🚀 go-pugleaf

On Tue, 10 Jan 2012 22:41:11 +0100, Jeroen <jeroen@nospam.please>
wrote:

>On 2012-01-10 17:50, John Larkin wrote:
>> On Tue, 10 Jan 2012 17:29:35 +0100, Jeroen Belleman
>> <jeroen@nospam.please> wrote:
>> 
>>> John Larkin wrote:
>>>> On Tue, 10 Jan 2012 01:28:22 -0800 (PST), Bill Sloman
>>>> <bill.sloman@ieee.org> wrote:
>>>>
>>>>> 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?
>> 
>> 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.
>> 
>> 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.
>
>Ah, yes, I agree with you there. Mostly anyway. Capacitance between
>planes should be plenty to take care of that.

Exactly. The planes are huge and essentially equipotential, so the
wavefront zooming down a trace doesn't know or care what its reference
plane originally was.

>
>I *did* have this amplifier with a ridiculously high gain that
>wouldn't keep still with its output trace running over the V+
>plane. The problem vanished when I made it run over a GND plane
>instead. Then again, there were a few other layout tweaks that
>may have contributed to fixing it.
>
>Jeroen Belleman

Noise on a power plane can certainly couple into a signal trace. And
planes can resonate and couple to things. But this "return current"
fixation is silly.

Here's a TDR test trace, bottom of page.

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

This is referenced to all sorts of power and ground planes.

John