🚀 go-pugleaf

On Tue, 10 Jan 2012 15:39:08 -0800 (PST), Bill Sloman
<bill.sloman@ieee.org> wrote:

>On Jan 10, 5:50�pm, John Larkin
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Tue, 10 Jan 2012 17:29:35 +0100, Jeroen Belleman
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>> <jer...@nospam.please> wrote:
>> >John Larkin wrote:
>> >> On Tue, 10 Jan 2012 01:28:22 -0800 (PST), Bill Sloman
>> >> <bill.slo...@ieee.org> wrote:
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>> >>> On Jan 10, 7:47 am, "David L. Jones" <altz...@gmail.com> 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[...]
>>
>> >> Yeah, his book is half nonsense. If you can tell which half is which,
>> >> you don't need the book. His ideas on "return currents" and bypassing
>> >> are absurd. And the writing is dreadful.
>>
>> >I haven't got the book handy, but I'll pick it up tomorrow or so.
>> >Do you mind expounding on why his "return currents" make no sense?
>> >Did he mean something different from image currents?
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>> He says that a transmission-line pcb trace must always be "referenced"
>> to the same "return current" plane it started with and shouldn't pass
>> through vias to become referenced to a different plane, or else the
>> return currents will get confused or go on strike or something. If
>> that were true, 98% or the multilayer boards on Earth would quit
>> working.
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>> He also says that you should add bypass caps and stitching vias to
>> conduct these return currents whenever a trace does via to a different
>> "reference plane."
>>
>> All silly.
>
>Not completely silly. One of the more avoidable hold-ups in the
>development of the electron beam tester at Cambridge Instruments from
>1988 to 1991 was when the drafting office sent out a six layer board
>for manufacture without specifying the order of the inner planes -
>they "knew" that the order didn't matter, despite the fact that I'd
>spelled it out, in detail, on the release note that I'd given them.
>
>Almost all of the very high frequency connections - from GaAs logic
>outputs to GaAs logic inputs - were supposed to have been routed
>directly above the -2V plane (which should have been directly under
>the components), and the terminating resistors were returned to that
>plane (through a via at the relevant end of the resistor).
>
>The board didn't work, and it took us a while to find out why.
>Whenever I walked past the engineer working on the board I'd remind
>him that he had the stuck of images of the various planes stuck
>together in the wrong order, and eventually this irritated him enough
>that he got out a drill and worked that he'd got them in the right
>order, which explained most of why the board wasn't working.
>
>We did get it to work eventually, but all the high-frequency
>connections through the board ended up being routed through tacked-on
>lumps of sub-miniature coaxial cable.
>
>The next board had the inner planes in the right order, amongst other
>changes, and worked a lot better.
>
>--
>Bill Sloman, Nijmegen


Of course the stackup and order matter, if not to get the DC
connections right, then to get the impedances right. But the obsession
with return currents and "reference planes" is mostly silly.

As is the admonition to never cross slits in planes, fr'instance
running a trace above a plane that has multiple pour pours.

John