[SI-LIST] Re: TDR and S-parameter
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Tue, 01 Oct 2013 09:26:24 -0700
Bala, the resistance gets added in both directions: Lossless Tx line Z
+ 2X resistance.
Steve
On 10/1/2013 8:57 AM, bala wrote:
> Scott,
> As per my understanding,At high frequencies due to skin effect my electrons
> are not utilizing my entire cross section and resistance increases.(As
> this is happening at high frequency I will call this as AC resistance).so
> my tdr impedance is my intrinsic impedance+ac resistance.
>
> bala
> On 30 Sep 2013 21:48, "Scott McMorrow" <scott@xxxxxxxxxxxxx> wrote:
>
>> Eric
>> This statement is not totally true:
>>
>> "Given this condition, the characteristic impedance read from the flat
>> region of the reflected signal will be independent of the rise time. It is
>> intrinsic to the interconnect."
>>
>> The characteristic impedance read from the flat region of the reflected
>> signal is independent of the rise time, and is the sum of the intrinsic
>> impedance of the flat region of the interconnect ... *plus ...* the
>> accumulated round trip DC resistance of the interconnect.
>>
>> This is is an important distinction. Unless accumulated interconnect
>> resistance is removed from a TDR measurement, a TDR measurement always
>> indicates an impedance that is higher than the actual intrinsic impedance
>> of an interconnect section. This accumulated resistance represents an
>> error in impedance measurements, and is often disregarded. It becomes a
>> significant error when TDR-ing structures which are fed by narrow traces.
>> We experience this issue all the time when correlating measurements of
>> semiconductor packages with modeling.
>>
>>
>> best regards,
>>
>> Scott
>>
>>
>>
>> On Mon, Sep 30, 2013 at 11:59 AM, Eric Bogatin <eric@xxxxxxxxxxxxxxx>
>> wrote:
>>
>>> Bala-
>>>
>>>
>>> Do not confuse characteristic impedance with "impedance" when displayed
>> in
>>> a
>>> TDR type measurement or simulation.
>>>
>>>
>>>
>>> The characteristic impedance of an interconnect only applies, by
>>> definition,
>>> to a uniform transmission line. If it is not uniform, it has no
>>> "characteristic" impedance. All you can do is sort of approximation the
>>> interconnect as a uniform transmission line and then estimate the
>>> approximate characteristic impedance.
>>>
>>>
>>>
>>> However, every interconnect will reflect some signal from a 50 Ohm source
>>> due to its "instantaneous" impedance encountered by the step edge.
>>>
>>>
>>>
>>> The amount of reflected signal depends on the rise time of the signal,
>> the
>>> impedance profile and the electrical length of the interconnect.
>>>
>>>
>>>
>>> If you have a uniform transmission line, and the rise time of your step
>>> edge
>>> is short compared to its time delay, you will see a flat top or flat
>>> bottom.
>>> From this reflected value, you can read off the front screen the
>>> characteristic impedance. Given this condition, the characteristic
>>> impedance
>>> read from the flat region of the reflected signal will be independent of
>>> the
>>> rise time. It is intrinsic to the interconnect.
>>>
>>>
>>>
>>> However, even if you have a uniform transmission line, but your TDR step
>>> edge is electrically long, the value of the reflected signal, and the
>>> extracted "impedance" will depend on the rise time of the edge. It is not
>>> intrinsic to the interconnect.
>>>
>>>
>>>
>>> When comparing the TDR response for different models, be sure to use the
>>> same rise times, and edge profiles. Unless there is a flat top or bottom,
>>> what you are comparing is the instantaneous impedance at a given rise
>> time,
>>> which is only a limit on the characteristic impedance.
>>>
>>>
>>>
>>> I cover this topic in my book, Signal and Power Integrity- Simplified
>>>
>>>
>>>
>>> --eric
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> *******************************************************
>>> Dr. Eric Bogatin, Signal Integrity Evangelist
>>>
>>> Bogatin Enterprises
>>>
>>> Setting the Standard for Signal Integrity Training
>>> web site: <http://www.bethesignal.com/> www.beTheSignal.com
>>>
>>> Blog: <http://www.bethesignal.com/blog> www.beTheSignal.com/blog
>>>
>>> Twitter @beTheSignal
>>> e: <mailto:eric@xxxxxxxxxxxxxxx> eric@xxxxxxxxxxxxxxx
>>>
>>> Laboratory: Connected Community Networks
>>>
>>> 105 S Sunset St, Suite J
>>>
>>> Longmont, CO 80501 USA
>>>
>>> cell: 913-424-4333 skype: eric.bogatin
>>> ***********************************************
>>>
>>>
>>>
>>>
>>>
>>> Msg: #1 in digest
>>>
>>> Date: Sun, 29 Sep 2013 23:49:39 +0530
>>>
>>> Subject: [SI-LIST] TDR and S-parameter
>>>
>>> From: bala <balaseven@xxxxxxxxx>
>>>
>>>
>>>
>>> Hi Experts,
>>>
>>> I am validating one high speed interface for my new design.I just wanted
>> to
>>> ensure differential impedance of my IC package+PCB interconnect+backplane
>>> connectors are in 90-110 ohm range.
>>>
>>> My connector vendor shared their s-parameter model and their TDR
>>> report.Their plot shows 85-102 ohm variation.They just applied 100 ps
>>> square
>>> pulse and generated their TDR plot, whereas in my tool i generated TDR
>>> directly from S-parameter(no pulse applied,this tool has this option),and
>>> my
>>> plot shows different result(95-103 ohm).I am just checking with my
>> vendors
>>> on this.My question is,no vendor always share their report along with
>> their
>>> model.Many of us would have decided their channel's TDR is good as it
>> shows
>>> 95-103.But actually different tool has different method to generate TDR
>> and
>>> shows different plot,like my vendors' report and my report.So if someone
>>> provide their model without their own correlation report,how do we
>> believe
>>> our results are correct?
>>>
>>> --
>>>
>>> bala
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> ------------------------------
>>>
>>>
>>>
>>> Msg: #2 in digest
>>>
>>> Subject: [SI-LIST] Re: TDR and S-parameter
>>>
>>> From: "Alfred P. Neves" <al@xxxxxxxxxxxxxxxxx>
>>>
>>> Date: Sun, 29 Sep 2013 11:31:06 -0700
>>>
>>>
>>>
>>>
>>>
>>> I would correspond the two TDR (one from S-parameters, the other directly
>>> measured) with a standard like stepped impedance Beatty standard, along
>>> with
>>> a 50ohm airline.
>>>
>>>
>>>
>>> Anritsu and Maury market them as calibration validation standards. They
>>> are a little tricky to use because they need to be connected with a bit
>> of
>>> care, since there is either only one or no beads supporting the airline.
>>>
>>>
>>>
>>> Our website has a DesignCon tutorial we did with Anritsu last year that
>>> discusses this measurement validation approach. " Methods of Improving
>> 3D
>>> EM Model Development and Associated Time/Frequency Domain Measurements",
>> we
>>> wrote it with Jon Martens, Bob Buxton at Anritusu and Josiah Bartlett at
>>> Tektronix.
>>>
>>>
>>>
>>>
>>>
>>> Both coaxial and planar standards (Channel Modeling Platforms) IMHO are
>> the
>>> most underutilized tools for the SI engineers. Coaxial have specific
>>> strengths for measurement validation (they are NIST traceable), versus
>>> planar standards (channel modeling platforms) have ability to also
>> include
>>> correspondence of measurement and 3D EM solver analysis.
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Products for the Signal Integrity Practitioner
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>> Alfred P. Neves
>>>
>>> Chief Technologist
>>>
>>>
>>>
>>>
>>>
>>> Office: 503-679-2429
>>>
>>>
>>>
>>> www.wildrivertech.com
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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>>
>> --
>>
>> Scott McMorrow
>> Teraspeed Consulting Group LLC
>> 16 Stormy Brook Rd
>> Falmouth, ME 04105
>>
>> (401) 284-1827 Business
>>
>> http://www.teraspeed.com
>>
>> Teraspeed® is the registered service mark of
>> Teraspeed Consulting Group LLC
>>
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>>
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