🚀 go-pugleaf

On Tue, 10 Jan 2012 18:27:01 -0800 (PST), George Herold
<gherold@teachspin.com> wrote:

>On Jan 10, 12:09�pm, Phil Hobbs
><pcdhSpamMeSensel...@electrooptical.net> wrote:
>> On 01/10/2012 01:47 AM, David L. Jones wrote:
>>
>> > Just thought some might be interested in hearing our recent guest on
>> > our Amp Hour radio show, Dr Howard Johnson of "Handbook of Black
>> > Magic" fame:
>> >http://www.theamphour.com/2012/01/10/the-amp-hour-77-winsome-waveform...
>>
>> > Lots of history of the book, signal integrity, Gigabit Ethernet
>> > development, 90 degree PCB bends, audiophoolery, and a whole bunch of
>> > other talk over almost 1 1/12 hours.
>>
>> > Dave.
>>
>> HJ has a mixture of good stuff and complete nonsense.
>>
>> I hope you grilled him a bit about his less sensible stuff, like that
>> hilariously erroneous video where he attempts to use electrostatics to
>> demonstrate that ground currents want to flow under traces. �Of course,
>> the impedance levels are millions of times too high for inductance to be
>> a factor in that demo, but never mind. � �Have a look athttp://www.sigcon.com/Pubs/news/8_08.htmfor the whole ridiculous thing.
>>
>> (The current path is determined by corona discharge at the sharp corners
>> of the metal squares--if you look at all closely, you'll see that the
>> discharges always want to go out the corners of the squares.)
>>
>> Cheers
>>
>> Phil Hobbs
>>
>> --
>> Dr Philip C D Hobbs
>> Principal Consultant
>> ElectroOptical Innovations LLC
>> Optics, Electro-optics, Photonics, Analog Electronics
>>
>> 160 North State Road #203
>> Briarcliff Manor NY 10510
>> 845-480-2058
>>
>> hobbs at electrooptical dot nethttp://electrooptical.net
>
>That is pretty silly.
>But certainly it's a good idea to keep a nice unbroken ground plane
>under the signal(s) that you really care about.
>
>George H.

That's often not practical. If the issue is a digital logic signal
cruising above a layer that has multiple power pours, with
design-rule-sized gaps between them, it's not likely to be a problem.
The power pours are big slabs of copper with a lot of capacitance to
the ground plane and to the universe, and are generally bypassed, so
as far as the signal trace is concerned, they may as well be over
ground. And a wavefront, even a 100 ps one, is not going to be much
disturbed by cruising over a narrow gap in the copper; it is filled
with epoxy. I've built test boards with this geometry, and with
deliberate slits in ground planes, and I've measured this stuff.

One day I should lay out a horrible-example multilayer board and
document what really happens. I do often put test traces on production
boards, to see how we're doing on the impedance calculations.

http://johnlarkin.yolasite.com/pc-boards.php


John