[ibis-macro] Re: Analog Model Isolation Definition
- From: <radek_biernacki@xxxxxxxxxxx>
- To: <bob@xxxxxxxxxxxxx>, <ibis-macro@xxxxxxxxxxxxx>
- Date: Wed, 21 Mar 2012 14:33:21 -0600
Hi Bob,
In your original e-mail you derived the S-parameters of a cascade of the
unity-gain amplifier (E-element, infinite input impedance, zero output
impedance) and a specific S-parameter matrix in the form
[Stx] = [0 0 ]
[S21 S22 ]
to take the form
[S-etx] = [1 0 ]
[2*S21 S22 ]
where S21 and S22 refer to the original matrix.
I have not verified your derivation but it looks correct. However, the fact
that the S-parameter 21 changes, that is S21' = 2*S21, does not mean that the
voltage gain (that's what we are talking about using an ideal voltage source Vs
at the input and measuring the open-circuit voltage Vout at the output) is
different. In fact, it can be shown that
Vout/Vs = 2*S21/(1+S22)
in the case of [Stx], and
Vout/Vs = S21'/(1+S22')
in the case of [S-etx]. Therefore, since S21' = 2*S21 and S22' = S22, the
voltage gain is the same in both cases.
Best regards, Radek
-----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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