[SI-LIST] Re: Resonant Modes, Ansoft presentation slides posted
- From: Istvan Novak - Board Design Technology <Istvan.Novak@xxxxxxx>
- To: weirsi@xxxxxxxxxx, xuzhengrong <xuzhengrong@xxxxxxxxxx>
- Date: Tue, 04 Sep 2007 14:13:44 -0400
FYI;
The slides Steve referred to in his mail below are available, either =
at
the ansoft FTP site:
ftp://ftp.ansoftsupport.com/download/workshop/emi_emc_boston_2007/Ans=
oft_EMI_Workshop_SUN_for_22Aug07_v2.pdf
or as the top item on
http://home.att.net/~istvan.novak/papers.html
Regards,
Istvan Novak
SUN Misrosystems
steve weir wrote On 08/29/07 04:57,:
>Zhengrong, generally the modal impedance peaks build up to a limit a=
nd
>then fall-off for the higher modes. If your board or plane patch are=
a 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 dielectri=
c
>loss correctly, have very small cavity areas, and/or very thick cavi=
ties.
>
>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 ar=
e
>evenly spaced and selected with the proper values then the resonance=
s
>only peak above the inductive line a little bit. This is a somewhat
>crude and expensive solution, but it does work. Bruce Archambeault h=
as
>also written extensively on this subject, usually from an EMI centri=
c
>point of view.
>
>The S11 impedance which is much easier to simulate than to measure t=
ells
>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 red=
uce
>the amount of energy you inject into it such as through return curre=
nts.
>HF impedance including resonant peaks always comes back to the
>inductance of the bypass caps and the interconnect which is why I gi=
ve
>inductance a lot of attention. If you are trying to build cheapy lin=
e
>cards on 4 or 6 layers or even 2 layers this takes a good deal of ca=
re.
>
>You can use bypass caps to drop the cavity impedance and suppress no=
ise
>across the card very effectively even with thick cavities. Where thi=
ck
>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 de=
sign
>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 w=
ork
>on any realizable PCB.
>
>Steve.
>
>xuzhengrong wrote:
> =20
>
>> 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 whi=
ch
>>decreases the resonance and makes the impedance smooth.
>>What I meant is that, for the same plane pair structure with the sa=
me
>>capacitors, when I set the number of modes and simulate, I will g=
et 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 t=
he
>>resonance from horizontal angle, Mode 2 from vertical angle, Mode=
3 from
>>diagonal angle, etc. Different mode analyzes the resonance from dif=
ferent
>>angle.
>>As my last mail said, every mode has a certain frequency and Q in=
creases
>>with the number of modes and corresponding frequency in term of the=
result.
>>Mode 1 is the lowest frequency among all. Therefore, with the inc=
reasement
>>of the number of modes, more resonant frequency will appear.
>>You mean, I only pay attention to the bandwidth of significant sig=
nal which
>>may excite the resonance. Ok, I see.
>>
>>My following question is, why do Qs increase with the number of mod=
es and
>>frequency for the same cavity and PDN?
>>Which imformation will I obtain from Qs?=20
>>For Q indicates the ratio of storage and loss, does it mean large=
r number
>>of Mode with higher resonant frequency will let more energy pass?
>>
>>Best Regards
>>Zhengrong Xu
>>
>>
>>-----=D3=CA=BC=FE=D4=AD=BC=FE-----
>>=B7=A2=BC=FE=C8=CB: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bo=
unce@xxxxxxxxxxxxx]
>>=B4=FA=B1=ED steve weir
>>=B7=A2=CB=CD=CA=B1=BC=E4: 2007=C4=EA8=D4=C229=C8=D5 14:02
>>=CA=D5=BC=FE=C8=CB: xuzhengrong@xxxxxxxxxx
>>=B3=AD=CB=CD: si-list@xxxxxxxxxxxxx
>>=D6=F7=CC=E2: [SI-LIST] Re: Resonant Modes
>>
>>Zhengrong,
>>
>>Pay attention to the resonances that are within the bandwidth of si=
gnificant
>>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 ca=
pacitors
>>and interconnect. You can manipulate modes and Q by manipulating yo=
ur
>>capacitors and cavity geometries.
>>
>>Steve.
>>
>>xuzhengrong wrote:
>> =20
>> =20
>>
>>>Oh, I am so sorry for that.=3D20
>>>I have sent another email in text format, but i don't know why it=
=3D=20
>>>hasn't been sent to all yet.
>>>When I sent to si-list@xxxxxxxxxxxxx, i received several autorep=
lies=20
>>>=3D for their out of office.=3D20 So I sent both to si-list@freel=
ists.org=20
>>>and =3D si-list-bounce@xxxxxxxxxxxxx this time.
>>>I'm new here and very glad to join in.=3D20 Could you tell me the=
=20
>>>relationship between si-list@xxxxxxxxxxxxx and=20
>>>si-list-bounce@xxxxxxxxxxxxx? When i want to join the discussion,=
=20
>>>which =3D mail address will i send to?
>>>
>>>Thank you for your reminder with best regards.=3D20 My question i=
s=20
>>>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=
=3D=20
>>>several values including freqency and Q. Every mode has a certain=
=20
>>>frequency and =3D Q increases with the number of modes.=3D20 =3D20=
Which=20
>>>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=
=20
>>>from =3D it?
>>>=3D20
>>>Appreciate any idea about that.
>>>=3D20
>>>Best Regards
>>>Zhengrong Xu
>>>
>>> =20
>>>
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