[time-nuts] Experience with THS788 from TI?

Attila Kinali attila at kinali.ch
Thu Mar 22 21:17:47 UTC 2012


On Thu, 22 Mar 2012 11:12:40 -0400
Ben Gamari <bgamari at physics.umass.edu> wrote:

> > If you are really going to build your own design, then i suggest you
> > read these papers:
> 
> Thank you very much for this list. While I have already stumbled upon a
> few of the FPGA papers, I'm largely ignorant of the other possible
> approaches. Given the limitations of FPGA TDCs, it will be nice to see
> what is possible by other means.

I'm quite sure there is much more around. I only dug a little bit a
weekend or so and got a couple of intersting papers. It is also a 
good idea, to dig trough old circuit descriptions, form the 70s and older.
You will find there many forgotten gems, that get increasingly relevant
when you leave the digital domain, especially when going high speed.
 
> That being said, I'm quite keen on bringing up something on the FPGA. I
> just got the power supplies on the PandaDAQ running last night (QFN is a
> pain without my shiny new hot air rework station), so it seems that soon
> enough I'll have a Spartan 6 at my disposal.

*g*
If you do QFN or any serious SMD stuff, get a Leiter HOT JET S
with fine nozzles (3mm and 5mm), or anything similar. Normal hot
air guns don't really work and a complete rework station is way
too expensive for anything a mere mortal does. On the other hand,
with such a Hot Jet S (or similar) you can even solder BGAs, reliably.


> > "Time-Interval Measurements Based on SAW Filter Excitation", by Petr Panek, 2007
> > "Time interval measurement device based on surface acoustic wave filter
> > excitation, providing 1ps precision and stability", by Panek and Prochazka, 2007
> > "Random Erros in Time Interval Measurement Based on SAW Filter Excitation",
> > by Petr Panek, 2008
> > A very nice idea on how to use a high frequency startable oscillator with
> > an ADC as phase detector. Panek claims to get below 1ps with a 200MHz clock
> > and a 525MHz filter/oscillator. His calculations indicate that the ultimate
> > limit of resolution is given by the sampling jitter of the ADC and the
> > frequency and bandwidth (ie the Q of the oscillator). There 

Oops.. "There" is a sentence missing... Interrupts are bad for emails.

There is even a report of a similar design, using an LC tank as resonant
circuit that got into the <10ps RMS region. See "High frequency, high time
time resolution time-to-digital converter emplying passive resonant circuits",
by Ripamonti, Abba and Geracy, 2010
http://risorse.dei.polimi.it/digital/products/2010/High%20frequency,%20high%20time%20resolution%20time-to-digital%20converter%20employing%20passive%20resonating%20circuits.pdf
 
> > But getting to below that will not be
> > easy. Mainly due to all those side effect, non-idealities and other
> > stuff you have to deal with. And be aware, that you are dealing with
> > an high frequncy/high speed circuit. Crudly said, you are in the
> > ballpark of a 1/10ps = 100GHz system. Everything has to be right to
> > get you there.
> 
> Sure. This is the real issue. I am a physicist by training, so the
> basics of high-speed design are largely a mystery to me. From
> application notes (in particular Jim Williams' old but very readable
> work) I've gleaned the following,
> 
> 1) Keep traces short and well impedence matched
> 2) Ample bypassing
> 3) Ground plane is essential

4) Know where thy return path is!

I recommend getting the Tietze-Schenk "Halbleiter Schaltungstechnik"
(resp "Electronic Circuits" in englisch). It's like Horowitz, just
with more theoretical background and more explenation how to design
stuff. Ie you get the formulas to calculate what you need if you want
to go to the limit. It's still very much practical (only as much
theory as needed) but covers enough of the theory if you want to have
more than just cookbook examples.

I'm still looking for a good high speed / high frequency book.
I've asked for literature in that area a few weeks ago on this
list, but have not gotten the time yet to read all those books.
You might want to check the answers too.
 
> But beyond these rough guidelines my intuition isn't so well
> honed. Do you have anything to add?

Unfortunatley not. I'm too young for much experience and don't have
a big lab to test things. Not to mention the constant lack of time.
The only thing i have at my disposal are tons of papers i've read
on train rides... If you've specific questions, i can see whether
i can dig something up in my collection. But i don't have any general
"learning" tools yet.

> One final question:
> In Williams' notes, you often see images of point-to-point wired
> circuits constructed over a copper clad board ground plane. While in the
> '90s I can see this being a very reasonable approach, is it still
> relevant in today's world of surface mount packages and 100GHz
> bandwidths?  Assuming fly-wiring isn't an option, is there any way to
> prototype a circuit capable of handling even 50ps signals short of
> etching a board and hoping for the best?

It's done as well. Just at an other level. You still have these
copper boards, but you have specialized "pads" for different
IC packages and you connect them using coax and short copper wires.
I've never used any of those myself, but i've heard that they are
good to 1-2GHz at least (mostly due to the good ground). But of
course you have to know what you are doing when you want do get
to that performance.

Also forget about 100GHz, that's not something you can do using
normal of the shelf components. If you get a couple GHz, feel lucky.
Anything beyond that will not reliably work. The 1/10ps = 100GHz
was more a figure of speach. Your signals will not have such high
frequency components. You cannot get slopes that steep to get there
with normal components and packages. But you have to design your
circuit thinking you have that frequency because the high frequency
handling ideas (correct termination, signal path etc) make it easier
to get where you want to go.


				Attila Kinali
-- 
Why does it take years to find the answers to
the questions one should have asked long ago?



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