[ibis-macro] Re: Replacing BIRD 122 with "Reserved_Model" keyword in [External Model]
- From: "Walter Katz" <wkatz@xxxxxxxxxx>
- To: <scott@xxxxxxxxxxxxx>, "'James Zhou'" <james.zhou@xxxxxxxxxx>
- Date: Tue, 17 Jan 2012 22:31:38 -0500 (EST)
All,
Please note that IBIS-AMI is only looking at the differential signal. So
the only thing that AMI assumes is a constant differential impedance. The
fact that IBIS AMI assumes that an Impulse Response is all that is
required to define the channel between the algorithmic Tx and Rx, and
since LTI assumes that if one doubles the input one doubles the output,
one must assume that the impedance of the Tx and the Rx is constant over
variations of the amplitude of the signal.
Of course drivers have non-linear behavior at the limits when they are
driving high or low, but there are several things that happen with a
SerDes driver. First, if one has a ~50 Ohm driver with a full swing
differential output of 1V, what is the voltage at the output of a driver
when driving a long 50 Ohm channel? And is this voltage level in the
non-linear range of a Tx Driver? Please remember the loss at Nyquist
frequency for these channels is often between 15dB and 30dB, so the return
signal is attenuated dramatically. Also, when pre- emphasis and
post-emphasis is used in the Tx, the output of the algorithmic model that
drives the Tx output buffer often does not reach the 1V swing. Finally,
any non-linear behavior is at frequencies around the rise time of the
input to the Tx output buffer. These frequencies are attenuated very
quickly. I think the characterization of my models being Zero Order is
quite incorrect. I would accept characterizing them as First Order, and
realize that any second order affects are quickly attenuated.
Walter
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Scott McMorrow
Sent: Tuesday, January 17, 2012 9:16 PM
To: James Zhou
Cc: IBIS-ATM; Arpad_Muranyi@xxxxxxxxxx
Subject: [ibis-macro] Re: Replacing BIRD 122 with "Reserved_Model" keyword
in [External Model]
James
if the driver output impedance is constant it is therefore "static", which
is a synonym. It is a non-real fictional entity. It is a zero order
model for output impedance. By definition a driver is not LTI. It is not
part of the interconnect. A driver is used to drive an interconnect, not
be part of the interconnect. If you want to perform LTI linearization, I
contend that it be done after the driver is used to "ping" the
interconnect. Otherwise, how will the real condition of differing rising
and falling output impedance of drivers be handled, along with other
non-ideal driver characteristics? IBIS driver models do a better job of
this than a simple output impedance.The model that Walter proposes is an
over-simplification.
Scott
On Tue, Jan 17, 2012 at 9:01 PM, James Zhou <james.zhou@xxxxxxxxxx> wrote:
Hi Scott,
Not quite sure what is "static driver output impedance". Walter's original
statement "the impedance is constant over the operating range of the
device" is just another way to say that the Tx driver is LTI.
Thanks,
James Zhou
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Tuesday, January 17, 2012 4:51 PM
To: James Zhou
Cc: IBIS-ATM; Arpad_Muranyi@xxxxxxxxxx
Subject: Re: [ibis-macro] Re: Replacing BIRD 122 with "Reserved_Model"
keyword in [External Model]
Consistent, but not complete. Small signal linearization does not
require a static driver output impedance.
On Jan 17, 2012 7:43 PM, "James Zhou" <james.zhou@xxxxxxxxxx> wrote:
Arpad,
This interpretation of "small signal" is consistent with and equivalent to
the assumption of LTI.
Thanks for the clarification,
James
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Muranyi, Arpad
Sent: Tuesday, January 17, 2012 4:27 PM
To: IBIS-ATM
Subject: [ibis-macro] Re: Replacing BIRD 122 with "Reserved_Model" keyword
in [External Model]
James,
I think by "small signal" Walter means what is described
in this article:
http://en.wikipedia.org/wiki/Small-signal_model
Thanks,
Arpad
===========================================================
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of James Zhou
Sent: Tuesday, January 17, 2012 5:25 PM
To: wkatz@xxxxxxxxxx; IBIS-ATM
Subject: [ibis-macro] Re: Replacing BIRD 122 with "Reserved_Model" keyword
in [External Model]
Hi Walter,
Please see my comments and questions below:
(1) "This implies that AMI modeling assumes a small signal interface to
the analog channel."
Can you please clarify the meaning of "a small signal interface"? The only
necessary assumption when representing a channel using impulse response is
that the channel must be LTI.
(2) "This also implies that the differential impedance of a Tx or Rx is
constant over the operating range of the device."
There are several possible cases of "impedance of a Tx or Rx". It could be
the input or, the output or, the load impedance of Tx analog block.
Please clarify which one is referred to by this statement. Same
questions exist for Rx.
Thanks,
James Zhou
QLogic Corp.
From: ibis-macro-bounce@xxxxxxxxxxxxx
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Tuesday, January 17, 2012 1:16 PM
To: IBIS-ATM
Subject: [ibis-macro] Replacing BIRD 122 with "Reserved_Model" keyword in
[External Model]
All,
I would like to extend BIRD 116 to allow the following for AMI [Model]s
[External Model]
Language ISS
Reserved_Model
AMI_Tstonefile_Tx|AMI_Tstonefile_Rx|AMI_Thevenin_Tx|AMI_Thevenin_Rx
[End External Model]
AMI modeling assumes that a channel can be represented by an Impulse
Response. This implies that AMI modeling assumes a small signal interface
to the analog channel. This also implies that the differential impedance
of a Tx or Rx is constant over the operating range of the device. Note
that any legacy IBIS model (IV, Vt curves, Series Pin), that when used in
the real operating range of a SerDes channel (where AMI modeling is
applicable) can be accurately represented by the following Thevenin Tx and
Rx models.
Reserved_Model AMI_Tstonefile_Tx
[External Model]
Language ISS
Reserved_Model AMI_Tstonefile_Tx
[End External Model]
Is equivalent to:
[External Model]
Language ISS
Corner Typ AMI_Tstonefile_Tx.cir AMI_Tstonefile_Tx
Parameters Tstonefile = AMIfile(Tstonefile) | Tstonefile must be an AMI
Model_Specific parameter
Converter_Parameters Vhi = AMIfile(Voh) | Voh must be an AMI
Model_Specific parameter
Converter_Parameters Vlo = AMIfile(Vol) | Vol is 0 if it is not an AMI
Model_Specific parameter
Converter_Parameters Trf = 1.0ps
Ports my_driveP my_driveN A_signal_pos A_signal_neg
D_to_A D_drive my_driveP 0 Vlo Vhi Trf Trf Typ Non-Inverting
D_to_A D_drive my_driveN 0 Vlo Vhi Trf Trf Typ Inverting
[End External Model]
AMI_Tstonefile_Tx.cir.
.SUBCKT AMI_Tstonefile_Tx my_driveP my_driveN A_signal_pos A_signal_neg
+ Tstonefile="NA.s4p"
Sdriver my_driveP my_driveN A_signal_pos A_signal_neg 0 MNAME=Smodel_name
.MODEL Smodel_name S N=4 TSTONEFILE=Tstonefile
.ends
Reserved_Model AMI_Tstonefile_Rx
[External Model]
Language ISS
Reserved_Model AMI_Tstonefile_Rx
[End External Model]
Is equivalent to:
[External Model]
Language ISS
Corner Typ AMI_Tstonefile_Rx.cir AMI_Tstonefile_Rx
Parameters Tstonefile = AMIfile(Tstonefile)
Ports A_signal_pos A_signal_neg A_int_pos A_int_neg
A_to_D D_receive A_int_pos A_int_neg -.02 .02 Typ
[End External Model]
AMI_Tstonefile_Rx.cir.
.SUBCKT AMI_Tstonefile_Rx A_signal_pos A_signal_neg A_int_pos A_int_neg
+ Tstonefile="NA.s4p"
Sdriver A_signal_pos A_signal_neg A_int_pos A_int_neg 0 MNAME=Smodel_name
.MODEL Smodel_name S N=4 TSTONEFILE=Tstonefile
.ends
Reserved_Model AMI_Thevenin_Tx
[External Model]
Language ISS
Reserved_Model AMI_Thevenin_Tx
[End External Model]
Is equivalent to:
[External Model]
Language ISS
Corner Typ AMI_Thevenin_Tx.cir AMI Thevenin_Tx
Parameters Rt = AMIfile(Rt)
Parameters Rs = AMIfile(Rs)
Parameters Cc = AMIfile(Cc)
Parameters Vt = AMIfile(Vt)
Parameters Cac = AMIfile(Cac)
Parameters Rac = AMIfile(Rac)
Parameters Rd = AMIfile(Rd)
Parameters Cd = AMIfile(Cd)
Parameters Trf = AMIfile(Trf)
| Only AMI Model_Specific parameters Rt Rs Cc Vt Cac Rac Rd Cd defined in
the
| .ami file shall be passed as parameters to the subckt AMI Thevenin_Tx
instance
Converter_Parameters Vhi = AMIfile(Voh) | Voh must be an AMI
Model_Specific parameter
Converter_Parameters Vlo = AMIfile(Vol) | Vol is 0 if it is not an AMI
Model_Specific parameter
Converter_Parameters Trf = AMIfile(Trf) | Trf is 1p if it is not an AMI
Model_Specific parameter
Ports my_driveP my_driveN A_signal_pos A_signal_neg
D_to_A D_drive my_driveP 0 Vlo Vhi Trf Trf Typ Non-Inverting
D_to_A D_drive my_driveN 0 Vlo Vhi Trf Trf Typ Inverting
[End External Model]
AMI_Thevenin_Tx.cir
.SUBCKT AMI_Thevenin_Tx my_driveP my_driveN A_signal_pos A_signal_neg
+ Rt=1Meg Rs=1Meg Cc=0. Vt=0. Cac=0. Rac=1Meg Rd=1Meg Cd=0.
E_H E_my_driveP 0 VCVS my_driveP 0 1.
E_L E_my_driveN 0 VCVS my_driveN 0 1.
R_Rs_H E_my_driveP A_signal_pos R='Rs'
R_Rs_L E_my_driveN A_signal_neg R='Rs'
R_Rd A_signal_pos A_signal_neg R='Rd'
C_Rd A_signal_pos A_signal_neg C='Cd'
R_Rt_H A_signal_pos Vt R='Rt'
R_Rt_H A_signal_neg Vt R='Rt'
V_Vt Vt 0 DC='Vt'
C_Cc_H A_signal_pos 0 C='Cc'
C_CC_L A_signal_neg 0 C='Cc'
R_ac A_signal_pos Nac R='Rac'
C_ac A_signal_neg Nac C='Cac'
.ends
Reserved_Model AMI_Thevenin_Rx
[External Model]
Language ISS
Reserved_Model AMI_Thevenin_Rx
[End External Model]
Is equivalent to:
[External Model]
Language ISS
Corner Typ AMI_Thevenin_Rx.cir AMI_Thevenin_Rx
Parameters Rt = AMIfile(Rt)
Parameters Cc = AMIfile(Cc)
Parameters Vt = AMIfile(Vt)
Parameters Cac = AMIfile(Cac)
Parameters Rac = AMIfile(Rac)
Parameters Rd = AMIfile(Rd)
Parameters Cd = AMIfile(Cd)
|Only AMI Model_Specific parameters Rt Cc Vt Cac Rac Rd Cd defined in the
|.ami file shall be passed as parameters to the subckt AMI Thevenin_Tx
instance
Ports A_signal_pos A_signal_neg A_int_pos A_int_neg
A_to_D D_receive A_int_pos A_int_neg -.02 .02 Typ
[End External Model]
[External Model]
AMI_Thevenin_Rx.cir.
.subckt AMI_Thevenin_Rx A_signal_pos A_signal_neg A_int_pos A_int_neg
+ Rt=1Meg Cc=0. Vt=0. Cac=0. Rac=1Meg Rd=1Meg Cd=0.
E_H A_int_pos 0 VCVS A_signal_pos 0 1.
E_L A_int_neg 0 VCVS A_signal_neg 0 1.
R_Rd A_signal_pos A_signal_neg R='Rd'
C_Rd A_signal_pos A_signal_neg C='Cd'
R_Rt_H A_signal_pos Vt R='Rt'
R_Rt_H A_signal_neg Vt R='Rt'
V_Vt Vt 0 DC='Vt'
C_Cc_H A_signal_pos 0 C='Cc'
C_CC_L A_signal_neg 0 C='Cc'
R_ac A_signal_pos Nac R='Rac'
C_ac A_signal_neg Nac C='Cac'
.ends
Walter Katz
wkatz@xxxxxxxxxx
Phone 303.449-2308
Mobile 720.333-1107
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--
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Teraspeed Consulting Group LLC
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