[SI-LIST] Re: proper Rref for return loss sim
- From: "White, Christopher" <White.Christopher1@xxxxxxxx>
- To: "agathon" <hreidmarkailen@xxxxxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 24 Feb 2011 16:04:40 -0600
Hello Agathon,
That is a question that really needs more than a simple answer.
The details are dependent on a number of factors including the ASIC I/O
structure, the maximum data rate and the low frequency cutoff. Briefly,
though, I would say that you should match the Zdd11 and Zcc11 of the
receiver or driver. If your coupling is -20dB or less, you can use:
Z0 = SQRT[ (Zdiff/2 * Zcommon*2)]
Or you can treat the microstrip as a match between the measurement and
the die:
Z0mstrip = SQRT[ Z0die * Z0smp] i.e. SQRT[ 42.5Ohm * 50Ohm] = 46Ohm
Regards,
Chris
Christopher White | Senior Engineer | SPPDG | 507-538-5480 |
white.christopher1@xxxxxxxx
Mayo Clinic | 200 1st St. SW | Rochester, MN 55905 |
http://www.mayo.edu/sppdg
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of agathon
Sent: Thursday, February 24, 2011 3:34 PM
To: si-list
Subject: [SI-LIST] proper Rref for return loss sim
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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