[ibis-macro] Re: Analog Model Isolation Definition

  • From: <fangyi_rao@xxxxxxxxxxx>
  • To: <james.zhou@xxxxxxxxxx>, <bob@xxxxxxxxxxxxx>, <ibis-macro@xxxxxxxxxxxxx>
  • Date: Wed, 21 Mar 2012 12:22:53 -0600

I agree with James. Spice simulations never need to make any assumption on the 
values of S-parameters, and it should be same case here for IBIS analog model 
specification. I don't think it's necessary to include examples of how to treat 
special S-parameters. Matrix stamping of S-parameters are standard textbook 
simulation techniques.

Regards,
Fangyi

-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of James Zhou
Sent: Wednesday, March 21, 2012 10:53 AM
To: bob@xxxxxxxxxxxxx; 'IBIS-ATM'
Subject: [ibis-macro] Re: Analog Model Isolation Definition

Hi Bob,

Thanks for providing the link of past references. It is helpful for those of us 
who were not on the list a couple of years ago.

With reference to: "the complete interface needs to be known and defined ... to 
get the proper gain (S21 term).", this is a proven and well-known principle in 
circuit and LTI network analysis. The conclusion at yesterday's meeting 
sufficiently satisfies this requirement. The conclusion as I interpreted was 
that, IBIS Specification only need to say that the Tx analog circuit input must 
be driven by ideal voltage sources (i.e. zero source impedance), and Rx analog 
circuit output must drive a load with infinite impedance. Once this is 
established, the Specification does not need to say anything about what goes 
into, or how to model, the "bigger blockbox" in the diagram (provided it 
satisfies other IBIS requirements such as LTI and IBIS-ISS compliant, etc.).

With reference to: "In both cases, the transfer function S21 term is changed by 
a factor of 2...", these two cases represent total reflection by open or short 
circuits (S11=1 or -1). In both cases, S21 must be 0. This result is consistent 
with the general formula of S-parameter modeling when S11 can have arbitrary 
values (including the cases of S11=1 or -1). The treatment for these specific 
cases of open and short circuits can be provided in the Specification as 
examples instead of requirements of the Specification.

On the issue of setting S12=0, there have been IBIS discussions or 
presentations recommending this. However, it is not necessary to impose this 
condition on the analog circuits and it should not be treated as a requirement 
of the BIS Specification.

Regards,
James Zhou
QLogic Corp.



-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx [mailto:ibis-macro-bounce@xxxxxxxxxxxxx] 
On Behalf Of Bob Ross
Sent: Tuesday, March 20, 2012 2:57 PM
To: 'IBIS-ATM'
Subject: [ibis-macro] Analog Model Isolation Definition

All:

Per the discussion today on the block interface definition, here is a 
derivation based on BIRD122 for just s2p 2-ports (but should apply to s4p files 
with more math).  It  shows the gain sensitivity to the explicit S11 term for 
TX and S22 term for RX from a reference impedance definition (e.g., TX: S11=0 
or 50 ohm to infinite impedance S11 = 1 and RX: S22=0 to zero impedance S11=-1).

In both cases, the transfer function S21 term is changed by a factor of 2 
(assuming no S12 term).  This is equivalent to specifying the isolation 
interfaces with a unity gain ideal voltage amplifier.

//www.freelists.org/post/ibis-macro/Eelement-in-analog-model-BIRD,1

Sorry, but some of the prior material may have involved some lost or private 
e-mails and also a presentation or e-mail  shown to the ATM committee according 
to questions and exchanges in the Dec. 14, 2010 minutes including:

Walter showed an email from Fangyi:
- Fangyi: The equation is at the bottom
- Bob: We have defined S12 as zero
  - The real voltage gain is a function of S11
- Todd: Is this because S11 can affect V1 and therefore V2?
- Fangyi: No
  - If V1 was fixed V2 would still be affected by S11
- Bob: The defaults should be zero for series impedances and
  infinity for impedances to ground.

http://www.vhdl.org/ibis/macromodel_wip/minutes/20101214/mikelabontecisco/Mi
nutes%20for%20the%2012-14-2010%20ibis-atm%20meeting/20101214.txt

The main point is that the complete interface needs to be known and defined in 
the Specification (for S-parameters and perhaps different for a SPICE model) to 
get the proper gain (S21 term).

Bob

--
Bob Ross
Teraspeed Consulting Group, LCC
http://www.teraspeed.com
bob@xxxxxxxxxxxxxx
Direct : 503-246-8048
Teraspeed Labs: 503-430-1065
Headquarters: 401-284-1827

Teraspeed is a registered service mark of Teraspeed Consulting Group LLC



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