Integration Guide

OEM TD-CDMA PCI Express Module Integration
May 2013 General Dynamics Broadband Proprietary and Confidential Page 9 of 20
Subject to Non Disclosure Agreement
f. Designers must be aware that PCB connectors contain exposed metal which will radiate and
design and shield accordingly. Provision should be provided for options to link connector
shields to PCB ground if the best way is not clear, or is not known.
3.4.2 Unit Level
The ideal way to suppress RF interferers is to enclose all of the active circuitry in a metal can without
holes or joins. Unit level shielding, either SMT or PTH aims go get as close to an ideally shielded metal
box as possible within the constraints of reality – i.e. functionality and cost.
a. SMT shielding should be used to cover all components on each side of any PCB that might
radiate into the Antenna System.
b. The shielding system should be fully integrated with the PCB surface flood fill ground planes
with small distances between connection points.
c. The shielding system should be un-lidded or have many mechanical connection points around
the perimeter of the lids used.
d. Special attention should be paid to signals that must travel outside of the metal shield to
suppress/filter high speed edges – e.g. R/C filters on LED driving signals.
e. Shielded PCB sockets must be chosen for all interconnect signals and the shield metal must
correctly routed and grounded, separate to any circuit ground or power signals carried within
the connection bus.
f. Wherever possible, always, filter every interconnect signal, inside the metal shields, with the
appropriate filtering components, including the power signals.
g. Whilst instances are much rarer, it should be noted that the UE Transmit signal is capable of
inducing false edges onto unshielded digital signals when they are operating in close proximity
to antennas carrying pulsed, maximum power Tx signals (notably GSM).
3.4.3 Connecting Cables
Choosing the unshielded version of an interconnect but connector system is a false economy if a high
sensitivity RF receiver antenna is nearby. Always use the shielded system from the outset and design
it in correctly as detailed above. The socket and connector shields are as much part of the shielding as
the flood-fill PCB ground planes.
a. If a cable end-connector does not encompass all of the cable cores with a shield then it will
radiate. Ensure the connector is fully shielded.
b. If the drain wire/foil/sheath of a shielded cable is not correctly connected to the metal shield
within the cable-end connector then it will radiate.
c. If both ends of a shielded cable cannot be shielded (e.g. 100m Ethernet) then ensure that it is
at the far end of the cable that the shield is not connected, not the TD-CDMA PEM Antenna
end.
3.4.4 System Level
Some choices made at the system level can directly affect the Receive performance on the TD-CDMA
PEM, as follows.
a. If possible, ensure that the main lobes of the PEM’s antenna system (particularly if it is
directional) are not pointing at either the Host Equipment electronics. Assume that all active
electronics radiate RF Blockers until proven otherwise. E.g. If the User Equipment is placed in
the trunk of a vehicle and the antennas are cabled onto the roof of a car, that’s good.