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Forums > Silicon-package-board > Signal Integrity and Modeling

Subject: Definition of Cut-off Frequency & How to use it correctly
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kingseiko
Posts: 1
Online: User is Offline
5/03/2006 4:51 PM  
According to the FAQ " Calculating Impedance in SPECCTRAQuest " the following is stated for when cutoff frequency in simulation preference is set to non-zero values: " The field solver usually provides the frequency dependent RLGC parameters up to 50GHz to cover the asymptotic behavior of those parameters. "

I would like to know how is this 50GHz boundary is being defined and how is this related to the non-zero value that you key into the cutoff frequency menu.

Is this cutoff frequency has anything to do with the rise/fall time of a signal and is this value has any relationship with the one that Howard Johnson has defined in his MAGIC book ( the knee frequency )?

For example, if I have a signal that has an edge rate of 100ps ( 10/90% ) so what will be the cutoff frequency that I should key-in in order to make sure the loss calculated in the field solver will cover the bandwidth of this signal?

Thanks very much.

Michael Chan
elie
Posts: 0
Online: User is Offline
5/04/2006 12:03 PM  
I'd take the cutoff frequency, in the simulation preference, as the max excitation freq contained in the signal edge.  This is approximated to :.35/tr.
So in your case, this would be .35/100e-12= 3.5 Ghz.
Donald Telian
Posts: 42
Online: User is Offline
5/09/2006 8:43 AM  

You might want to learn to look at the RLGC tables generated in the file "ntl_rlgc.inc" down in the /sim subdirectory.  That will give a practical feel for what the tool is doing.  It is a text file - watch for the FREQUENCY=xx lines.

The help info cited in the first post is saying that when you use a non-zero value the tool will likely decide to solve up to 50 GHz.  For example, yesterday I was doing sims set at 3 GHz cut-off yet I find tables up to 50 GHz in the .inc file.

The edge-speed "rule of thumb" is helpful.  Others also decide to care about up to the 5th or 7th harmonic of the simulation frequency (5x or 7x).  Whatever you decide, be advised that the tool is likely to go higher to ensure the asymptotes are correct.  That's what the manaul is saying.

Donald
Donald Telian
SI Consultant
telian@sti.net
559-760-5793
elie
Posts: 0
Online: User is Offline
5/10/2006 8:40 AM  
"when you use a non-zero value the tool will likely decide to solve up to 50 GHz.  For example, yesterday I was doing sims set at 3 GHz cut-off yet I find tables up to 50 GHz in the .inc file."

I don't understand why would the tools goes higher than it was setup for especially to 50 Ghz...?
I check my rlgc file all the time.  So far it went to the frequency I set it up
and I always go to the highest harmonics contained in the fastest signal edge; because I want the worst case.
Yet, this is not good news for those who are doing postlayout simulation with all the traces involved, would not this be time consuming?  It must be....
elie
Posts: 0
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5/10/2006 12:49 PM  
"Yet, this is not good news for those who are doing postlayout simulation with all the traces involved, would not this be time consuming? It must be...."
Clearly, I am talking about the case when the simulator goes to 50Ghz and ignoring the set frequency.
From my experience, and if it is the case( I have Not witnessed what Donald is describing)
Cadence Folks are quick into issuing a PCR and correcting the issue.
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Forums > Silicon-package-board > Signal Integrity and Modeling > Definition of Cut-off Frequency & How to use it correctly


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