[SI-LIST] Re: TDR impedance measurement and rise time

  • From: Mick zhou <mick.zhou@xxxxxxxxx>
  • To: istvan.novak@xxxxxxx
  • Date: Fri, 28 Aug 2009 09:38:00 -0600

This is just the math part. It is valid only for w>>WLC as claimed.  I doubt
how it is valid in all Z(t) components. Unfortunately, all plots are
artificial, not real.
On Fri, Aug 28, 2009 at 9:34 AM, Mick zhou <mick.zhou@xxxxxxxxx> 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> 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>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>
>>>>> 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>
>>>>> To: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>
>>>>> Cc: "Istvan Nagy" <buenos@xxxxxxxxxxx>; <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>:
>>>>>>> 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
>>>>>>> -----Original Message-----
>>>>>>> From: 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
>>>>>>> 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>
>>>>>>> To: "Nagy István" <buenos@xxxxxxxxxxx>
>>>>>>> Cc: <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>
>>>>>>>> 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]
>>>>>>>>> On Behalf Of Nagy István
>>>>>>>>> Sent: Tuesday, April 07, 2009 4:59 AM
>>>>>>>>> To: 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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