🚀 go-pugleaf

On Thu, 12 Jan 2012 02:11:46 -0800 (PST), Bill Sloman
<bill.sloman@ieee.org> wrote:

>On Jan 12, 3:13�am, John Larkin
><jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
>> On Wed, 11 Jan 2012 16:49:18 -0800 (PST),BillSloman
>>
>> <bill.slo...@ieee.org> wrote:
>> >On Jan 11, 8:22 pm, Phil Hobbs
>> ><pcdhSpamMeSensel...@electrooptical.net> wrote:
>> >> On 01/11/2012 01:43 PM, John Larkin wrote:
>>
>> >> > On Tue, 10 Jan 2012 18:02:50 -0800 (PST),BillSloman
>> >> > <bill.slo...@gmail.com> wrote:
>>
>> >> >> On Jan 10, 4:55 pm, John Devereux<j...@devereux.me.uk> wrote:
>> >> >>> Robert Macy<robert.a.m...@gmail.com> writes:
>> >> >>>> On Jan 10, 2:28 am,BillSloman<bill.slo...@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: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.
>>
>> >> >>>>> "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.
>>
>> >> >>>> Perhaps, it is time for the first ever 'conference' book written by a
>> >> >>>> consortium of engineers/scientists.
>>
>> >> >>>> Put my name in for proffreading. :)
>>
>> >> >>> Great idea. Perhaps John Larkin, Jim Thompson andBillSlomancould
>> >> >>> collaborate on it.
>>
>> >> >>> Sure that will work out well... :)
>>
>> >> >> I'm not sure what Jim Thompson might contribute. I've seen papers that
>> >> >> worry about the characteristic impedance of connections on the silicon
>> >> >> (or whatever) inside integrated circuits, but I don't have the feeling
>> >> >> that Jim designs stuff that's big enough and fast enough for him to
>> >> >> need to know much about it.
>>
>> >> > I get the impression that most interconnects inside ICs are primarily
>> >> > distributed R-C things, not real transmission lines at all.
>> >> > Propagation speed of interconnects is a big deal in routing fast logic
>> >> > chips, but the speed of light isn't the issue.
>>
>> >> >> Motorola's ECL applications notes on board layout were rather useful,
>>
>> >> > Just don't trust their microstrip equation!
>>
>> >They looked okay in the range 50R to 75R.
>>
>> Yup. And they give silly, even negative, impedances for traces outside
>> a narrow width zone.
>
>Which is more or less what I went on to say in the next paragraph -
>you seem to suffer from premature ejeculation.


You suffer from nostalgia for the times, decades ago, when you
actually did things... most of which didn't work.


>
>> >Specialised textbooks give
>> >longer and more complicated expressions that give rather different
>> >results for microstrips and strip-line with higher and lower
>> >characteristic impedances.
>>
>> >> Yup, all RC transmission lines with repeaters, speed roughly c/10.
>>
>> >I wonder what Dr. Hobbs thinks he is talking about
>>
>> The fact that on-chip interconnects are usually dominated by
>> resistance and capacitance, and don't behave like the transmission
>> lines you're used to. Since the risetime of a distributed R-C line
>> degrades as length-squared or something horrible like that, it makes
>> sense to insert active buffers in long runs.
>
>Which didn't have much to do with the point that the Motorola ECL data
>books used formulas for trace impedance that only worked well over a
>narrow - if useful - range of trace impedances. It's entirely relevant
>to the point that Jim Thompson doesn't have to know much about laying
>out printed circuit boards for signals with short transition times,
>but Phil didn't bother to make it clear that he was addressing that
>particular question.
>
>> I believe he has a bunch of direct experience in this area.
>
>> >�- the Motorola �ECL
>> >application notes discussed proper microstrip and stripline
>> >transmission lines, where the propagation delay is basically that of
>> >electromagenetic waves in dielectric - something around two-thirds of
>> >the speed of light in vacuum (depending on the dielectric constant of
>> >the substrate).
>>
>> They also cautioned against sharp corners, a silly warning at ECL
>> speeds.
>
>At current ECLinPS speeds everything gets averaged over a few
>millimetres of track. The extra capacitance implicit in a right-angle
>bend on a 5OR track might be perceptible, but you can pretty much
>eliminate it by chamfering the corner, or making two 45-degree bends
>in quick succession.

That's what the ancient Moto book said. It was stupid at 10K speeds
and it remains stupid at EclipsPlus speeds. I can barely discern a
right angle with a zoomed-up 30 ps TDR edge. The weave of the
fiberglass changes the impedance more than a sharp corner. Vias are
hugely more important than right angles.

John