Dr Howard Johnson, this weeks guest on The Amp Hour

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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-wizardry /

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.

Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Mon, 9 Jan 2012 22:47:01 -0800 (PST), "David L. Jones"

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That Dr. Howard looks awfly familiar !

He also lives awfly close to us here in the PNW...

Also saw a link that shows your new videos.

Thx David  !

boB



Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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"Black Magic" is more notorious than famous. I own both it and it's
follow-up, and both have got some useful content but they are badly
organised and what useful content they do contain is hard to find.

The field is still waiting for a decent textbook on high speed circuit
design and layout.

This is one of the few matters on which John Larkin and I seem to
agree.

--
Bill Sloman, Nijmegen


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Tue, 10 Jan 2012 01:28:22 -0800 (PST), Bill Sloman

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fame:http://www.theamphour.com/2012/01/10/the-amp-hour-77-winsome-waveform ...
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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.

His articles in EDN (or ED, whatever) are horrible too. He ran out of
material years ago, but he keeps writing.

I started highlighting the really bad, flat silly parts of "Black
Magic" but gave up less then halfway through.

John


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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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?

Jeroen Belleman

Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Tue, 10 Jan 2012 17:29:35 +0100, Jeroen Belleman

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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.

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.

John


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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Ah, yes, I agree with you there. Mostly anyway. Capacitance between
planes should be plenty to take care of that.

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

Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Jan 10, 5:50A0%pm, John Larkin
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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

--
Bill Sloman, Nijmegen


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Tue, 10 Jan 2012 15:39:08 -0800 (PST), Bill Sloman

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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



Re: Dr Howard Johnson, this weeks guest on The Amp Hour

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Did you mean "multiple poor pours."?  :-)


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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I think "multiple pour pours" is acutally rather self-fulfilling.

Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Thu, 12 Jan 2012 08:51:42 -0800 (PST), Bob

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Our cat makes noises like that.

John


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Jan 11, 1:49A0%am, John Larkin
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<snip>

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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.

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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.

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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


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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A track across a slit is an antenna. Unless there's another plane
underneath covering the slit, in which case it probably doesn't
matter.

Jeroen Belleman

Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Wed, 11 Jan 2012 08:30:48 +0100, Jeroen Belleman

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On a multilyer board, there usually is. But I've done test slits on
microstrips on 2-sided boards, clear across the board, and not much
happens. I didn't measure radiated EMI, but signal integrity wasn't
much affected at 30 ps risetimes.

John




Re: Dr Howard Johnson, this weeks guest on The Amp Hour

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right,  just like a coax cable, a disruption of the shield  won't make a
difference to the signal integrity to one signal in the cable

 but can still make a big difference to the shielding effectiveness or
transfer impedance and therefore crosstalk between traces.

If you run TWO traces over the  gap in the plane, the coupling between the
traces will be increased dramatically.

Mark






Re: Dr Howard Johnson, this weeks guest on The Amp Hour
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You're just spoiled...

8-layer boards aren't cheap.  Heck, four layer boards aren't cheap.  If I
can avoid it, I'll pick 2 layer whenever possible.  Great proto prices and
turn arounds.  With two layers to choose from, you have no choice but to
stitch like a carpet.  You only have room for one ground plane as such.
Heck, if you think you have room for two (maybe VCC and GND), you're being
lazy and could get by with 1 layer. :)

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms



Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Tue, 10 Jan 2012 21:10:15 -0600, "Tim Williams"

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8's do get expensive, and you can't do serious stuff on 4, so we try
to use 6 layers when we can. Routing a BGA FPGA to a BGA ARM and then
to a lot of ADCs and such can be a challenge on 6 layers.

Having a solid ground layer is great for signal integrity, mandatory
for fast stuff. And the power pours make nice heat sinks for voltage
regulators and power opamps and things.

I haven't done a single-sided board in decades. We do do some doubles
for adapters and breakouts, things like that.

But a bare board, 6 or 8 layers, is a per cent or two of our selling
price. It makes more sense to pack on features and sell a few more.

John


Re: Dr Howard Johnson, this weeks guest on The Amp Hour
On Tue, 10 Jan 2012 21:10:15 -0600, the renowned "Tim Williams"

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These days even a house thermostat design requires a 432 pin BGA (plus
scores of other parts), so 1 or 2 layer boards are not practical. 8-(


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: Dr Howard Johnson, this weeks guest on The Amp Hour

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Anything complicated at all needs 4-layers (I've only done one 2-layer board
in 35+ years).  More layers can often save money with smaller boards and other
packaging costs.  


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