Post subject: Shielding and grounding Posted: Mon Sep
29, 2008 12:20 pm
Joined: Mon Sep 29,
2008 11:50 am
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.
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
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.
Post subject: Posted: Thu
Oct 02, 2008 3:24 pm
Joined: Mon Jun
27, 2005 2:02 pm
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.