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Shielding and grounding - RF Cafe Forums
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jcosper
Post subject: Shielding and grounding Posted: Mon Sep 29,
2008 12:20 pm
Lieutenant
Joined: Mon Sep 29, 2008 11:50
am
Posts: 1
Being new to RF design from a digital background,
my first design at a new company has me a little troubled. The RF engineers
want to ground the shield around the RF cavities to the ground referenced
by the RF traces (analog ground). The shield is then physically connected
to the chassis for thermal dissipation. From a signal quality perspective,
this creates a ground loop with the ground tied at multiple points to
the chassis.
The thermal dissipation is required, as this unit
will generate lots of heat. The RF engineers were not happy when I wanted
to isolate chassis ground from analog ground since it would compromise
the Faraday cage (the analog and chassis ground would be connected at
the output of the power supply -- standard "star" system layout). They
seem perfectly happy with the grounding situation, which contradicts
common practice.
This cannot be the first time that an issue
like this has arisen. How is shielding and grounding typically handled
in RF design? How should the design be handled if the shield is also
being used as a thermal path to the chassis? Are there electrically
isolated (but thermally shorted) shields that can be constructed or
COTS? Thanks in advance for relaying your experiences.
Jay
Top
IR
Post subject: Posted: Thu Oct 02, 2008 3:24
pm
Site Admin
Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Hello Jay,
You think in
digital concept, which is not true for RF.
In RF the ground
is VERY important and crucial for the good operation of the circuit.
A good design practice is to ground any plane which does not
contain RF signal (As wide as possible) and a complete uniterrupted
GND layer underneath the RF signals. The GND planes on the top layer
should be connected to the GND layer with as many vias as possible.
The shielded cans which you describe should also be connected
to GND planes and GND layer through vias. The cans are a good mean to
dissipate the heat. There are also shielded cans with holes on the cover
to better dissipate the heat generated inside the can.
If you
want to separate digital GND from RF GND, it is a good idea to form
2 different planes for both grounds (Either on the same layer or in
different layers) and to connect them near the power supply. If there
are high current flowing, you can do the connecton in several locations
on the board by using 0-ohm resistors or coils.
Posted 11/12/2012
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