FreeLists / si-list / [SI-LIST] Re: General Trace Impedance

[Adrian Udenze - Sun Uk - NSUK - Signal Integrity Engineer <Adrian.Udenze@xxxxxxx>]

Hi Kedar,
it depends on the technology you are working with - 

- you will normally have a range of impedance for most technologies, check to 
make sure you are violating the impedance specs. you may not be.

- if you are then some pre-layout simulations would be in order as this will 
give you a feel for what signals will look like

- your results should reveal how much reflections you'll have with the new 
impedance value which will depend primarily on the rise time of your driver, 
your topology, frequency and timing.

- if you have reflections and they are a probelm (depending on  and the 
frequency / clocking scheme of technology, Max AC specs) you may want to 
experiment with termination.

- You simulations will also reveal what the result of the voltage / current 
divider formed with your new impedance is like - this should generally be OK 
but 
if not termination / timing can help.

In brief there are quite a few things to take into account before one can say 
whether the design will be OK or not.
Good news is I think it is workable with good design / thoughts.

Adrian Udenze
Sun Microsystems UK


> Delivered-To: si-list@xxxxxxxxxxxxx
> From: "Kedar P. Apte" <kedar@xxxxxxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Subject: [SI-LIST] General Trace Impedance
> Date: Fri, 31 Aug 2001 12:06:27 +0530
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> 
> Hi Gurus
> 
> I am designing a board in which I want 50 ohms Zo for some 60% traces
> and rest of the traces 65 ohms 
> but I am getting the trace width difference for both the impedance 
calculations which is a bit problem in that I cannot complete the board 
> and I cant also restrict the signals to specific layers
> 
> So if I use 60ohms and 65 ohms traces will it make much of a difference in 
operation
> 
> actually 65 ohms is for CPCI which I cant change and 50 ohms is for all TTL 
level logic if I make this 50 as 60 ohms what difference will it make to the 
operation
> 
> Thanks in advance
> 
> -------------------------------------------------------
> With Regards,
> Have A Nice Day,
> 
> Bye,
> 
> KEDAR P. APTE
> RTSDG, C-DAC Bangalore
> Off :+91-080- 5341909/1874 - 145
> Res :+91-080- 5269939
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