Sigh.
Again, at an edge-to-edge distance between trace edge and via pad of 4x the
trace-to-plane thickness There is essentially zero RF disturbance out to 70
GHz. At a distance less than that there will be a disturbance as the field
passes by each via, lowering impedance within that region. This will create a
repetitive discontinuity, which will slightly lower the effective average
impedance, but more importantly, will give rise to a repetitive notch
discontinuity. You will need a 3D solver to fully evaluate the impact in your
design, but, if you are using a 1/20 lambda spacing between the vias, I would
not worry much. The first notch resonance will be 10x your operating frequency.
Scott McMorrow, CTO Signal Integrity Group
Samtec
Office 401-284-1827 | +1-800-726-8329
www.samtec.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Cotton, Chris
Sent: Wednesday, July 3, 2019 2:08 PM
To: farhilltramper@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: [EXTERNAL] The gap b/w stitching vias and tline in
serpentine routing
Hi Tramp,
I think it is difficult to answer your question as there are many missing
details such as frequency of interest, spacing relative to dielectric height,
etc. Also, what kind of SNR are you trying to achieve? Is this a digital or
analog/RF application. Generally for digital applications, if you have the
luxury of sufficient spacing to aggressors then forget the GND vias and the
spacing will suffice.
Best regards,
Chris
From: si-list-bounce@xxxxxxxxxxxxx <si-list-bounce@xxxxxxxxxxxxx> On Behalf Of
Tramp
Sent: Wednesday, July 3, 2019 7:36 AM
To: si-list@xxxxxxxxxxxxx
Subject: [EXTERNAL] [SI-LIST] The gap b/w stitching vias and tline in
serpentine routing
[EXTERNAL SENDER: This email originated from outside of Stratus Technologies.
Do not click links or open attachments unless you recognize the sender and know
the content is safe.]
Dear SIlisters,
I did some quick sims sweeping different gaps b/w the stitching ground vias and
tline (Note here the gap refers to the spacing b/w the via and the tline, not
two adjacent ground vias for which I set to twentith lambda) using 2.5D tool
and the results are almost the same. Does anyone have this experience, such as
whether I need to do the sim in 3D, or what distance is recommended? I saw some
implementations put the via at the distance of merely 4mil while the others do
it w/ 20mil. So how on earth is the impact?
--
Best Regards,
Tramp
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