[SI-LIST] Re: Via stub math help needed....
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 12 Jan 2012 06:15:26 -0800
No, there is no trivial way to get an answer. You can get parasitics
piece-wise to form an estimate. Generating a reasonably accurate answer
is what the tools are good at doing.
Steve
On 1/12/2012 5:18 AM, Ralph Wilson wrote:
> Steve,
>
> And this (we're not in TEM mode) is because...........(it's been 35 years
> since I've dealt with Maxwell's math)
>
> 1 ?? The via is not in "free space" (or uniform material)? Since the via
> stub passes perpendicularly through various copper planes (ground,
> power, and signal planes), these create boundary conditions that
> cause E and M fields in the "z" direction (parallel to the via
> orientation)
> and, hence, affect the propagation speed. ???
>
> 2 ?? We should be in TEM mode and I didn't set some parameter in the
> simulation ???? (probably not)
>
> 3?? other ??
>
> Beyond actually invoking 3D simulators and the like, or manually solving
> Maxwell with the boundary conditions of a specific application, are there
> any "simple" formulas for calculating propagation speed in non-TEM
> conditions?
>
> Thanks!
> Ralph
>
> On 1/12/2012 6:42 AM, steve weir wrote:
>> Ralph, the null frequency will be Vprop/(4 * Lstub) as you expect.
>> Vprop is different because you are not in TEM mode.
>>
>> Steve.
>>
>>
>> On 1/12/2012 4:24 AM, Ralph Wilson wrote:
>>> All,
>>>
>>> While working on some SERDES nets, specifically trying to quantify
>>> the effects of some via stubs, I ran across something that has me
>>> stymied. I expect a null in S21 where the stub length (delay, actually)
>>> is 1/4 wavelength. However, the simulations are showing a null at
>>> half the frequency I predict. I've subsequently run the via model through
>>> several different tools, and although the null frequency varies a little
>>> bit due to modeling / parasitic issue, they all come up with a null
>>> frequency roughly half of what my "math" predicts. So my fundamental
>>> question is where is my theory or my math wrong?
>>>
>>> Null frequency = 1/wavelength = 1 / (4 x via-stub-delay)
>>>
>>> via-stub-delay = d / s, where d = distance (length of stub) and
>>> s = wave propagation speed
>>>
>>> s = c / sqrt(Dk), where c = 299,792,458 m/s, or
>>> c = 299,792,458 m/s x 1/0.0254 in/m x 1E-9 s/ns = 11.8 in/ns
>>>
>>> So, if I pick a via stub length of 80 mils in FR4 with a Dk of 4...
>>>
>>> s = 11.8 in/ns x 1/sqrt(4) = 5.9 in/ns
>>> via-stub-delay = 80 mils x 1/5.9 ns/in x 1E-3 in/mil = 0.0136 ns
>>>
>>> Hence, the predicted null frequency = 1 / (4 x 0.0136 ns) = 18.44 GHz
>>>
>>> However, all of my simulation tools (Hyperlynx, IE3D, CST MWS) show
>>> a null in the range of 9-10 GHz. Digging deeper, they show the via
>>> delay to be in the range of 27 ps rather than the 13.6 that my math shows.
>>>
>>> What gives? Why is my delay calculation off by (roughly) a factor of 2?
>>> Is the lumped capacitance of the via stub somehow affecting the
>>> propagation delay in the via stub? That's somehow mixing t-line
>>> theory with lumped model approximations... I'm at a loss.
>>>
>>> Thanks for any insight.
>>> Ralph Wilson
>>> Alcatel-Lucent
>>>
>>>
>>>
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