Interesting experiment

Interesting demonstration of the principle that the return current for a high speed signal line over a ground plane follows the top trace even if it's not the shortest path, to minimize inductance.

A gent took an experiment by Benjamin Franklin and turned it into a visual demonstration of that principle.

They bend the trace on the top layer and you can see the return current flow change to follow the top layer high speed signal.

And they do it with a Marx Generator.

Robert

Sounds like HoJo is saying that it takes current to charge a capacitor, and that if I show up at some trade show, and pay him a lot of money to attend one of his seminars, he's willing to argue with me about it.

Hey, I already paid for his silly book! In the Xilinx ads, this guy is referred to as "the world's foremost authority on signal integrity."

John

I'm sure he's perfectly humble in person. :-) He's got plenty of company too... Eric Bogatin, Doug Smith, and even Henry Ott, etc. all tend to overlap his area of expertise to some degree. My experience has been that what those guy teachs at seminars really is useful information to a lot of people -- there are a lot of, e.g., BSEE programs that are completely lacking in some course along the lines of "practical high speed design" or somesuch. I've never tried it myself, but I'm willing to bet you that a significant fraction of new graduates couldn't correctly sketch the return current path of say, a

100MHz toggling output.

(And to demonstrate just how little I knew out of college, I failed an interview question along the lines of "In a typical inverting or non-inverting op-amp setup, how does the amount of noise at the output relate to that at the input?" Oops...)

---Joel

I design gigahertz/picosecond/microvolt/kilovolt stuff, and I probably couldn't either. Never tried, actually.

John

Some people have naturally good instincts? :-) I'm sure you'd admit there's plenty of crappy designs out there!

not

Much like those (british?) homing pigeons that follow highways and roundabouts to find their way home instead of going in a straight line. Hmm... so maybe homing pigeons are in fact homing electrons? That would explain why there are so many of them. But how do they put them in batteries? Honey, I shrunk the pigeons?

--DF

Yeah, and it's a totally bogus demo anyway--a complete snake-oil job. The inductance you'd need to make any significant difference to a 150 kV spark isn't in the 1-microhenry range of that tiny loop--1 uH at 1 GHz is only 6k ohms, and I doubt that Marx generator gets anywhere near that, even in the high harmonics.

What's really going on is that the E field near the corners of the squares is huge when the other conductor is underneath, which causes a corona discharge on those squares, which causes the return path to follow the signal.

Notice that the return wants to go diagonally when the trace turns a corner. This is pure electrostatics, not inductance. Honestly, Howie, can't you do any better than that?

(Anyone want to buy a slightly used copy of the "black magic" book? Cheap?)

Cheers,

Phil Hobbs

Don't let Phil rip you off! Whatever he offers to sell for, I can beat his price.

John

I've been wondering about those homing pigeons they supposedly used to transmit messages back and forth - how do they do that? Take a cage full of pigeons out to the field, and send them home when they need to send a message home? Then how does the HQ send messages out to the field?

Thanks, Rich

"Noise? I was never very good with noise - my first guess is that it's amplified, either by AVol or by (R1 + R2)/R2."

So, what was the right answer, or do they just tell you "You got this one wrong, you don't get the job"?

I blew a test question once in an "interview" - actually it was just the app test, I never did interview on that one. The question was, "write a program that prints out the primes between 1 and 1000." I panicked, as I usually do when the pressure's on, and two hours later I hadn't finished it.

The next day, of course, at the bus stop, I wrote a sieve in three lines, on the back of an envelope or something. (in C, of course. :-) )

Thanks, Rich

Well that's quite impressive, especially considering it takes a minimum of five lines just to print "hello f****ng world" in C ;-)

Tim

-- Deep Fryer: a very philosophical monk. Website:

That was a good question to check creativity. The question as posed did not preclude the existance of a file containing the primes from 1 to 1000. The requested program need only read the file and print it.

I believe they were looking for an answer along the lines of, "Oh, it's about

1/beta," where beta -- the feedback factor -- is R2/(R1+R2) for both the non-inverting AND inverting case...

I imagine if I'd done better, I suspect that next up would have been something along similiar lines such as, "For an otherwise ideal op-amp with a 100MHz GBW product, with a gain of -1, what's the resulting system GBW?" -- It's 50MHz, although right out of college I'd have flubbed that too.

I did get a few questions regarding the return loss going through some mismatched terminations correct... :-)

They were in "polite" mode where there wasn't any direct feedback of whether or not you were giving them the correct answers. In the end, I wasn't offered a job, and I had meanwhile conceded I wasn't qualified for it anyway, so I wasn't particularly unhappy. (They were looking for someone to do high speed op-amp IC design, and that really wasn't me...)

---Joel

But the book is valuable for its humor. Reread section 7.4, "Return Current And Its Relationship To Vias" for a good chuckle.

John

Tim Williams skrev:

nah, only two lines. one line to include stdio.h and one line for the code

-Lass

Half of it is good stuff and half is nonsense. If you know enough to tell which is which, you don't need the book.

John

In PowerBasic, it's

PRINT "Hello, cruel world!"

or maybe

TCP OPEN PORT 2000 AT "192.345.678.001" AS # 1 TIMEOUT 500

PRINT #1, "Hello, internet!"

Either compiles to a single, few-kilobyte .EXE file.

John

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Hello:

So, "High-speed digital design : a handbook of black magic / Howard W. Johnson, Martin Graham " is not a good reference??

Best Regards

Steve Sousa