[SI-LIST] Re: Resonant Modes
- From: steve weir <weirsi@xxxxxxxxxx>
- To: xuzhengrong <xuzhengrong@xxxxxxxxxx>
- Date: Wed, 29 Aug 2007 01:57:07 -0700
Zhengrong, generally the modal impedance peaks build up to a limit and
then fall-off for the higher modes. If your board or plane patch area is
small the high frequency limit where this happens might be very high. If
you do not see this fall-off you are probably not modeling dielectric
loss correctly, have very small cavity areas, and/or very thick cavities.
There are a number of ways to suppress Q. I think Istvan just put on a
presentation for Ansoft on this subject. I haven't checked to see if his
presentation is available to the public or not. A gross method is to
flood the board with bypass capacitors. If many bypass capacitors are
evenly spaced and selected with the proper values then the resonances
only peak above the inductive line a little bit. This is a somewhat
crude and expensive solution, but it does work. Bruce Archambeault has
also written extensively on this subject, usually from an EMI centric
point of view.
The S11 impedance which is much easier to simulate than to measure tells
you how much noise will appear at a given location for a given
excitation at that point. Higher Qs result in higher impedance peaks,
and consequently higher voltage noise for a given amount of current
injected at the frequency of interest. Basically to combat the noise
voltage, you either have to bring the PDN impedance down, and/or reduce
the amount of energy you inject into it such as through return currents.
HF impedance including resonant peaks always comes back to the
inductance of the bypass caps and the interconnect which is why I give
inductance a lot of attention. If you are trying to build cheapy line
cards on 4 or 6 layers or even 2 layers this takes a good deal of care.
You can use bypass caps to drop the cavity impedance and suppress noise
across the card very effectively even with thick cavities. Where thick
cavities can be very ugly is when using ICs that were not engineered to
work well on high PDN impedance PCBs. Getting IC manufacturers to design
ICs with the application PCB in mind is still largely a work in
progress. In extreme cases an IC may not be engineered properly to work
on any realizable PCB.
Steve.
xuzhengrong wrote:
> Steve,
> I'm very glad to receive your reply and have learnt a lot from your papers
> issued on DesignCon.
> I know increasing ESR and decreasing inductance can lower Qs which
> decreases the resonance and makes the impedance smooth.
> What I meant is that, for the same plane pair structure with the same
> capacitors, when I set the number of modes and simulate, I will get datas
> of different mode such as Mode 1, Mode2 and so on.
> According to the figure of the result, it seems that Mode 1 shows the
> resonance from horizontal angle, Mode 2 from vertical angle, Mode 3 from
> diagonal angle, etc. Different mode analyzes the resonance from different
> angle.
> As my last mail said, every mode has a certain frequency and Q increases
> with the number of modes and corresponding frequency in term of the result.
> Mode 1 is the lowest frequency among all. Therefore, with the increasement
> of the number of modes, more resonant frequency will appear.
> You mean, I only pay attention to the bandwidth of significant signal which
> may excite the resonance. Ok, I see.
>
> My following question is, why do Qs increase with the number of modes and
> frequency for the same cavity and PDN?
> Which imformation will I obtain from Qs?
> For Q indicates the ratio of storage and loss, does it mean larger number
> of Mode with higher resonant frequency will let more energy pass?
>
> Best Regards
> Zhengrong Xu
>
>
> -----邮件原件-----
> 发件人: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> 代表 steve weir
> 发送时间: 2007年8月29日 14:02
> 收件人: xuzhengrong@xxxxxxxxxx
> 抄送: si-list@xxxxxxxxxxxxx
> 主题: [SI-LIST] Re: Resonant Modes
>
> Zhengrong,
>
> Pay attention to the resonances that are within the bandwidth of significant
> signal energy from your loads and/or injected into your return path.
>
> Q depends on the capacitors and materials you use for your PCB. Low mounted
> inductance capacitors such as X2Ys exhibit lower Qs than capacitors with
> higher mounted inductances and similar ESRs.
>
> Cavities that are thinner and/or have higher tangent losses exhibit lower Qs
> than do thicker cavities and/or cavities with lower tangent loss.
>
> At the end of the day you are always fighting inductance in your capacitors
> and interconnect. You can manipulate modes and Q by manipulating your
> capacitors and cavity geometries.
>
> Steve.
>
> xuzhengrong wrote:
>
>> Oh, I am so sorry for that.=20
>> I have sent another email in text format, but i don't know why it =
>> hasn't been sent to all yet.
>> When I sent to si-list@xxxxxxxxxxxxx, i received several autoreplies
>> = for their out of office.=20 So I sent both to si-list@xxxxxxxxxxxxx
>> and = si-list-bounce@xxxxxxxxxxxxx this time.
>> I'm new here and very glad to join in.=20 Could you tell me the
>> relationship between si-list@xxxxxxxxxxxxx and
>> si-list-bounce@xxxxxxxxxxxxx? When i want to join the discussion,
>> which = mail address will i send to?
>>
>> Thank you for your reminder with best regards.=20 My question is
>> listed as follows.
>>
>>
>> Hi all,
>> I am new in using EM tools to analyze the plane pairs' resonance.
>> After inputing the number of modes and computing, the results have =
>> several values including freqency and Q. Every mode has a certain
>> frequency and = Q increases with the number of modes.=20 =20 Which
>> mode should i pay most attention to?
>> Why does Q value increase with the number of modes and frequency?
>> How to analysze with these data and which information can i obtain
>> from = it?
>> =20
>> Appreciate any idea about that.
>> =20
>> Best Regards
>> Zhengrong Xu
>>
>>
>>
>> -----=D3=CA=BC=FE=D4=AD=BC=FE-----
>> =B7=A2=BC=FE=C8=CB: olaney@xxxxxxxx [mailto:olaney@xxxxxxxx]=20
>> =B7=A2=CB=CD=CA=B1=BC=E4: 2007=C4=EA8=D4=C229=C8=D5 12:00
>> =CA=D5=BC=FE=C8=CB: xuzhengrong@xxxxxxxxxx
>> =D6=F7=CC=E2: Re: [SI-LIST] Resonant Modes
>>
>> Just so you know what I actually received...
>> I think jpg files need to be sent as attachments.
>>
>> On Wed, 29 Aug 2007 11:02:59 +0800 xuzhengrong <xuzhengrong@xxxxxxxxxx>
>> writes:
>>
>>
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--
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