[IBIS] Recap of BIRD74.2


Subject: [IBIS] Recap of BIRD74.2
From: deBurgh, Guy (guy_deburgh@mentorg.com)
Date: Mon Apr 07 2003 - 14:01:28 PDT


BIRD 74 - recap
===============

April 7, 2003
Please direct comments, questions to the author listed below:
Guy de Burgh, mail to: guy_deburgh@mentor.com (805) 278-6823

1) Introduction

This document is a recap of progress and discussions on BIRD74
"EMI Parameters". It outlines the reasoning behind the BIRD, what was
proposed, and comments and changes that were made.

2) History

* The first presentation was given at the IBIS Summit Meeting at DAC on
  June 21, 2001.
* A second presentation was given at DesignCon on January 28, 2002.
* The BIRD was submitted in November 2001.
* Current version, 74.2, was submitted in March 2002.
* There has been little or no discussion on the BIRD since March 2002, due,
  in part to the focus of IBIS on the multi-lingual extensions.

3) Why the BIRD was introduced

EMI is becoming increasingly important in PCB design. All electronic
equipment
has to pass an FCC emission test. More and more manufacturers are looking
towards some kind of simulation/analysis that will help them solve EMI
problems
before finding them in testing. As clock frequencies go higher and higher
EMI
becomes more of a problem.

However EMI simulation is not an exact science. The problem is too
complicated
for today's computers. You will probably have to simulate for 10 years to
get
and accurate answer. Obviously unacceptable. So simulation is out, but some
kind of analysis that is able to identify which signals radiate more than
others (relatively), and show where the energy flows in a system, can help
in
identifying potential EMI problems.

No longer requiring absolute accuracy makes the analysis considerably
easier,
and, more importantly, possible. With that in mind there is more freedom in
choosing the parameters needed for the analysis. So the parameters that have
been chosen are ones that are readily available in data books. This makes it
very easy to create models.

There is also a choice, too, in how to use these parameters. The Reference
section at the end of this document provides the technical documents behind
these parameters and their usage.

4) EMI Mechanisms

The EMI mechanisms considered are:

* Differential Mode
  Direct radiation from signal loops.

* Current-Driven Common Mode
  Signal return currents create noise voltages across finite return planes
  that drive attached cables.

* Voltage-Driven Common Mode
  Signal voltages and noise voltages due to crosstalk drive attached cables.

* Power Bus
  Digital switching creates power bus noise.

* Heatsinks
  Act as antennas driven by potential differences on the return plane.

* Crosstalk
  Coupling among nearby traces.

* Immunity
  Electro-Static Discharge (ESD), Magnetic (differential model) and Electric
  (common mode) Field Susceptibility.

5) The Parameters

Here are some of the topics that were discussed, but not necessarily
resolved,
at the teleconference meetings and by email:

* The parameters cannot be expressed using the multi-lingual extensions.
  The proposed parameters are of the form "name = value" rather than an
  expression.
* The parameters should occupy their own chapter.
* The parameters are to be bounded by [Begin EMI ...] and [End EMI ...]
  statements.
* Store SI and EMI parameters in the same place, i.e. in an IBIS model,
  as the SI part of the IBIS model already contains information that can be
  used in EMI analysis (e.g. rise/fall time, max. current).
* There is no specific version of IBIS that this BIRD is targeted for.

6) Here are the proposed parameters in BIRD74.2:

The following parameters are valid in a [Component] statement:

* Type
  This parameter indicates whether the component is a connector. It
identifies
  the I/O signals which act as monopole antennas.

* Domain
  Defines whether the component is Analog, Digital or both. Analog circuits
  operate a very low signal levels (mV or uV) and can contain high gain
  amplifiers. Digital circuits, in contrast, operate at relatively large
  signal levels (compared to Analog circuits). See page 276 of reference
[5].

* Family
  Describes the logic family. This can be one of UNDEF, TTL, CMOS or ECL.
  This parameter allows you to make reasonable assumptions regarding missing
  information (e.g. source impedance, input impedance, max. transient
switching
  current). E.g. an unused, floating, CMOS input is likely to pick up noise
  because of its high input impedance. This is true for all logic families
  with high input impedance. This noise could cause unintentional switching.
  In CMOS, however, an unused gate whose inputs are floating may bias itself
  into the linear region. This will increase the dc current drawn by the
  circuit. See page 295 of reference [5].
  With this parameter you can make reasonable assumptions for all the other
  parameters.

* Cpd
  Power Dissipation Capacitance.
  Cpd includes both internal parasitic capacitance (e.g., gate-to-source and
  gate-to-drain capacitance) and through currents present while a device is
  switching and both n-channel and p-channel transistors are momentarily
  conducting.
  Simultaneous switching of outputs is one of the main causes of transient
  noise voltages on the Power Bus resulting in common mode radiation. From
the
  EMI point of view we need to know the high frequency noise on each DC
Power
  Bus due to the switching of digital circuits. Fourier transform the
transient
  current drawn from the power bus. Now knowing the current spectrum, the
  magnitude of each harmonic of the power bus noise voltage can be
calculated.
  (See references [6] and [7].)
  National Semiconductor, Texas Instruments, and Fairchild have datasheets
and
  application notes on Cpd.
  Some catalogs give the value of "Dynamic Power Supply Current Iccd."
  Where Iccd = Vcc * Cpd
  As a side note, Cpd is also used for Thermal calculations.

The following parameters are valid in a [Pin] statement:

* CSPEC name
  Describes a user definable name that corresponds to a model listed in the
  [Model] keyword. It is used to define connector pin impedances.

* Clock Domain
  Specifies the % of power used by each clock domain. A clock domain is a
  group of pins on a component.

The following parameters are valid in a [Model] statement:

* Ferrite
  Indicates that the model for this pin is a ferrite. Ferrite beads are very
  effective when used to dampen high frequency oscillations (caused by
  switching transients). They prevent high frequency noise from leaving or
  entering a circuit.

* CSPEC
  Assigns parameters to the pin CSPEC name. Possibilities are: the pin is
  Unshielded, Shielded (ground pins provide the shield), Shielded_pwr (power
  pins provide the shield), or Con_to_shield (pin connected to connector
shell).
  In addition the pin can have an explicit filter capacitance. This is used
to
  calculate an antenna impedance for the pin.

7) Using the Parameters

There are many ways in which these EMI parameters can be used. Reference [8]
explains one way of using all of these parameters. Like all technical papers
this reference explains the algorithm from a 10,000 foot (~3,048m) height.
So it is up to the reader to develop their own algorithms based on this
data.
Reference [5] gives a number of equations (e.g. page 301) that are useful in
the estimation of EMI. Some of the other references discuss a particular EMI
mechanism. E.g. references [6] and [7] discuss power bus noise.

8) Some topics to discuss

* Family keyword
  o The list should really be unlimited. If this list is fixed what do you
do
    about new or missing technologies?

  o What's the intended use?

* Additional parameters (not limited to):
  o Clock frequency of the Clock Domain keyword.
    Suggested by Zuken. This data would be helpful in improving the accuracy
    for those ICs with a high pin count.

  o Heatsink capacitance
    They act as antennas. They can be a significant EMI source.

  o Voltage Range
    This parameter describes the voltage range of a driver (or
bi-directional
    output) so that the radiation is not overestimated.
    Note that these voltages are not necessarily the same as the power
rails.
    E.g. for ECL the power rails are 0V and 3.3V, but the voltage range
    is 1.625V to 2.375V.

* Connector related parameters
  o Should these become part of a more general IBIS connector specification?
    The bad side to this is that the parameters are no longer in one file.

* The BIRD
  o Clean up syntax and errors.
  o Perhaps write an EMI cookbook to go along with the BIRD.
  o Add additional references as they are published or come to my attention.
  o Solicit input from interested parties. Already IBM, Siemens and Zuken
    have made suggestions. Cadence and Intel have also provided valuable
input.

9) Reference Documents

* [1] J.L. Drewniak, T.H. Hubing, and T.P. Van Doren, "Investigation of
  Fundamental Mechanisms of Common-Mode Radiation from Printed Circuit
Boards
  with Attached Cables", Proceedings of the 1994 IEEE International
Symposium
  on Electromagnetic Compatibility, Chicago, IL, August 1994, pp. 110-115.

* [2] C.R. Paul, Introduction to Electromagnetic Compatibility, John Wiley
  Interscience, New York (1992).

* [3] D.M. Hockanson, J.L. Drewniak, T.H. Hubing, T.P. Van Doren, F. Sha,
  C.-W. Lam, and L. Rubin, "Quantifying EMI Noise Sources Resulting from
  Finite-Impedance Reference Planes", IEEE Transactions on Electromagnetic
  Compatibility, vol. EMC-39, no. 4, November 1997, pp. 286-297.

* [4] C.R. Paul, "Printed Circuit Board EMC." 6th Symposium on EMC, Zurich,
  March 1985.

* [5] H.W. Ott, Noise Reduction Techniques in Electronic Systems, 2nd
Edition,
  John Wiley Interscience, New York (1988).

* [6] S. Radu, R.E. DuBroff, T.H. Hubing, and T.P. Van Doren, "Designing
Power
  Bus Decoupling for CMOS Devices", Proceedings of the 1998 IEEE
International
  Symposium on Electromagnetic Compatibility, Denver, CO, August 1998,
  pp. 375-380.

* [7] J. Mao, B. Archambeault, J.L. Drewniak, and T.P. Van Doren,
"Estimating
  DC Power Bus Noise", IEEE EMC Symposium, Minneapolis, MN, August 2002,
  pp. 1032-1036.

* [8] UMR web page ( http://www.emclab.umr.edu/consortium )
  Click relevant selection under Public Domain Documentation.

* [9] An http://www.google.com search for "Power Dissipation Capacitance"
  shows articles by Texas Instruments, National Semiconductor and Fairchild.

* [10] Katja Koller, Gerald Bannert, "Crossbar Current out of CMOS IBIS
  Models", IBIS meeting at DATE2002.

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