[SI-LIST] Re: Power Integrity (was: UltraCAD ESR and Bypass Capacitor Caculator)

Vishram - I agree.  My general approach is to address the PI problem first
and make sure that the silicon circuits have clean power.  This involves
management of capacitance and inductance at the PCB, package and chip levels.
Next, address the SI problems by making sure that all high speed signals have
a good return current path.  After doing these two things, many EMI problems
will be eliminated.  

I have also been able to fix EMI problems at multiple 100's of MHz by
using decoupling capacitors.  But the higher the frequency gets, the
harder it is to do this.  Capacitors at this frequency usually will not affect
the quality of the power as measured at the silicon circuit terminals (PI),
but they might effect emmissions. 

After we began using thin power plane 
dielectrics, I don't believe we have found any EMI problems that can
be fixed with discrete decoupling capacitors.  But if your product does
not have thin power plane dielectrics (4 mil or less) for cost or
other reasons, EMI problems can _sometimes_ be fixed with caps.  If
this works, it is usually not a very robust solution.  If some little
thing changes, the EMI problem often crops back up again.

regards,
Larry Smith
Sun Microsystems

Vishram Pandit wrote:
> 
> 
> Larry,
> 
> Very nice explanation. PI influences SSN, and SSN influences EMI. EMI is
> influenced by PI and SI. If we have sound PI and also, reduce the SSN, then
> EMI (due to that aspect of the circuit) is mitigated. Would you agree?
> 
> As mentioned in my pevious mails, I have seen improvements in EMI at higher
> frequencies (as high as 800MHz) with decoupling capacitors, and changing the
> P/G structure to improve the impedance. Your email states that PI is
> characterized by P/G impedance and decaps for PI are effective up to 100MHz.
> However, in my case, I reduced the 800MHz impedance further by decaps
> betweenP/G, and by improving the P/G strucutre, and it helped improve the
> EMI. Thus, improving PI at 800MHz improved the EMI.Apart from chaning the
> structure of P/G, decaps (value, ESL, locations) played important part in
> it.
> 
> I will appreciate your comments.
> 
> Thanks,
> 
> Vishram Pandit
> 
> Senior Member Techincal Staff
> 
> Hughes Network Systems
> 
>   >From: Larry Smith >Reply-To: Larry.Smith@xxxxxxx >To:
> si-list@xxxxxxxxxxxxx, Charles.Grasso@xxxxxxxxxxxx >Subject: [SI-LIST] Power
> Integrity (was: UltraCAD ESR and Bypass Capacitor Caculator) >Date: Fri, 15
> Aug 2003 14:04:39 -0700 (PDT) >>I changed the thread name to better reflect
> the subject.. >>Some of us at Sun have begun using a different word for the
> power >distribution problem, "power integrity." This phrase helps to
> >distinguish three major topics: power integrity (PI), signal integrity
> >(SI)and EMI. Power integrity is the issue that Charles is addressing >and
> signal integrity is what Kim is addressing in his very nice web >posting. A
> lot of the confusion could be eliminated by using clearer >terminology. >>I
> think of the "power integrity" problem as having only two nodes: Vdd >and
> Gnd. There are no signals involved. For the power integrity >problem, we are
> concerned with delivering many watts of power, often at >low voltage and
> highcurrent, to modern digital technology. The big >issues are transient
> current and DC loss. A good example is an >advanced micro processor that
> draws as much as 100 watts of power at 1 >volt (100 amps). The processor can
> go from an idle state to a fully >active state in just a few clock cycles (1
> nSec). The silicon circuits >may consume 50 amps and then 100 amps just a
> fewcycles later. >Delivery of this 50 watt transient through the various
> timeconstants, >which range from nSec to mSec (chip, package, PCB, VRM, AC
> toDC >converter), is very much a part of the power integrity problem. Note
> >that 1 mOhm of DC resistance in this circuit consumes 10 watts of power
> >(I^2*R) and renders our delivery system only 90% efficient. Power
> >Integrityinvolves delivering high current with huge transients. It is >best
> understood and managed by the concept of target impedance in the >frequency
> domain. >>Signal integrity, on the other hand, always involves signal nodes.
> A >few years ago, at the 50 MHz level, signal integrity basically meant >the
> waveform quality and timing on ideal transmission lines. Before >that, all
> wehad to worry about (at the 5 MHz level) was RC time >constants. Now we are
> beyond 500MHz where we must be concerned with >frequency dependent loss and
> return current paths. Several years ago, >SSN (simultaneous switch noise)
> wasmostly an L*di/dt problem that >created ground bounce in the DIP's (dual
> inline packages, lead >frames). After we started including ground planes in
> our packages, >replaced wire bonds with solder bumps and started using just
> as many >ground pins as signal pins, the SSN problem changed to a power
> plane>bounce and return current problem. This is how power integrity keeps
> >getting mixed up with signal integrity. The return current for signals >is
> on power and/or ground planes. But we can avoid a lot of confusion >if we
> usethe term "power integrity" for topics that involve just Vdd >and ground
> and reserve "signal integrity" for topics that involve >signal nodes.
> >>Decoupling capacitors play a role in all three topics. For the power
> >integrity problem, they are energy storage devices that mitigate power
> >transients. They deliver energy when the voltage droops and store >energy
> when the voltage spikes. For the signal integrity problem, they >enable
> return current to jump from one node to another (i.e. Vdd1 to >Vdd2 or Vdd
> toGnd) when packages, vias or connectors require signal >return current to
> make the jump. For the EMI problem, they provide low >impedance and energy
> absorption at frequencies where the product >naturally has a lot of energy
> (clock) or frequencies where the product >has a very efficient resonator or
> radiator. >>Decoupling capacitors are effective for the power integrity
> problem in >the 100 kHz to 100 MHz frequency band. Below 100 kHz it takes
> toomany >uF for them to be effective and above 100 MHz their inductance gets
> in >the way. However, decoupling capacitors may be used to complete return
> >current paths (SI) or absorb noise (EMC/EMI) up to much higher
> >frequencies.Below 50 MHz, position on the PCB is not very important >but
> above 200 MHz, position often becomes critical. Thin power plane
> >dielectricsare a good replacement for discrete decoupling >capacitors that
> are aimed at frequencies above 100 MHz. Power plane >capacitance is "broad
> band" but the Q of discrete capacitors becomes >sharp and limits their
> effectiveness as frequency increases. >>Very few topics on SI-list seem to
> evoke as many emotions as decoupling >capacitors. That is probably because
> people view them from so many >different perspectives. Vastly different
> conclusions can be drawn for >decoupling capacitors depending on the problem
> you are trying to solve >(PI, SI or EMI) and other variables such as power
> plane dielectric >thickness. Some of this can be helped by clearly defining
> the >terminology and use conditions. >>regards, >Larry Smith >Sun
> Microsystems >>>Delivered-To: si-list@xxxxxxxxxxxxx >>From: "Grasso,
> Charles">>To: "'si@xxxxxxxxxxxx'" , "'si-list@xxxxxxxxxxxxx'" >>>Subject:
> [SI-LIST] Re: UltraCAD ESR and Bypass Capacitor Caculator >>Date: Thu, 14
> Aug2003 15:39:34 -0600 >>MIME-Version: 1.0 >>Content-Transfer-Encoding: 8bit
> >>X-archive-position: 7937 >>X-ecartis-version: Ecartis v1.0.0
> >>X-original-sender: Charles.Grasso@xxxxxxxxxxxx >>X-list: si-list >>>>Hi
> Kim, >>First - thanks for putting the slides up on the bweb for >>all to
> see.I think that you may have missed the point >>a little. In your scenario
> (a signal trace switching >>planes )the location of the caps is vital.
> >>>>The discussion was centered on the location of caps >>wrt power
> distribution. The location of the capacitors >>(within reason) will not
> affect a S11/S21 measurement >>that much. >>>>Fancy tackling that little
> problem? >>>>Best Regards >>Charles Grasso >>Senior Compliance Engineer
> >>Echostar Communications Corp. >>Tel: 303-706-5467 >>Fax: 303-799-6222
> >>Cell: 303-204-2974 >>Email: charles.grasso@xxxxxxxxxxxx; >>Email
> Alternate:chasgrasso@xxxxxxxx
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