[SI-LIST] Re: TDR impedance measurement and rise time
- From: Mick zhou <mick.zhou@xxxxxxxxx>
- To: Istvan Nagy <buenos@xxxxxxxxxxx>
- Date: Tue, 5 May 2009 09:12:35 -0600
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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