[SI-LIST] Re: VIA stubs
- From: Dmitry Klokotov <dmitry.klokotov@xxxxxxxxx>
- To: orphanou@xxxxxxxxxxxx
- Date: Mon, 20 Aug 2012 17:58:47 -0700
Hi,
Is it correct though to use e_r of the dielectric (substrate) to estimate
the resonance frequency of a via stub?
If the board has multiple layers with multiple ground and power planes the
via will not behave as a trace in a solid dielectric. Even in a solid
dielectric a via might have a different e_r associated with it due to
material anisotropy. The resonance can happen much sooner than 31.25 GHz..
However, for the case in point (6Gbs, 43 mil) the stub resonance will still
be too far to be a concern.
Best,
Dmitry
On Mon, Aug 20, 2012 at 11:19 AM, Antonis Orphanou <orphanou@xxxxxxxxxxxx>wrote:
> The via stub will resonate at quarter wavelength; at that frequency, from
> a capacitor the via appears to be an inductor.
> If you assume er=4 then for a 43mil trace you will see a resonance deep at
> on your insertion @ 31.25Ghz
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Knut Gabrielsen (kngabrie)
> Sent: Monday, August 20, 2012 7:58 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] VIA stubs
>
> Hello Experts!
> I am involved in a design where we need to transfer ~6Gbit/s diff-signals
> between a processor and an FPGA. If I route these signals as striplines
> inside the PCB, there will be a stub on the (through-hole) VIA, stub-length
> ~1100um = 43mil.
>
> My question to you all: Should I be concerned? At what frequencies will a
> VIA stub seriously affect SI?
>
> And - will the signal see the stub itself, or just "experience" the VIA as
> a stray capacitance on the transmission line?
>
> Rgds,
> Knut
>
>
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