[SI-LIST] Re: High Impedance Traces are prone to Radio Frequency Interference

I am not sure where your information comes from but I don't really agree =
with it. Heres some food for thought...

Draw two simple series circuits near each other one consisting of =
source, source resistance a track then a load resistasnce. The other =
circuit having a source resistance a track then a load resitance. Lets =
call one the source and the other the victim. Now first off link the two =
tracks in the middle of the circuits together with a capacitor to model =
parasitic capacitance or electric field coupling between the two =
circuits. You will find that the maximum signal transfer from one =
circuit to the other occurs with a high impedance circuit at the victim.

Now change the coupling so you have an inductor in series in each =
circuit which is coupled to the other circuit (transformer coupling if =
you like). This time when you do the sums you will find that the low =
impedance at the victim circuit is the one to avoid. This coupling =
mechanism is magnetic of H field coupling. This occurs with low =
impedance high current things near each other.

There is one other type of coupling which you need to be aware of. This =
is antenna coupling. In very simple terms you will have structures on =
your board that are resonant at certain frequencies. At this frequency =
the structure behaves like a good antenna to signals at the resonant =
frequency and is therefore prone to pick up or to radiate energy at that =
frequency.=20

The High impedance problem is normally quite easy to avoid given that =
electric fields are easily screened by thin metal screening cans around =
any circuits prone to this affect. The low impedance magnetic field =
problem on the other hand can be a little more problematic. If the =
energy is low frequency energy the shielding required can be =
inpractically large.=20

In all cases the best way out is to put physical distance between source =
and victim. You will also note if you have done the sums on the circuits =
mentioned above that the source and load impedances can be changed to =
help.=20

We could go of forever on this one

The subject is covered will in all good EMC / RF books

Hope my input gave you a start

Steve Rogers B.Eng (Hons) C.Eng IEE
RF Design Engineer

Micromill Electronics Limited
Leydene House
Waterberry Drive=20
Waterlooville Hampshire
PO7 7XX

Tel: +44 (0) 23 9236 6600
Fax: +44 (0) 23 9236 6673

Registered No. 1456922 (England). =20
Registered Office Brook Road
Wimborne, Dorset BH21 2BJ

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-----Original Message-----
From: Lim Guan Choon [mailto:guanchoon.lim@xxxxxxxxxxxxxxxx]
Sent: Friday, September 26, 2003 9:31 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] High Impedance Traces are prone to Radio Frequency
Interference


Hi, all.   Could anyone help to explain why high impedance traces are =
=3D
prone to Radio frequency interference?  I have come across this kind of =
=3D
remark in articles,  but no explanation why it is so. Please help.

Warmest regards

Lim
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