[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 f=data_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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