Also, can we use a crystal from our own
manufacturer
A18
: The crystal for the T7264 is tuned to a particular
load capacitance that does not include external
capacitors. The advantage to this is that no exter-
nal components are required. The disadvantage
is that board parasitics must be very small. The
data sheet notes that the board parasitics must
be within the range of 0.6 pF
±
0.4 pF. Lucent
does not require that a particular crystal be used,
but we strongly recommend adhering to the crys-
tal parameters specified in the data sheet. A
crystal which deviates from these parameters
can work under most conditions, but we cannot
guarantee that it will start up and/or meet the
±
100 ppm requirement under all operating condi-
tions.
Q19
: Is there a test pin available for generating a +3/–3
sequence so that pulse templates can be
measured easily
A19
: There is no such pin. But a sequence of four +3s,
followed by four –3s (and so on) is the 10 kHz
wakeup tone (TN, TL in ANSI T1.601, Figure 17)
that each transceiver outputs when initiating a
start-up. This tone is produced when the K2 bit,
istp in the dc octet, is asserted. At the NT, this
tone lasts for 9 ms (90 pulses), and at the LT, it
lasts for 3 ms (30 pulses). This should be long
enough to capture the data on a digitizer or stor-
age scope for analysis.
Q20
: What are the average cold start and warm start
times
A20
: Lab measurements have shown the average cold
start time to be about 3.3 s—4.2 s over all loop
lengths, and the average warm start time to be
around 125 ms—190 ms over all loop lengths.
Q21
: What is the U-interface’s response time to an
incoming wakeup tone from the LT
A21
: Response time is about 1 ms.
Q22
: What is the minimum time for a U-interface
reframe after a momentary (<480 ms) loss of
synchronization
A22
: Five superframes (60 ms).
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