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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.
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.
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.