[ibis-macro] Re: Analog Model Isolation Definition
- From: "Bob Ross" <bob@xxxxxxxxxxxxx>
- To: <radek_biernacki@xxxxxxxxxxx>, <ibis-macro@xxxxxxxxxxxxx>
- Date: Thu, 22 Mar 2012 14:23:19 -0700
Hi Radek:
It looks like you are right at least when s12=0.
I did not know this.
According to HP Application Note 95-1:
http://www.hpmemory.org/an/pdf/an_95-1.pdf
page 58 for 2-ports shows the voltage gain:
Av = V2/V1 = s21(1+LL)/((1-s22*LL)(1+s11'))
where
s11' = s11, if s12=0 and
LL is the fixed reflection coefficient of an arbitrary output
load ZL.
Note: s11' = s11 + s12*s21*LS/(1-s22*LL), where LS is
the source reflection coefficient, but I am simplifying this
with s12=0.
So I believe s11 remains a factor in the overall voltage gain,
but it can be cancelled exactly when:
s11 = 0, Av = s21(1+LL)/(1-s22*LL)
s11 = 1, Av = 2*s21*(1+LL)/(1-s22*LL)(1+1)), which is the same.
(at least when s12=0).
Bob
-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of
radek_biernacki@xxxxxxxxxxx
Sent: Thursday, March 22, 2012 10:49 AM
To: bob@xxxxxxxxxxxxx; ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: Analog Model Isolation Definition
Hi Bob,
A small correction. I made a typo when I composed my previous e-mail without
looking at my scrap paper derivation. The signs in the denominators should
be negative ( (1-S22) and (1-S22'), respectively), but that does not change
the conclusion.
Best regards, Radek
-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of
radek_biernacki@xxxxxxxxxxx
Sent: Wednesday, March 21, 2012 1:33 PM
To: bob@xxxxxxxxxxxxx; ibis-macro@xxxxxxxxxxxxx
Subject: [ibis-macro] Re: Analog Model Isolation Definition
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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