Re: IBIS BIRD41.2 Modelling Series Switchable Devices

Bob,

Excellent work! The new Bird seems to cover all important issues,
especially how to model those infamous FET switches.

I'm not competent enough to give detailed comment on your
HSpice model: just happy that the model is derived from simple
tables. (Bench measurements of a FET device will be trickier
than for simple I/V measurements with a fixed load, but I can live
with that).

I hope that, before voting tales place, at least one other
simulator company will confirm to the reflector that they will
use this model.

Comments on some minor points are included below.

Once again, thanks agin for pushing though this Bird.

All the best,
John

_________________________________________________________________

Comments:

1) Voltage ranges

| [Series Current] GND - POWER GND + POWER
| [Series MOSFET] GND GND + POWER

   Ok for [Series Current]. This is exactly as for [GND Clamp], with
   the same problems of excessively large current values.
   (Would it be useful in IBIS 3 to introduce the notion of a maximum
    current, possibly set by the model creator?)

   However, for [Series MOSFET], the limits should be from
   GND-POWER to POWER+POWER. The upper range is important,
   because a series MOSFET is often used to clip signals
   e.g. in 5V to 3.3V conversion.

2) Parallel elements

   The Bird does not explain how to treat parallel elements e.g.
   C_comp, GND and POWER clamps, RC terminations. In fact, it would
   appear to disallow the combination of series and parallel elements
   (with the exception of C_comp?).

   I think that parallel elements should be allowed, and be defined
   to exist at Pin 1:

          -+- POWER
           |
         +-+-+
         | | clamp or RC termination
         | |
         | |
         +-+-+
   Pin 1 | +-----------+ Pin 2
     ------+---------------+--+ Series +---------
         +-+-+ | +-----------+
         | | Clamp or |
         | | RC term- --+--
         | | ination --+-- C_comp
         +-+-+ |
           | |
          -+- GND -+- GND
               

   Aside:
   I think it would be complicated to allow parallel elements on
   both sides of the series element. There would be however one
   potential application for this: the simple connector model we
   all use:

               +--------+
      ------+--+ L, R +-----+-------
            | +--------+ |
            | |
          --+-- --+--
          --+-- C --+--
            | |
            | |
           -+- GND -+- GND
 

3) Function table

   Two names are used for the function table:
      [Series Function Table] and [Series Pin Table]

   I think the first name is the better, but because the defined
   states are ON or OFF, the name should include the word "switch"
   e.g. [Series Switch Groups]

4) RLC model

   In the example, it is clear that, if not defined, R_c_series,
   L_c_series and R_l_series are short-circuits. Perhaps this should
   also be explained in the text?

5) [On] and [Off]
 
   State that keywords common to both [On] and [Off] should
   appear before the keywords [On] and [Off].

   Example:
   If a switch has a constant [C_series] part, must it appear twice
   (after each of the [On] and [Off] keywords) or once (before the
   [On] and [Off] keywords)?

-- 
John Fitzpatrick   <John.Fitzpatrick@ln.cit.alcatel.fr>    
Alcatel Telecom, 4 rue de Broglie, 22304 Lannion, France
Tel: +33(0)2.96.04.79.33  Fax: +33(0)2.96.04.85.09