AC Decoupling pad design

Hello all,My question has to do with pad design for AC Decoupling caps (but I suppose can be translated toany high speed signal line that has to terminate into a component).Untreated, the pad presents a very low impedance at its location (albeit for a short time) to the intended signal impedance.Common practice is to cut out the ground plane underneath to increase the pad impedance to match the intended signalimpedance. So - if one just examines the numbers then we would have a 50ohm  trace terminating into a 50 pad.However the structures are quite different dimensionally (thin trace over a plane vs a thick pad over a hole and a distant plane).It is this disproportion that leads to my questions:1.       Will the EM wave propagate undisturbed through the combined structure?2.       Is this structural mismatch (but electrically matched) significant? (Or should I park this in the right angle bend trash pile!!??)3.       Given that the pad is usually 4x the trace width would "tear dropping" (i.e gradually increasing the trace width  to matchthe pad with a corresponding void in the plane underneath) be an advantageous design feature?Before I launch on an extensive simulation exercise, getting the opinions of experts in the industry will point me in theright direction. Thank you all for your generosity in sharing your expertise.Best RegardsCharles GrassoCompliance EngineerEchostar Communications(w) 303-706-5467(c) 303-204-2974(t) 3032042974@xxxxxxxxx(e) charles.grasso@xxxxxxxxxxxx(e2) chasgrasso@xxxxxxxxx
Charles.Grasso 5 years 7 months 28 days

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Answered byweirsi 5 years 7 months 28 days
On 2/24/2015 7:42 AM, Grasso, Charles wrote:> Hello all,> My question has to do with pad design for AC Decoupling caps (but I suppose > can be translated to> any high speed signal line that has to terminate into a component).>> Untreated, the pad presents a very low impedance at its location (albeit for > a short time) to the intended signal impedance.> Common practice is to cut out the ground plane underneath to increase the pad > impedance to match the intended signal> impedance. So - if one just examines the numbers then we would have a 50ohm  > trace terminating into a 50 pad.> However the structures are quite different dimensionally (thin trace over a > plane vs a thick pad over a hole and a distant plane).> It is this disproportion that leads to my questions:>> 1.       Will the EM wave propagate undisturbed through the combined > structure?As your intuition is suggesting to you, there is not a plane of demarcation where to one side is a microstrip and on the other is a 50 Ohm pad / capacitor structure.  If you have a tool like HFSS, Simbeor, or others you can design a well-behaved transition.>> 2.       Is this structural mismatch (but electrically matched) significant? > (Or should I park this in the right angle bend trash pile!!??)At 6Gbps or higher I would attend to it.  Below 1Gbps I wouldn't give it much concern.>> 3.       Given that the pad is usually 4x the trace width would "tear > dropping" (i.e gradually increasing the trace width  to match> the pad with a corresponding void in the plane underneath) be an advantageous > design feature?I think it is easier to manage the transition by modifying the RF plane opening from a straight rectangle.>> Before I launch on an extensive simulation exercise, getting the opinions of > experts in the industry will point me in the> right direction. Thank you all for your generosity in sharing your expertise.>> Best Regards> Charles Grasso> Compliance Engineer> Echostar Communications> (w) 303-706-5467> (c) 303-204-2974> (t) 3032042974@xxxxxxxxx> (e) charles.grasso@xxxxxxxxxxxx> (e2) chasgrasso@xxxxxxxxx>>>