[SI-LIST] Re: Copper Fill
- From: Jim Freeman <freeman@xxxxxxxxxxx>
- To: gtang@xxxxxxxx
- Date: Mon, 17 Jun 2002 11:51:26 -0700
Hi George,
You are correct that megnetic fileds don't terminate to anything, but
remember that the (Closed integral of B(dot)dl=u0I, Where I is the total
current enclosed by the closed integral.
Thanks
Jim Freeman
George Tang wrote:
> 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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