[SI-LIST] Re: Via stub math help needed....
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Thu, 12 Jan 2012 04:42:22 -0800
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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