[SI-LIST] Re: Resonant Modes
- From: xuzhengrong <xuzhengrong@xxxxxxxxxx>
- To: weirsi@xxxxxxxxxx
- Date: Wed, 29 Aug 2007 19:42:46 +0800
Steve,
As you said, there's more chance to emerge PI problems on less layers' =
PCB
which has less or no gnd plane. =20
Recently there's a 2 layer board without gnd plane, which has evident
voltage noise on the gnd greatly , namely ground bounce, affecting the
signaficant signs. So I want to locate this problem with EM tools.
Generally speaking, large system boards have less problems like gnd =
bounce,
for they have integral gnd planes and conduct noise to the earth ground =
from
metallic bolts.=20
However, terminal devices such as cell phone have no connect to the =
earth
ground. Noise can be only dissipated by resistors(including ESR) or
radiation and still left much on the board leading to more PI and SI
problems.
But I find that the resonance caused by cavities seems to be lesser =
affected
by placing several HF caps. =20
Could you give me some idea about minishing the ground bounce in such
boards?
-----=D3=CA=BC=FE=D4=AD=BC=FE-----
=B7=A2=BC=FE=C8=CB: si-list-bounce@xxxxxxxxxxxxx =
[mailto:si-list-bounce@xxxxxxxxxxxxx]
=B4=FA=B1=ED steve weir
=B7=A2=CB=CD=CA=B1=BC=E4: 2007=C4=EA8=D4=C229=C8=D5 16:57
=CA=D5=BC=FE=C8=CB: xuzhengrong
=B3=AD=CB=CD: si-list@xxxxxxxxxxxxx
=D6=F7=CC=E2: [SI-LIST] Re: Resonant Modes
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=20
> papers issued on DesignCon.
> I know increasing ESR and decreasing inductance can lower Qs which=20
> decreases the resonance and makes the impedance smooth.
> What I meant is that, for the same plane pair structure with the same=20
> capacitors, when I set the number of modes and simulate, I will get=20
> 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=20
> resonance from horizontal angle, Mode 2 from vertical angle, Mode 3=20
> from diagonal angle, etc. Different mode analyzes the resonance from=20
> different angle.
> As my last mail said, every mode has a certain frequency and Q=20
> 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=20
> 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=20
> 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 larger=20
> 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=20
> [mailto:si-list-bounce@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=20
> 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=20
> mounted inductance capacitors such as X2Ys exhibit lower Qs than=20
> capacitors with higher mounted inductances and similar ESRs.
>
> Cavities that are thinner and/or have higher tangent losses exhibit=20
> 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=20
> capacitors and interconnect. You can manipulate modes and Q by=20
> manipulating your capacitors and cavity geometries.
>
> Steve.
>
> xuzhengrong wrote:
> =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=20
>> autoreplies =3D for their out of office.=3D20 So I sent both to=20
>> si-list@xxxxxxxxxxxxx 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 is=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
>>
>>
>>
>> -----=3DD3=3DCA=3DBC=3DFE=3DD4=3DAD=3DBC=3DFE-----
>> =3DB7=3DA2=3DBC=3DFE=3DC8=3DCB: olaney@xxxxxxxx =
[mailto:olaney@xxxxxxxx]=3D20
>> =3DB7=3DA2=3DCB=3DCD=3DCA=3DB1=3DBC=3DE4: =
2007=3DC4=3DEA8=3DD4=3DC229=3DC8=3DD5 12:00
>> =3DCA=3DD5=3DBC=3DFE=3DC8=3DCB: xuzhengrong@xxxxxxxxxx
>> =3DD6=3DF7=3DCC=3DE2: 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=20
>> <xuzhengrong@xxxxxxxxxx>
>> writes:
>> =20
>> =20
>>> Content-type: text/plain; charset=3D3Dgb2312
>>> Content-transfer-encoding: base64
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>>> =20
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>>> IGFuZCBkZWxldGUgaXQhDQoNCiANCg=3D3D=3D3D
>>> =3D20
>>> =3D20
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--
Steve Weir
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882=20
California office
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Main office
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