[SI-LIST] Re: Copper Fill

  • From: "George Tang" <gtang@xxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 17 Jun 2002 10:21:23 -0700

Hi Mike,

Yes, you can reduce the radiated emission of a microstrip by using ground
guards.  This method increases the mutual inductance between the signal
conductor and its return current conductor(s).  But I do not call this as
"terminating the fields."  Electric field start from a positive charge and
terminates to a negative charge, but magnetic fields do not terminate to
anything.  Further, if you have a monopole above a ground plane, the charges
and image current on the ground plane increases the radiation efficiency.  I
believe the phenomenon you are talking about is not "terminating the
fields," but minimizing the current loops.

George



-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of MikonCons@xxxxxxx
Sent: Sunday, June 16, 2002 12:23 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Copper Fill


In a message dated 6/14/2002 7:57:23 PM Pacific Standard Time,
gtang@xxxxxxxx
writes:


> 5. Some also mentioned that the copper fill can be used to terminate
> electromagnetic waves to reduce emi emission.  This idea rarely works.  We
> know that a signal on a transmissionline can be terminated by a matched
> impedance load.  A copper fill with vias tied to ground has an impedance
> varying with frequency.  This is a resonating structure at best.  EMI
> emissions from digital circuits occur at multiple frequencies, and these
> frequencies can drift with temperature and / or change due to
manufacturing
> tolerances of discrete components (capacitors).  Using a resonating
> structure for impedance matching at multiple frequencies that may drift
> with
> temperature is a task that few have even attempted.  And even fewer have
> succeeded.
>
Hi, George:

As you know, it is sometimes difficult to place all-inclusive comments in
the
difficult and complex SI domain. One would have to write a comprehensive
technical article for each response to be complete. To eleborate on my
earlier comments (in one area only), I offer the following.

Your (item 5) comments are understood and appreciated as to their cautions.
However, the nearby presence of a conducting structure that is connected to
the reference of an emitting source indeed reduces emissions considerably.
One must indeed take care to have sufficient ground vias, unevenly spaced to
avoid resonant symmetries, on any grounded fill. The dimensions must
consider
the highest frequencies (harmonics) of interest. A good illustration of this
suppressive effect can be observed by comparing a simple 50-Ohm microstrip
(one inch long will do just fine) carrying a 66 MHz clock with 1 ns
rise/fall
times. Using any number of radiated emission computation methods, you can
predict maximum radiated emissions (ignoring any enclosure) from this
surface
trace that exceed the CISPR Class A limit at 66 MHz by approximately 5 dB.
Simply by having multiple adjacent circuits (as is common on normal PC
boards, the emissions can be reduced by 8 to 15 dB. A similar (but directly
related) demonstration is to measure an emissions reduction of 6 to 8 dB
(typical) offered by the addition of grounded guard traces on each side of
the emitting trace. The crosstalk between multiple signal traces with and
without guard traces exhibit a 12 to 14 dB reduction because of the field
termination effect.

Mike

Michael L. Conn
Owner/Principal Consultant
Mikon Consulting
Cell: (408)821-9843

                   *** Serving Your Needs with Technical Excellence ***


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