[SI-LIST] Re: plane cutouts under ac-coupling cap pads
- From: "Alfred P. Neves" <al@xxxxxxxxxxxxxxxxx>
- To: MSeverini@xxxxxxxxxxxx
- Date: Thu, 11 Jul 2013 08:31:13 -0700
Great thread, any details you can share would be appreciated.
We create platforms for 3D EM, SERDES testing, measurement standards, etc., to
32Gbsec.
Products for the Signal Integrity Practitioner
Alfred P. Neves
Chief Technologist
Office: 503-679-2429
www.wildrivertech.com
On Jul 11, 2013, at 8:23 AM, Matthew Severini <MSeverini@xxxxxxxxxxxx> wrote:
> First off thank you all for your responses. What a wonderfully supportive
> community we have here. As it turns out Scott hit the nail on the head. We
> had a resonance on these lanes, that I managed to tune to a harmonic of the
> nyquist by adding the cutouts. The resonance was high enough in frequency
> that I hadn't noticed it before, but when I plotted the s-parameters out far
> enough, there it was, staring me in the face. Shifted ever so slightly by the
> cutouts under the capacitors, it fell perfectly on a harmonic that it hadn't
> before.
>
> Thanks again everyone,
>
> Matt Severini
>
> Signal Integrity Engineer
> mseverini@xxxxxxxxxxxx
> (801) 987-7127
>
>
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Scott McMorrow
> Sent: Wednesday, July 10, 2013 7:42 PM
> To: weirsi@xxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: plane cutouts under ac-coupling cap pads
>
> Matt
>
> Look closely at your TDR profile and compute the resonant frequencies from
> all the peak( and/or troughs) of the discontinuity on one side of the
> capacitor vs the one on the other side. Compare those resonances between the
> cap with and without the cutout. I suspect that you've managed to tune the
> capacitor to have a resonance at either the fundamental or a harmonic that
> exhibits ringing.
>
> In HFSS did you actually model a capacitor, or did you model a short across
> the pads? If you model the capacitor body and plates correctly, you will
> obtain an accurate result. If you model just the shorted pads, you can cause
> additional half and quarter wave resonances. I've used HFSS to optimize
> about 20 different capacitor transitions up to 30 Gbps and achieved +/- 1.5
> ohms with a sub 15 ps TDR.
>
> On 7/10/13, steve weir <weirsi@xxxxxxxxxx> wrote:
>> Matthew, the first thing that comes to mind is masking by any number
>> of other impairments. A bit of a check-list:
>>
>> 1. How have you established your measurement reference? Are you
>> confident of your measurement deembedding?
>> 2. How closely do the simulations match measurements?
>> 3. Are there other discontinuities that you know about, or can
>> perceive from measurements close to the DC blocking capacitors?
>>
>> Physics always prevails.
>>
>> Steve.
>> On 7/10/2013 4:10 PM, Matthew Severini wrote:
>>> Content-Type: text/plain; charset="us-ascii"
>>> Content-Transfer-Encoding: quoted-printable Hello experts,
>>>
>>> I have a problem that has me totally stumped, so I thought you may
>>> have som= e ideas. We have some signal lanes with dc-blocking caps,
>>> that run at about>>> 10ghz. I noticed that the planes had not been cutout
>>> underneath the
>>> pads o= f these caps, which has obvious issues. I simulated the cap
>>> structure in HF= SS to find an optimum cutout size, and rolled it
>>> into the next spin of the = board. All of my simulations claim that
>>> the new structure should have a hug= e positive influence on the si
>>> of these lanes. Unfortunately, when we measu= re the eye, it seems to
>>> be about the same, or ever so slightly worse! I did>>> a tdr on the cards
>>> with and without the cutout, and the cutouts
>>> definitely>>> seem to flatten the impedance curve along the trace. I
>>> haven't been
>>> able t= o do a vna measurement yet because I cannot access one side
>>> of the trace.
>>> I>>> have ordered boards with only those caps on them so that I can make
>>> this m= easurement, but all the initial data seems to say that the
>>> transmission mea= surements will also be better with the cutouts. I
>>> was wondering if anyone h= ad some possible explanations as to why
>>> the eye would be a little worse, in>>> almost every measurement (according
>>> to sig test), when the
>>> impedance is fl= atter.
>>>
>>> Thanks for your help,
>>>
>>> Matt Severini
>>>
>>> Signal Integrity Engineer
>>> mseverini@xxxxxxxxxxxx<mailto:mseverini@xxxxxxxxxxxx>
>>> (801) 987-7127
>>>
>>>
>>> [signature_image]
>>>
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>>
>>
>> --
>> Steve Weir
>> IPBLOX, LLC
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>
>
> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 16 Stormy Brook Rd
> Falmouth, ME 04105
>
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> Teraspeed(r) is the registered service mark of Teraspeed Consulting Group LLC
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