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The original drawing was a bit hard to read, I've attached another...
Kevin
-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
Kevin Stallard
Sent: 2009-03-04 18:15
To: lml@lancaironline.net
Subject: [LML] Re: TruTrak autopliot and RF
>The principle is that each pin has a capacitor between the top of the
pin >(input) and ground (case) to shunt rf energy to ground.
Ground can be very ambiguous. What you are doing is creating a low
impedance path back to the source (generator) of the signal. It is
returning to the unit from which the signal originated, and it's trying
to do it with the least impedance, *not necessarily the shortest path*.
When signals have a frequency greater than 10 Khz, the return signal
likes to be as close as possible to the outbound signal. If fact, on a
circuit board, with a copper ground plane, HF (> 10 Khz) return signals
will follow the traces carrying the outgoing signal above the plane,
they don't fill the entire plane. I've attached a drawing illustrating
this.
If the return signal is forced to move away from the outgoing signal, a
loop is created. This loop becomes an antenna and will radiate the
signal outwards to be picked up by other devices.
My understanding is that shielding is the only way to protect against
radiated noise as it affects both the source and return paths of signals
on victim device.
If you are creating a low impedance path for signals to return to their
source, I wonder if the noise isn't conducted in some way.
Kevin
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