comments on BIRD 31.1

To: ibis@vhdl.org

Subj.: Mated connector model, BIRD 31.1

Regarding the proposed BIRD31.1 specs, I have the following questions/comments

1. Power and ground pins assignment will affect L,C values

1. Ground (and power) pins assignment will affect both the self LC values
of a pin and the LC coupling matrix, based on the position of each pin
relative to the ground pins.
Consider the following three connector pins example:

                o o o
                ^ ^ ^
                | | |
               gnd 1 2(or gnd)

The pins are 50 mils apart.

When only the center pin is a signal and the other two are ground I get:

SINGLE CONDUCTOR LC:
____________________

 i j Lij Cij Ze Zo Se So Fwdx Rvsx
from to (nh/in) (pf/in) (ohms) (ohms) (ns/ft)(ns/ft) (s/s) (v/v)
--------------------------------------------------------------------------
  1 1 10.537 3.066 58.63 - 2.16 - - -
;

and when only one side pin is a ground I get:

DUAL CONDUCTOR LC:
_________________

 i j Lij Cij Ze Zo Se So Fwdx Rvsx
from to (nh/in) (pf/in) (ohms) (ohms) (ns/ft)(ns/ft) (s/s) (v/v)
--------------------------------------------------------------------------
  1 1 15.880 3.066 88.36 - 2.16 - - -
  1 2 10.903 1.502 130.90 33.01 2.46 1.81 0.303 0.597
  2 2 22.247 2.188 123.78 - 2.16 - - -
;

Which clearly demonstrates the fact that you can not assign LC values to
connector pins just by their physical location in the connector. You also
need to know their position relative to the ground pins.

This can be accomplished by depicting several most likely to be used signal-gnd
configurations and providing the LC matrices to all of them. In a dual
row connector you may end up with about 10 different combinations.
This kind of information is precisely what's needed for high speed
simulations and will make the modeling effort worthwhile from system
engineering point of view.

2. Another level of complication in a mated model is the plug-in daughter
card. The fingers themselves are part of a PC board. How long are those
fingers in the mated model? is there any definition for it?
The electrical characteristics can be affected by the presence (or absence)
of a ground plane in between the daughter cards' top and bottom layers (the
finger layers). How is this going to be reflected in the model?

3. Another issue is measurements and verifications of the model itself.
Since we are talking about relatively small LC values, the test set-up
can greatly affect the measured LC values.
You may build a very elaborate test fixture at your facilities and obtain
very precise figures. But can every vendor or user do the same?
How do we decide which values are the right ones. I believe that we need a
well defined measurement methodology to be included as part of the
specs.

4. Another method for model development is to use the mechanical drawings and
feed the physical dimension to a field solver. If precise mechanical dimensions,
including the ER of the connector body, can be provided, then a field solver can
be used to extract the LC matrices. This can be an alternative to the above methods.
The issue here is which field solver to use? 2D? 3D? from which CAE vendor?
etc.

David Fogel
Received on Thu Apr 11 16:47:51 1996