V23832-T2431-M101

V23832-R421-M101

Description

Data Sheet

11

2003-11-19

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.

micro strip, 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

PAROLI module 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

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 module 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 fiber optic module 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 huge

systems.

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.

The bezel opening for a transceiver should be sized so that all contact springs of the

transceiver cage make good electrical contact with the face plate.

Please consider that the PCB may behave like a dielectric waveguide. With a dielectric

constant of 4, the wavelength of the harmonics inside the PCB will be half of that in free

space. Thus even the smallest PCBs may have unexpected resonances.