[SI-LIST] Re: PEC in 3D field solver?
- From: <J_Dong@xxxxxxxxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Fri, 15 Apr 2011 09:27:38 -0400
I personally think that the simulation should be accurate as long as the
following issues are clearly considered. It does not matter what tools you
use to simulate the package (assuming the solvers are accurate).
1) The cascaded network's ports should have same/similar modes. For
instance, you have two subsystems optimized as two wave-port networks, the
first one with single mode, and 2nd one with more than one dominant mode.
Cascading those two networks might lead to unexpected system response. Most
hybrid solvers now can take into account multiple modes, but you have to
give it the right ones first in order to see correct results.
2) Even the ports have similar modes. When they are cascaded, the interface
should not generate much unexpected mode. For example, two systems may both
have 50-ohm coaxial outputs, if their diameters are too different from each
other, it could generate considerable amount of high order modes, which may
couple back to your dominant mode resulting high return loss etc. This is
typically true at high frequencies.
3) Impedance vs. modes. When we talking about impedance, normally we are
talking about it for a certain mode. Even the two systems have same
input-output impedance, it does not mean they will always follow the return
loss = (z1-z0)/(z1+z0) at system level. Choosing the right way of
calculating impedance (e.g. v/i, v**2/p, p/i**2) and properly choose the
mode (e.g. avoid cascading diff type of ports like microstrip - stripline)
helps when doing simulation.
4) The impact on system due to cross-talk or mutual coupling between the
subsystems should be negligibly small. Otherwise, your hybrid solver may
give you a prediction that is way-off from reality. Of course, it is a
unnecessary worry if the solver is completely full wave.
Please correct me if something is missing here. Thanks.
Regards,
Junwei Dong
Microwave Engineering Corporation(MEC)
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Brad Brim
Sent: Thursday, April 14, 2011 11:13 AM
To: 'Inmyung'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: PEC in 3D field solver?
hello Delta,
Based on your text it is impossible to know what you are doing with your 3D
EM solver so it is not possible to answer your general question. However, it
is possible to explain the 3D EM analogy to pin grouping in a hybrid solver.
First, pin grouping is not required in a hybrid solver nor in a 3D EM
solver..
Next, pin grouping really only makes sense for port definitions, so I will
focus the response in this manner.
In hybrid solvers as well as 3D EM solvers, ports are 2-terminal devices. In
a hybrid solver your port is a logical connection, not physical object. You
can select multiple pins as each the (-) or (+) terminal of a port. You are
simply providing an electrical short circuit amongst those multiply-selected
pins for each terminal. In a 3D EM solver your port is a physical 2D object.
As most 3D EM solvers are formulated this object will represent a surface
through which current flows between the (-) and (+) sides of the port. (or
for so-called wave ports, less common to SI/PI simulations, a face through
which energy flows into the circuit) In a 3D solver, if you wish to group
pins, then you must draw a physical object to short them together.
Take for example the die-side connection of a BGA package. Assume you choose
a VDD pin in the core power area to define as a port and you wish to use a
group of local VSS pins as a return path. In a hybrid solver you simply link
this single VDD pin to the (+) terminal of the port and you link each of the
multiple local VSS pins to the (-) terminal of the same port. You have
electrically short circuited these the local VSS pins to use as the single
(-) terminal of the port. Current will flow as it desires into each of these
VSS pins. If one of them has a higher impedance path than the others, then
it will carry less return current for the port. For a 3D solver you must
physically connect the local VSS pins you wish to use as a return path for
the single VDD pin. What many people do is draw a 2D PEC object in the XY
plane at the top of the solder bumps. The shape is not critical but it must
touch all the VSS solder bumps. The port is then defined between this 2D PDC
object and the single VDD bump. You can make a hole in this 2D object to
define the port as an annular ring in the plane of the 2D object or you can
shorten the VDD solder bump slightly and define a vertical port between the
bottom of the 2D object above and the top of the shortened VDD solder ball
below.
If a VDD port bothers you, then simply replace "VDD" in the text above with
"signal". Exactly the same concept applies.
Based on this example, the analogy between pin grouping in a hybrid solver
and a 3D EM solver should be more clear. In a hybrid solver multiple pins
may be electrically linked to a single pin to provide a virtual short
circuit amongst those pins to define a single circuit electrical node.
However, in a 3D EM solver there must be made an explicit physical
connection in the 3D geometry prior to port definition.
A common concern that you did not express is: Your driver models may well be
multi-pin circuits with signal pins, VSS and VDD/VCC pins. The drivers may
be single or dual referenced. How do you hook-up these multi-pin, multiple
power domain driver models to your simulation ports? The summary answer is
that you must define multiple ports to which to connect your multi-terminal
driver circuit. Sorry, but a single port will not be adequate to represent
dual referenced drivers.
best regards,
-Brad Brim
Sigrity
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of Inmyung
> Sent: Thursday, April 14, 2011 12:45 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] PEC in 3D field solver?
>
> Hi everyone,
>
> Some engineer try to analyze the package by using 3D full
> wave field solver such as HFSS.
> But as you know, the power and ground nets was disconnected generally.
> So they put the PEC metal to short the disconnected ground nets.
> Is it a right method?
> Is there any other methodology to simulate package?
>
>
> How about Hybrid tools?
> They do not use PEC but pin grouping.
> I guess pin grouping means equi-potential so that is similar
> with PEC right?
>
> Thanks,
> Delta
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