[SI-LIST] Re: proper Rref for return loss sim
- From: steve weir <weirsi@xxxxxxxxxx>
- To: agathon <hreidmarkailen@xxxxxxxxx>
- Date: Fri, 25 Feb 2011 00:38:31 -0800
If you are seeing peaks at specific distances, as Jory has already
pointed out, then you are seeing modal effects. No matter what you
should not be introducing new discontinuities. The portion of the
channel that is under your control, including its termination should be
self-consistent, and it should match your best evaluation of the portion
that you do not control.
Steve
agathon wrote:
> Steve! Thanks for the input.
> Yes, I expect the optimal mstrip Z0 to end up being < = pkgZ0 but not
> necessarily since the RL spec profile overrules all.
> The question I had, though, is different -- because my RL results do a
> 180deg change DEPENDING ON Rref of the measurement (in simulation).
> The goal is to optimize RL, mainly doing a mstrip Z0 sweep.
>
> --->> But I need the "proper" Rref first because the results depend on
> it.
>
> The sweep results completely REVERSE their order of worst to best at the RL
> peaks @ 11 and 18GHz. The data rate is 17Gbps. The mstrip is 0.35 to 0.55
> in. The pkg diffpair is ~18mm.
> Some of the dependence on Rref arises from reflections and SWR at the
> measurement node, no doubt, so I think it's best to use Rref=mstrip_Z0.
> Right???
>
> So, I need to determine the proper measurement/sim Rref before I sweep
> mstrip Z0 and make a judgement on the RL results!!
>
> Again, the results reverse their worst-to-best order. The cases of
> measurement Rref:
> Rref=50 then I have mstrip Z0=50 is optimal and Rref < 50 is worse.
> Rref=Z0 of mstrip, I get the OPPOSITE. Then, Z0 is optimal, even when it's
> 40 ohms. THAT'S 10 OHMS DIFFERENCE. I'm sure I could continue the sweeps
> and get even worse contradiction.
>
> These are given / cannot change:
> Die & pkg sparam models use Rref=50.
> Pkg Z0 = 42.5 nom. and pkg Zeven ~ Zodd.
>
> Thanks for revisiting this.
>
>
> On Thu, Feb 24, 2011 at 5:08 PM, steve weir <weirsi@xxxxxxxxxx> wrote:
>
>
>> If by chance the die is using 50 Ohms while the package is 42.5, then
>> unless the package Tx line is very short, there is little you can do about
>> that discontinuity. Your best bet is to keep the remainder of the channel
>> at a constant impedance matched to that of the package. The ustrip and
>> termination should both match the package at 42.5 Ohms.
>>
>> Steve.
>>
>> agathon wrote:
>>
>>
>>> Hello All,
>>> It's real simple.
>>>
>>> ................ BACKGROUND ....................
>>>
>>> I'm asked to optimize the RL to be measured on a test board (tb) that will
>>> evaluate an asic serdes io cell.
>>> In general, RL depends on the measurement Rref, since some signal at
>>> measurement node will include that node's own reflections.
>>>
>>> asic -- pkg --|--- microstrip -- cap --- microstrip (taper if
>>> necessary) -- SMP
>>>
>>> The asic diffpair cell model is a 2-port (p/n) with Rref=50. The pkg is
>>> s-param with Rref=50 and the diffpair in the pkg has Zdiff=85 loosely
>>> coupled (Z12 ~ 1ohm). First, I'm just looking at the die/pkg/mstrip
>>> without
>>> any cap or rightside mstrip or SMP. I really things will change w/ those
>>> in
>>> the sim.
>>>
>>>
>>> ............... THE QUESTION .....................
>>>
>>> In simulation, just with mstrip to left of cap + pkg + asic, the RL is
>>> found
>>> with Rref termination at mstrip end = 50 and various mstrip Z0
>>> ([Zodd+Zeven]/2).
>>> The best to worst, against a template, is for mstrip Z0= 50, 45, 42.5, 40.
>>> Ie: max RL.
>>>
>>> asic -- pkg --- mstrip <--- Rref---gnd
>>>
>>> If I change the mstrip Rref to = mstrip Z0 then the best to worst reverses
>>> order. So, there's an issue in my mind... WTF is the "proper" Rref for
>>> the
>>> sim? The term "50 ohm system" doesn't really apply, it seems.
>>>
>>> The die Zdd11 smoothly goes: 75.6.. ~54.7 ..~94.6 ohms
>>> (@200Mhz..10GHz..18GHz) and Zcc11 (18.9.. 15.. 23.8ohms). So, for the die
>>> the coupling is loose and the "Z0 equivalent" ranges approx. 38 .. 29 ..
>>> 47
>>> ohms.
>>>
>>> Since I'm optimizing mstrip for asic+pkg, shouldn't I be using Rref =
>>> Z0(mstrip), instead of Rref=50? Obviously, we're lowering mstrip Z0 below
>>> 50 to better match the die & pkg and will recommend that to customers.
>>> This
>>> is for eval and for customer demo. We're not trying to make RL
>>> unrealistically good, just trying to support accurate measurement.
>>>
>>> Thanks for the input.
>>>
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>>>
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>> --
>> Steve Weir
>> IPBLOX, LLC 150 N. Center St. #211
>> Reno, NV 89501 www.ipblox.com
>>
>> (775) 299-4236 Business
>> (866) 675-4630 Toll-free
>> (707) 780-1951 Fax
>>
>>
>>
>>
>
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--
Steve Weir
IPBLOX, LLC
150 N. Center St. #211
Reno, NV 89501
www.ipblox.com
(775) 299-4236 Business
(866) 675-4630 Toll-free
(707) 780-1951 Fax
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