People doing stuff like cell phones and wireless internet stuff are learning
a lot about how to control RF on PC boards.  They have to so that their
processors won't kill their own radio receivers, and their radio
transmitters won't kill their own processors.

The basic trick is to keep the high frequencies where they are needed and
never let them out any further than necessary.  I'll bet that if you look at
the boards in the iMac, you won't see very many traces because the outer
layers of the multilayer boards are all power and ground planes, which
effectively shields all the interconnects.  That alone, plus a little care
around the wires going in and out is often enough to meet the FCC emission
requirements, which are, after all, not really all that stringent.

Another trick that seems like cheating but is legal is "spread spectrum"
clocks.  Here the clock frequency is intentionally dithered a little (i.e.
frequency modulated with noise).  That way the emissions show up on a
spectrum analyzer as a broad hump rather than a narrow line.  Since the same
energy is spread over a wider bandwidth, its peak intensity is lower, and
that's what is spec'ed by the FCC.  Supposedly that is OK because it is the
peak intensity that matters most as far as interference with intentional
signals is concerned.

> -----Original Message-----
> From: Randy Glenn [mailto:picxpert@YAHOO.COM]
> Sent: Saturday, August 05, 2000 9:40 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [EE]: Grinning, Macs, and the FCC
>
>
> I know for certain that the analog board is shielded. It's located
> underneath.
>
> Also, the bus isn't running at 450 MHz - it's running at 100MHz, as I
> recall. The chip (and, with the G3 processor) cache are
> running at 450MHz
> and 225MHz, respectively (a G3's cache can run at the speed
> of the chip, but
> you'd have to pay for memory that fast)
>
> Those factors mean that the problem of shielding probably
> isn't as bad as it
> would be otherwise, but yes, it still does need some
> shielding. My theory is
> that the carbon in the polycarbonate shell is helping with
> that - carbon is
> conductive, right?
>
> -Randy Glenn
> PICxpertANTISPAM@picxpert.com (remove ANTISPAM)
> http://www.picxpert.com/ << WORKING NOW!
>
> Those packing a big grudge, usually pack a big mouth along with it.
>
> -----Original Message-----
> From: pic microcontroller discussion list
> [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Dan Michaels
> Sent: Monday, July 31, 2000 9:46 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: [PICLIST] [EE]: Grinning, Macs, and the FCC
>
>
> Anybody who has seen Steve Jobs lately, of course, notes his
> trademark ear-to-ear grin. Apple is doing well, and he keeps
> birthing sleek new Mac designs. Getting to the point:
>
> Apple's new marketing point is "transparent" iMacs and iCubes.
>
> Question is, how does a 450 Mhz machine with a transparent
> plastic case pass FCC EMC requirements? I may be wrong, but my
> impression is that you could not embed enough conductive material
> into something that "appears" to be transparent to be an effective
> shield. So, possibly, internal shielding in exactly the critical
> places?
>
> Also, on a more philosophical level, is it ever possible for a
> "bare" unshielded pcb [eg, multilayer or not], with chips
> running at say 20+ Mhz, to pass FCC reqs? Or does it almost
> always require some local shielding or a shielded case? For
> that matter, will "local" shielding ever actually do it?
>
> best regards,
> - Dan Michaels
> Oricom Technologies
> http://www.sni.net/~oricom
> ==========================
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