[SI-LIST] Re: TDR impedance measurement and rise time
- From: Mick zhou <mick.zhou@xxxxxxxxx>
- To: istvan.novak@xxxxxxx
- Date: Mon, 31 Aug 2009 09:54:03 -0600
Istvan,
You are absolutely right.
But seems they authors do apply the transformation to (3.87) and arrives
(3.88). Even though, I cannot get (3.88) from (3.87) using inverse Laplace
transformation. However, the dimension looks right. Would you advice?
Thanks,
Mick
On Sat, Aug 29, 2009 at 3:42 PM, Istvan Novak <istvan.novak@xxxxxxx> wrote:
> Hi Mick,
>
> Not sure what you want to say with this... The note for [3.87] states that
> the approximation used to get [3.87] (from [3.86]) is valid only well above
> the wlc corner frequency. When you apply Fourier or Laplace transform, they
> require (or rather use) an infinite frequency range, so to me it means that
> you can apply the transformations to [3.86] if you wish, but not to [3.87].
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
>
> Mick zhou wrote:
>
>> All,
>>
>> This discussion has been there for a while. I reviewed the section in
>> Johnson's book (Advanced Black Magic) about the slope in Zc(t). From [3.87]
>> to [3.88], I checked Fourier transform and Laplace transform pairs,
>> 1/(j*omega) <-->1, not t. So the rest is incorrect, unless we go to the
>> second order. Can somebody help double check?
>>
>> Thanks,
>>
>> Mick
>>
>> On Wed, May 6, 2009 at 11:12 AM, Mick zhou <mick.zhou@xxxxxxxxx <mailto:
>> mick.zhou@xxxxxxxxx>> wrote:
>>
>> Istvan,
>>
>> I cannot agree with you more. That's why I emphasized "general
>> cases".
>> There are also elements that Laplace transformation can be
>> performed correctly, for example ideal L and C.
>>
>> Yep, fortunately we live in "Newton's world" most likely.
>>
>> Best regards,
>>
>> Mick
>>
>>
>>
>> On Wed, May 6, 2009 at 9:18 AM, Istvan Novak <istvan.novak@xxxxxxx
>> <mailto:istvan.novak@xxxxxxx>> wrote:
>>
>> Hi Mick,
>>
>> I see two related, but independent statements/questions in
>> your message. The answer to "How can a
>> complex value (f-domain) equals a real value (TDR, t-domain)
>> in general cases?" the answer is: it is
>> doable without loss of accuracy, if done correctly. The
>> sentence you quote "Severe degrees of tilt
>>
>> make it very difficult to define one correct measurement
>> procedure that is best for all appications"
>> may apply to some laminate materials, but a) it has nothing to
>> do with the complex or real nature of
>> the data, and b) luckily today the typical materials used by
>> the PCB industry dont fall into this
>> category, not at least in the frequency range of common
>> interest for digital people.
>>
>>
>> Regards,
>>
>> Istvan Novak
>> SUN Microsystems
>>
>>
>>
>>
>>
>>
>>
>> Mick zhou wrote:
>>
>> Istvan,
>> Well, I should say fortunately we have no serious problems
>> in most
>> interconnect design practices since we intend to make them
>> low loss etc.
>>
>> How can a complex value (f-domain) equals a real value
>> (TDR, t-domain) in
>> general cases? In High Speed Signal Propagation: Advanced
>> Black Magic by
>> H.Johnson, sect. 3.6.3, the authors touched the surface of
>> the difficulty by
>> concluding " Severe degrees of tilt make it very difficult
>> to define one
>> correct measurement procedure that is best for all
>> appications" (p.172).
>> Actually I think it is a fundamental problem: how can we
>> define Z in
>> t-domain that is compatible with Z in f-domain
>> scientifically in general
>> cases? f-dependent, nonlinear etc.
>>
>> What we have done is to map Z(f)=V(f)/I(f) into
>> Z(t)=V(t)/I(t), and so
>> reflection etc. Obviously, these definitions are not
>> compatible even
>> mathematically. Laplace transformation is not satisfied in
>> general cases.
>>
>> My point is the current TDR theory has limitations that
>> make some of our
>> interpretations (struggles) meaningless. However,the
>> difficulty does not
>> stop our engineering work until it is absolutely necessary
>> to correct the
>> theory. We don't employ relativity to solve most of our
>> engineering
>> problems, but it is good to know the limitations of
>> Newton's theory to avoid
>> unnecessary struggles if we run outside of the territory.
>> Cheer!
>>
>> I don't think it is easy to solve this difficulty by
>> emails in this list
>> unless we want to confuse people more. I'd leave it to
>> theorists again.
>>
>> Best regards,
>>
>> Mick
>>
>>
>> 2009/4/23 Istvan Nagy <buenos@xxxxxxxxxxx
>> <mailto:buenos@xxxxxxxxxxx>>
>>
>>
>>
>> Hi
>>
>> So, does it mean that we can not do anything useful
>> about frequency
>> dependent impedance control on digital boards?
>> Impedance can vary 5% from 100MHz (analog VGA,
>> reference clocks) to few GHz
>> (PCI-express, SATA, XAUI), so it can cause a problem.
>> Or that is the maximum
>> accuracy what we can get?
>> 5% unaccuracy is 5% extra mismatch for the
>> termination, if we have other
>> sources of a mismatch already (component tolerance).
>> Isn't 5% bad, or is it
>> acceptable?
>>
>> Another aspect is what single frequency to substitute
>> for a digital signal
>> for impedance/trace_width calculations/simulations?
>> I thought it would be the knee frequency based on the
>> signal's rise time,
>> but i am not shure anymore.
>> For 8b10b encoded signals, there should be a lower
>> frequency (data_rate/10)
>> limit in the signal's spectrum, since maximum 5 zeroes
>> or ones can follow
>> each other.
>> Where do we need best matching for terminations, at
>> the highest frequency
>> components, or at the mean of the spectrum, or at the
>> highest peak...?
>> I was trying to do some simulations with different bit
>> patterns in QUCS and
>> cadence SigExplorer, then do FFT, but the result looks
>> mostly meaningless
>> garbage with some negative slope.
>> Anyway, how does the spectrum looks like for real data
>> signals, especially
>> at the lower end of the spectrum?
>>
>> How does the TDR determine the impedance? Does it
>> measure the reflected
>> signal voltage peak?
>> And at what frequency? if we check the impedance
>> characteristics from DC to
>> infinite Hz, the impedance varies a lot. In theory, if
>> both a simulation and
>> a TDR measurement gives a number, then at what
>> frequency should they be
>> equal, and why?
>>
>> regards,
>> Istvan
>>
>>
>> ----- Original Message ----- From: "Mick zhou"
>> <mick.zhou@xxxxxxxxx <mailto:mick.zhou@xxxxxxxxx>>
>> To: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx
>> <mailto:shlepnev@xxxxxxxxxxxxx>>
>> Cc: "Istvan Nagy" <buenos@xxxxxxxxxxx
>> <mailto:buenos@xxxxxxxxxxx>>; <si-list@xxxxxxxxxxxxx
>> <mailto:si-list@xxxxxxxxxxxxx>>
>>
>> Sent: Monday, April 20, 2009 11:21 PM
>>
>> Subject: [SI-LIST] Re: TDR impedance measurement and
>> rise time
>>
>>
>> Yurily,
>>
>> Nice study.
>> I'd like to bring it deeper if not re-invent the
>> wheels.
>>
>> Except some practical issues, I think there is a
>> fundamental issue
>> that is the definition of Z in t-domain and
>> f-domain. The same
>> formula rho=(ZL-Z0)/(ZL+Z0) (or its V(t) form) is
>> simply used in both
>> t- and f-domains. It does not matter if Z is
>> f/t-independent,
>> otherwise it is questionable Unfortunately, it is
>> the foundation of
>> most TDR algorithms so far. You can simply apply
>> Fourier
>> transformation, convolution must be involved even
>> we assume Z0 is a
>> constant. I don't know there is a good solution so
>> far until we make
>> necessary corrections in the math.
>>
>> We may conclude that one to one match from
>> f-domain to t-domain is
>> meaningless in general cases. That's probably the
>> root cause of many
>> confusions. We can always find a point we like to
>> have a "match".
>> For weak f-/t- dependent, it should be OK.
>> Fortunately, most cases in
>> out community are weak f-/t- dependent? We don't
>> need to worry as much
>> as we should?
>>
>> Thanks,
>>
>> Mick
>>
>>
>>
>>
>>
>>
>> 2009/4/8 Yuriy Shlepnev <shlepnev@xxxxxxxxxxxxx
>> <mailto:shlepnev@xxxxxxxxxxxxx>>:
>>
>>
>> Hi Istvan,
>>
>> Looking through this thread, I finally decided
>> to spend a couple of hours
>> and to do a simple numerical TDR experiment
>> with a broad-band model of a
>> micro-strip line segment, to see at least
>> theoretical effect of the rise
>> time and to correlate frequency-dependent
>> characteristic impedance of the
>> line with the values that can be observed on
>> TDR. The results of this
>> simple
>> experiment are available as App. Note #2009_04 at
>> http://www.simberian.com/AppNotes.php (no
>> registration required). The
>> conclusion is that the observed TDR impedance
>> depends on the rise time
>> and
>> can be correlated with the characteristic
>> impedance at different
>> frequency
>> bands (well, at least theoretically).
>>
>> Best regards,
>> Yuriy Shlepnev
>> www.simberian.com <http://www.simberian.com>
>>
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx
>> <mailto:si-list-bounce@xxxxxxxxxxxxx>
>> [mailto:si-list-bounce@xxxxxxxxxxxxx
>> <mailto:si-list-bounce@xxxxxxxxxxxxx>]
>> On
>> Behalf Of Istvan Nagy
>> Sent: Tuesday, April 07, 2009 1:34 PM
>> To: si-list@xxxxxxxxxxxxx
>> <mailto:si-list@xxxxxxxxxxxxx>
>> Subject: [SI-LIST] Re: TDR impedance
>> measurement and rise time
>>
>> Hi
>>
>>
>> Peter from LeCroy wrote:
>> "short impedance discontinuities... if you
>> limit the frequency content
>> ...,
>> the bumps get smeared out by the slower
>> risetime and they don't look so
>> bad"
>>
>> - i think for these Test Coupon measurements
>> is the point not to measure
>> a
>> real PCB trace with the lots of
>> discontinuities, but to get the impedance
>> based on the cross section. otherwise we would
>> need different trace
>> widths
>> for every trace segment and we would need
>> real-time 3D simulationd during
>> PCB layout design.
>>
>> Exploring discontinuities on a real PCB (not
>> on a test coupon) is is
>> another
>>
>> story. I was asking about the measurements for
>> the test coupons (maybe I
>> forgot to mention). Normally (our) boards have
>> hundreds of controlled
>> impedance interconnects, those at the first
>> place should be correct based
>> on
>>
>> the cross section and test coupons. The rest
>> is design practices, to make
>> shure we dont deviate too much with
>> discontinuitise. Of course its
>> probably
>> nice to characterise a full board, but in
>> short development cycles, it
>> wouldn't work very well. but i dont know,
>> maybe it would...
>>
>> "Howard Johnson had an excellent video "
>> - if anyone knows where to find it, i would
>> appreciate...
>>
>>
>> Jeff Loyer wrote:
>> "The TDR will report the same characteristic
>> impedance of your trace
>> regardless of risetime"
>>
>> - which impedance? the impedance at 1 GHz? or
>> at 10 GHz? or at xxx GHz?
>> The characteristic impedance of a PCB trace
>> depends on the frequency,
>> since
>> Er and the loss tangent are frequency
>> dependent, and there is skin effect
>> and others... so Z0(1GHz) is not equal to
>> Z0(xxxGHz). So if a signal
>> (lets
>> simplify it) is at xxx GHz, then its
>> terminations should be best matched
>> at
>> xxx GHz, and not at yyyGHz, so the board
>> impedance should be correct at
>> xxx
>> GHz, and not at yyyGHz.
>>
>>
>> Rob Sleigh wrote:
>> "Yes, it's a very common practice to
>> characterize a PDB with a TDR whose
>> rise time is similar to the signal's rise
>> time. It's up to the designer
>> to
>> decide, but usually pick a faster rise time
>> than the system rise time to
>> provide yourself with some margin."
>>
>> -most of the PCB manufacturers we talked to,
>> they never asked about
>> rise_time or frequency information of our
>> signals, and when we tried to
>> provide these to them they said they have
>> deleoped their super-duper test
>> setup which is based on tonns of measurements
>> and it is accurate, and
>> they
>> dont care about our signal's frequency or rise
>> time, and we should just
>> pay
>> and shut up... We tried In europe, north
>> america and china. And the best
>> what they say is they compensate for
>> frequencies up to 10GHz, without
>> knowing anything about our signal's freq/Tr.
>> The last one said they can't or don't change
>> rise times on their TDR...
>>
>>
>> Kihong (Joshua) Kim wrote:
>> "maximum frequency that may capture the
>> bandwidth of imformation in
>> digital
>> world."
>>
>> - I was trying to estimate rise times and
>> bandwidth. Especially at the
>> receiver. I can't explain why it would be
>> better than at the transmitter
>> if
>> they are both matched terminated to Z0, but I
>> have a feeling like that...
>> Normally at the receiver we have slower rise
>> times. For example for PCIe
>> and
>>
>> SATA, the signal looks sinusoid, not that
>> rectangular as at the
>> transmitter.
>>
>> So at a pattern 1010101010 the frequency would
>> be fÚta_rate/2. For
>>
>> other
>> interfaces, like DDR2/3, we can get rise times
>> from simulation. So, I
>> would
>> provide these to the PCB manufacturer to
>> calculate trace widths and
>> verify
>> by TDR/test-coupon measurements.
>>
>>
>>
>>
>> regards,
>> Istvan Nagy
>> CCT, UK
>>
>>
>> ----- Original Message -----
>> From: "Kihong Joshua Kim" <joshuakh@xxxxxxxxx
>> <mailto:joshuakh@xxxxxxxxx>>
>> To: "Nagy István" <buenos@xxxxxxxxxxx
>> <mailto:buenos@xxxxxxxxxxx>>
>> Cc: <si-list@xxxxxxxxxxxxx
>> <mailto:si-list@xxxxxxxxxxxxx>>
>> Sent: Tuesday, April 07, 2009 4:51 PM
>> Subject: [SI-LIST] Re: TDR impedance
>> measurement and rise time
>>
>>
>> Nagy,
>>
>> Couple of TDR measurements experience for
>> real boards with known trace
>> models and physical data will give you
>> good sense of what TDR means.
>> However, if you do not have time to build
>> sample boards nor have TDR
>> equipment...here is my help.
>>
>> Risetime conversion to frequency needs to
>> be dealt with in-depth
>> understanding of the topic. The quick rule
>> of thumb equation mentioned
>> in one of threaded mails is the maximum
>> frequency that may capture the
>> bandwidth of imformation in digital world.
>> This is weird part because
>> one
>> might has question on why I am talking
>> about digital bandwith when
>> others
>> discuss about analog nature of signal
>> (rise time). Some excercise to
>> uderstand Fourier analysis would give you
>> an idea about what it meant.
>>
>> Anyhow, to get out of math and get the
>> real sense of TDR with variety of
>> sample boards.
>> I had developed couple of years ago a
>> virtual TDR head (IBIS TDR
>> model) working just fine in any IBIS
>> simualtion tools and I found out
>> the
>> paper in the internet (wow!). You could
>> try sample boards as long as you
>> have real board file and connector models
>> and etc....
>>
>> If you google key words for IBIS TDR or
>> TDR IBIS, you will find it
>> easily.
>> But just in case I attached here...
>>
>>
>>
>>
>>
>> http://www.cadence.com/rl/Resources/conference_papers/stp_TDR_in_IBIS_Kim.pd
>> f
>>
>>
>> Regards,
>>
>> Kihong (Joshua) Kim
>> http://www.linkedin.com/in/joshuakh
>>
>>
>>
>> On Tue, Apr 7, 2009 at 10:39 AM, Loyer,
>> Jeff <jeff.loyer@xxxxxxxxx
>> <mailto:jeff.loyer@xxxxxxxxx>>
>>
>> wrote:
>>
>> Concerning measuring Z0:
>>
>> The TDR will report the same
>> characteristic impedance of your trace
>> regardless of risetime, assuming your
>> trace is long enough and there
>> aren't
>> significant variations in impedance
>> along its length.
>>
>> Typically, we have very similar 6"
>> coupons for all our controlled
>> impedances. The board manufacturer
>> will typically measure them with an
>> HVM-compatible TDR, probably about 200
>> ps risetime. We verify the
>> impedances with our ~17ps TDR.
>>
>> For simulations, on the other hand,
>> you'll probably want a risetime
>> faster
>> than the projected risetime of your
>> device (I'd guess about 2x; I don't
>> remember seeing it quantified). I
>> typically see folks just go with the
>> risetime of the equipment, ~17ps, and
>> ensure simulation match those
>> measurements. They may be a little
>> conservative, but probably less work
>> in
>> the long run than trying to exactly
>> justify any particular risetime.
>>
>> The advantages/disadvantages I can
>> think of off-hand for fast risetimes
>> are:
>> 1) fast R.T. = resolution of finer
>> features (discontinuities).
>> Unfortunately, this can also
>> erroneously lead you to believe you need
>> to
>> fix things that are "invisible" at
>> your risetime of interest. Filtering
>> to
>> your risetime of interest can help you
>> decide whether a discontinuity
>> is
>> significant or not.
>> 2) fast R.T. = smaller probing
>> geometries. It doesn't make sense to try
>> to
>> maintain a 15 ps risetime through a
>> launch structure with 30 mil vias
>> spaced
>> 100 mils apart (such as might be used
>> for manufacturing testing).
>> Living
>> with slower risetimes can allow you to
>> adopt much more HVM-friendly
>> launch
>> structures, including pogo-pinned
>> probe connections.
>> 3) fast R.T. = less ESD protection.
>> It's very easy to damage a TDR head
>> from static discharge - HVM-compatible
>> TDR machines with slower
>> risetimes
>> have ESD protection.
>>
>> If the scope or post-processing
>> software doesn't have the ability to
>> slow
>> your risetimes, you can buy filters
>> from Picosecond Pulse labs (buy a
>> filter
>> at 0.35/RT). They also sell hardware
>> to put out very fast risetimes.
>>
>> Jeff Loyer
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx
>> <mailto:si-list-bounce@xxxxxxxxxxxxx>
>> [mailto:
>> si-list-bounce@xxxxxxxxxxxxx
>> <mailto:si-list-bounce@xxxxxxxxxxxxx>]
>> On Behalf Of Nagy István
>> Sent: Tuesday, April 07, 2009 4:59 AM
>> To: si-list@xxxxxxxxxxxxx
>> <mailto:si-list@xxxxxxxxxxxxx>
>> Subject: [SI-LIST] TDR impedance
>> measurement and rise time
>>
>> hi
>> If we measure PCB test coupons with a
>> TDR to determine characteristic
>> impedance, should we set the rise time
>> to be the same as the signal's
>> rise
>> time? is it possible to set it at all?
>>
>> what i found on the internet, the TDR
>> manufacturers try to make rise
>> time
>> to be as low as possible, like
>> 15ps..., and thats it.
>>
>> If the rise time is always 15ps, then
>> i think it will always measure
>> the
>> impedance on a very high frequency,
>> 2/t_rise or something, so several
>> gigahertz. Usually on a board we have
>> different signals, some are
>> running
>> 100MHz analog, some other are 800MT/s
>> digital, or 2.5Gb/s digital.
>> shouldn't we do different setups for
>> these, to get impedances on the
>> signal's operating frequency?
>>
>> Someone from a Fab told me, that the
>> "TDR is not frequency dependent".
>> so
>> they dont take the signal's frequency
>> into account.
>>
>> what is the correct handling of
>> signaling frequency for impedance
>> measurements, and simulations?
>>
>> regards,
>>
>> Istvan Nagy
>> CCT
>>
>>
>>
>>
>>
>>
>>
>>
>
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