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參數(shù)資料
| 型號： | IBM42F10SNNAA20 |
| 廠商： | IBM Microeletronics |
| 英文描述： | SFF-1063/1250N-SW PTH Serial Optical Transceiver(SFF-1063/1250N-SW PTH串行光纖收發(fā)器) |
| 中文描述： | SFF-1063/1250N-SW普通話系列光收發(fā)器（SFF-1063/1250N-SW普通話串行光纖收發(fā)器） |
| 文件頁數(shù)： | 4/7頁 |
| 文件大?。?/td> | 465K |
| 代理商： | IBM42F10SNNAA20 |
Application Note
SFF-1063/1250N-SW PTH Serial Optical Transceiver Design & Layout Guide
Page 4
optxcvr.fm
July 2000
EMI will be radiated through the bezel and bezel
opening if the SFF transceiver does not make good
multipoint contact to a well-grounded host bezel.
Pin Convention
At this time there is not complete agreement among
optical transceiver manufacturers on the use of
ground pins SC through SF. Some manufacturers
do not offer split-chassis grounding, thus SC through
SF are connected to a one-piece chassis. In this
case, all ground pins CA through SF (although some
pins might be omitted in some industry SFF prod-
ucts) are connected to box chassis ground. Some
manufacturers omit SC through SF from the trans-
ceiver chassis, while others float CA and CB and
retain SC through SF. IBM and several other manu-
facturers offer the split-chassis grounding scheme
whereby the pins are internally connected to logic
ground (See
Split-Chassis Grounds
on page 1.
Host PC board designers must consider these differ-
ences when choosing SFF transceiver suppliers. To
maintain compatibility among the various manufac-
turers, pins SC through SF should be floated to sep-
arate the chassis and logic grounds. For designs
using the IBM transceiver or an IBM-like transceiver
exclusively, SC through SF along with pins 1 and 7
should be connected to chassis ground to provide
maximum shielding and a ground connection for the
high speed logic circuits.
Isolation
Good design practice normally suggests that chas-
sis and logic grounds be tied together at only one
point or in close proximity in an electrical system.
Historically, this tiepoint has been at the power sup-
ply. However, when designing high speed digital
transceiver circuits, one must also consider the
effects EMI, EMC, and ESD in addition to traditional
ground loop problems. To minimize the effects of
crosstalk, radiation, etc., it is sometimes necessary
to connect the grounds at an alternate point in the
system. The proper design of system grounding is
complex and therefore must be carefully considered
during the early stages of system design, whether it
be a small box or many interconnected units. A big
advantage of optical fiber coupling is that from a
complex system standpoint, grounding can be bro-
ken into electrically isolated boxes.
Because the logic and chassis grounds are normally
tied together in the power supply, they should be
isolated at the transceiver whenever possible to
avoid ground loops. If this is not feasible, a 10-15
ohm resistor can be substituted for the direct con-
nection at the power supply, with direct connection
being made elsewhere in the chassis box. This
grounding technique is currently being used by
some computer manufacturers in their enclosure
designs.
If the logic and chassis grounds are not tied together
in the power supply, they can be joined at the com-
munication ports to minimize ground loops. This
approach requires the logic and chassis grounds to
be tied together at the transceiver in split-chassis
designs. Connecting the logic and chassis grounds
at the communication ports has also proven effec-
tive in some designs to minimize EMI radiation and
EMC/ESD susceptibility from the ports.
Enclosure
Because the host enclosure box uses copper inter-
face cables and contains an exposed transceiver
chassis, it poses the greatest risk for EMI/EMC/ESD
problems. To minimize these effects, the enclosure
must make good electrical contact with the conduc-
tive leaf springs around the opening for the trans-
ceiver chassis.
The opening must be in conductive material and be
sized to match the dimensions shown in the
1063/1250 MBd Small Form Factor Transceiver with
Signal Detect engineering specification. Enclosure
ventilation holes must not exceed 0.157in/4mm in
diameter, and internal wiring should be kept away
from the holes.
Cables and Connectors
All metallic interface cables should be shielded or
use in-line filters to prevent EMI and to provide good
EMC/ESD compatibility. To support the use of
unshielded cables outside the enclosure box, UTP
or similar connectors must be used. When using
unshielded cables, wiring inside the enclosure must
be shielded and kept away from the transceiver to
prevent EMI from radiating from the external inter-
face cables.
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