[SI-LIST] Re: trouble simulating with AC coupling capacitors in topology

Hi Roopesh,

                I am not a SI guy. I have faced similar issue while using 
circuit simulation (spice) SW.
We would connect a very high value resistor (several Megs) in parallel with 
caps.
I am just drawing parallel from circuit simulation, try if it works

Thanks
siddesh

________________________________________
From: si-list-bounce@xxxxxxxxxxxxx [si-list-bounce@xxxxxxxxxxxxx] on behalf of 
Yuriy Shlepnev [shlepnev@xxxxxxxxxxxxx]
Sent: Saturday, July 09, 2011 12:17 AM
To: 'prathap sirishe'
Cc: 'Newbie Pugal'; 'Roopesh Badala'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: trouble simulating with AC coupling capacitors in 
topology

Prathap,

You can expect faster convergence with S-parameters only if you do not have
sufficient number of samples at lower frequencies and your S-parameter model
have wrong asymptotes at DC. It often happens with S-parameters measured for
a channel with AC coupling capacitors. The insertion loss goes to very small
values at lower measured frequencies and "intuitive" extrapolation may be
something close to 0 dB loss at DC. In reality the insertion losses increase
at extremely low frequencies (depends on the values of the capacitors).  It
can also happen with modeled S-parameters if you do not set up frequency
sweep from DC with sufficient resolution at lower frequencies. Restriction
on lower frequencies in the frequency-domain analysis of a channel with AC
coupling caps may be a good alternative to the use of resistors instead of
caps discussed here. Though, I have not seen investigations of accuracy for
such analysis.

Best regards,
Yuriy Shlepnev
www.simberian.com


-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of prathap sirishe
Sent: Friday, July 08, 2011 10:49 AM
To: Andy Haas
Cc: Newbie Pugal; Roopesh Badala; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: trouble simulating with AC coupling capacitors in
topology

Hello Roopesh,
I remember facing same issue before. From what I recollect, apart from all
the solutions mentioned so far, you can try to generate a s-parameter model
for an ac coupled differential trace pair using another simulation tool like
Hyperlynx and use the resultant ".s4p" file in Cadence tool set, replacing
differential trace segment with 4-port s-parameter model.
Hope this helps!

Regards,
Prathap

On Fri, Jul 8, 2011 at 11:31 AM, Andy Haas <andyhaas@xxxxxxxxxxx> wrote:

> Hi guys,
>
> There should be no issue in simulating nets with series caps within
> the Cadence SI environment. Initial conditions can also be used if you
> use the right model. Feel free to contact me offline and I'd be happy to
help.
>
> Regards,
> Andy
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx
> [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Newbie Pugal
> Sent: Friday, July 08, 2011 9:50 AM
> To: Roopesh Badala; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: trouble simulating with AC coupling capacitors
> in topology
>
> Hi Roopesh,
> I have faced similar problem in the past and went to cadence for the
> support but did not worked out. With the AC caps in the cadence the P
> and N waveforms will not come as expect. Later I simulated using
> HSPICE with initial condition settings or else you need to run a long
> bits for the settled waveforms.
>
> Regards
> Pugal
>
>
> ________________________________
> From: Roopesh Badala <roopesh.badala@xxxxxxxxx>
> To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
> Sent: Friday, 8 July 2011 4:54 PM
> Subject: [SI-LIST] trouble simulating with AC coupling capacitors in
> topology
>
> Hi All,
> I am trying to simulate a topology which has AC coupling capacitors in it.
>
> Using Cadence Allegro PCB SI tool set , i am unable to get the
> expected output.
>
> but when I simulate the same topology by removing the AC coupling
> capacitors in signal path, I get a perfect waveform.
>
> Is this a tool limitation..? can some body suggest me if they have
> faced similar issue.
>
>
> Thank you
>
> Roopesh
>
>
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