🚀 go-pugleaf

On Jan 11, 1:49 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Tue, 10 Jan 2012 15:39:08 -0800 (PST),BillSloman
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> <bill.slo...@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),BillSloman
> >> >> <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[...]

<snip>

> >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.
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> >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).
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> >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.
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> >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.
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> >The next board had the inner planes in the right order, amongst other
> >changes, and worked a lot better.

> Of course the stackup and order matter, if not to get the DC
> connections right,

Our suppliers back then didn't do buried vias, and the boards weren't
so tightly packed that we had any need of them, so the DC connections
were going to be fine, independent of the stack-up, but the return
current under the transmission-line tracks did end up following rather
more convoluted routes than we'd had in mind.

> then to get the impedances right. But the obsession
> with return currents and "reference planes" is mostly silly.

Return current routing does matter, and you should keep it in mind.
Half-baked rules of thumb aren't a good substitute for thinking about
what is going on, but thye can be better than nothing.

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

It certainly changes the transmission line impedance in the vicinity
of the slit, if the slit is long enough vis a vis the transition time
of the edge travelling along the transmission line.

--
Bill Sloman, Nijmegen