V23848-M15-C56 Datasheet, PDF (17/29 Pages) Infineon Technologies AG – iSFP-Intelligent Small Form-factor Pluggable 1.25 Gigabit Ethernet 2.125/1.0625 Gbit/s Fibre Channel Single Mode 1310 nm Transceiver with LC Connector

V23848-M15-C56
Application Notes
Application Notes
EMI Recommendations
To avoid electromagnetic radiation exceeding the required limits set by the standards,
please take note of the following recommendations.
When Gigabit switching components are found on a PCB (e.g. multiplexer,
serializer-deserializer, clock data recovery, etc.), any opening of the chassis may leak
radiation; this may also occur at chassis slots other than that of the device itself. Thus
every mechanical opening or aperture should be as small as feasible and its length
carefully considered.
On the board itself, every data connection should be an impedance matched line (e.g.
strip line or coplanar strip line). Data (D) and Data-not (Dn) should be routed
symmetrically. Vias should be avoided. Where internal termination inside an IC or a
transceiver is not present, a line terminating resistor must be provided. The decision of
how best to establish a ground depends on many boundary conditions. This decision
may turn out to be critical for achieving lowest EMI performance. At RF frequencies the
ground plane will always carry some amount of RF noise. Thus the ground and V_CC
planes are often major radiators inside an enclosure. As a general rule, for small systems
such as PCI cards placed inside poorly shielded enclosures, the common ground
scheme has often proven to be most effective in reducing RF emissions. In a common
ground scheme, the PCI card becomes more equipotential with the chassis ground. As
a result, the overall radiation will decrease. In a common ground scheme, it is strongly
recommended to provide a proper contact between signal ground and chassis ground at
every location where possible. This concept is designed to avoid hotspots which are
places of highest radiation, caused when only a few connections between chassis and
signal grounds exist. Compensation currents would concentrate at these connections,
causing radiation. However, as signal ground may be the main cause for parasitic
radiation, connecting chassis ground and signal ground at the wrong place may result in
enhanced RF emissions.
For example, connecting chassis ground and signal ground at a front
panel/bezel/chassis by means of a fiber optic transceiver/cage may result in a large
amount of radiation especially where combined with an inadequate number of grounding
points between signal ground and chassis ground. Thus the transceiver becomes a
single contact point increasing radiation emissions. Even a capacitive coupling between
signal ground and chassis ground may be harmful if it is too close to an opening or an
aperture. For a number of systems, enforcing a strict separation of signal ground from
chassis ground may be advantageous, providing the housing does not present any slots
or other discontinuities. This separate ground concept seems to be more suitable in large
systems where appropriate shielding measures have also been implemented. The return
path of RF current must also be considered. Thus a split ground plane between Tx and
Rx paths may result in severe EMI problems.
Data Sheet
17
2004-06-23