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[Paparazzi-devel] Re: HF interference problems

From: Helmut Wabnig
Subject: [Paparazzi-devel] Re: HF interference problems
Date: Mon, 18 Feb 2008 19:15:30 +0100

On Mon, 18 Feb 2008 12:00:02 -0500, you wrote:

For example:
>Is there a better way to mount the Maxstream 9Xtend, to avoid the RF problem?

Very hard to give a good advice.
Vagabonding HF due to bad grounding design. 

<humor on>
When electronic circuits experience interference problems, 
we take a handful of small capacitors, throw them across the
boards and where the caps land, we solder them.
<\humor off>

All small-signal cables should be shielded, power cables twisted.
Signal inputs should have small capacitors to ground for
HF filtering, or an inductor + cap or a resistor + cap.
Power cables must have HF capacitors on both ends!
Electrolytic caps are not HF qualified.
Cables could be connected via "feed through capacitors"
and loaded with  ferrite beads.
Ground connections between the boards should be short
and made of litz wires. Card cage should be lined  with
copper foil and connected to common ground on  many places.
And so on. Much to  be tried out experimentally.

I would start to shield and filter the most sensitive circuitry.
Find the point of maximal sensitivity.
Switch off and on the various transmitters to identify the source.

Wrap the boards in thin copper sheet, and so on.
Line the complete electronics compartment with thin copper
and connect to ground.
Put a conductive "ground plane" where the antennas are mounted.
Adhesive copper shielding tape is available but you must put
solder dots all over the tape joints. Conductive paint spray
is also available. Create a HF "zero reference" plane where all ground
connections meet. "Star" grounding configuration is less important 
to HF designs, but we must avoid wire loops.

Even the A380 suffers from HF interference problems.

Good luck,

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