[SI-LIST] Re: S11 or S21

  • From: "Lee Ritchey" <leeritchey@xxxxxxxxxxxxx>
  • To: "Larry Smith" <larry.smith@xxxxxxx>, adsurevv@xxxxxxxxx
  • Date: Fri, 27 May 2005 14:53:50 -0700

Virenda,

To add to what Larry has said, if you are interested in the parasitic
inductance of a capacitor as it relates to using the part as part of your
power bypassing, it will turn out that the mounting inductance will
dominate over the parts parasitic inductance.  Part of this is due to the
inductance of the vias needed to connect the capacitor to the power planes
it is used with and part of it is due to the size of the mounting structure
needed to solder the part to the PCB.

Tests have shown that once you have to reach planes deeper into the PCB
than 10 or so mils, the value of ultra low inductance parts such as IDC is
masked to the point they are not worth the extra cost.  (The inductand of
vias is about 35 pH per mil of length.  With two of these, reaching just 20
mils into the PCB results in almost .7 nH just from mounting vias.  Doesn't
make sense to spend lots of money on super low inductance capacitors with
this as a handicap.)  

You would do better by your design by using simple 0603 0r 0402 capacitors
with two vias per mounting pad.  In the bargain, you will avoid making your
design single sourced- and imprortant consideration.

Lee W. Ritchey
Speeding Edge
P. O. Box 2194
Glen Ellen, CA 95442
Phone- 707-568-3983
FAX-    707-568-3504

I just used the energy it took to be angry to write some blues.
Count Basie


> [Original Message]
> From: Larry SMITH <Larry.Smith@xxxxxxx>
> To: <adsurevv@xxxxxxxxx>
> Cc: <si-list@xxxxxxxxxxxxx>
> Date: 5/27/2005 2:36:21 PM
> Subject: [SI-LIST] Re: S11 or S21
>
> Virenda - The ESL of a ceramic capacitor is a rather elusive parameter.
>  It depends a lot on how you define it, how you intend to use it and how
> you measure it.  In any case, most of the inductance will be in the
> mounting structure for the capacitor and a less significant inductance
> will be associated with the capacitor itself.
>
> A ceramic capacitor might be used as a DC blocking device (AC coupler)
> for high speed serial links.  In that case, it is probably in a
> signaling environment with 50 Ohm microstrip traces leading up to the
> pads.  For this application, you would probably want to measure the ESL
> by mounting it on pads connected to 50 Ohm traces so that the AC current
> completes the path between the 50 Ohm traces during the measurement.  A
> TDR, TDT, VNA S11 or S22 measurement will be able to detect an inductive
> discontinuity associated with the mounted capacitor.  The inductance
> value might be called ESL.
>
> The same ceramic capacitor might be used in a decoupling application
> where it is tied to Vdd and Gnd planes of a package or PCB.  The power
> plane impedance is likely to be much less than 1 Ohm, far different than
> a 50 Ohm signaling environment.  The capacitor will be connected in
> shunt (parallel) with the planes (rather than in series) so it should be
> measured that way.  The most interesting frequency band is at series
> resonance (1 to 100 MHz), where the impedance of the capacitor is
> approximately the ESR, (10 mOhm range).  S11 measurements do not work
> very well under these conditions because of the inductance of the
> fixture.  Even a 1 nH fixture, which is difficult to make, will give
> 62.8 mOhms in series with your capacitor at 10 MHz, 6 times more than
> the impedance you are trying to measure.  For that reason, S21 is your
> best bet.
>
> What is the nature of your 2" test board?  Is the capacitor mounted in
> series with 50 Ohm traces or is in parallel with power planes?  A third
> possibility is a continuous 50 Ohm microstrip trace touching one pad and
> the other pad shorted to the ground plane below.  In this case, the
> capacitor shorts the 50 Ohm trace to ground.  Each of these three
> fixtures will give you a different ESL for your capacitor.  Actually,
> you are measuring the mounting inductance of the fixture more than you
> are measuring the ESL of the capacitor.
>
> I like to define the ESL of the mounted capacitor in terms of it's
> capacitance and resonant frequency:  ESL = 1/((2*pi*f0)^2*C) .  The ESL
> can be further broken down into the mounting inductance, filler
> inductance at the bottom of the cap and plate inductance.  (See Sun
> Microsystems SI documents associated with SI-list for references).  But
> most of the inductance is probably in the mount unless you have taken
> great effort in your fixture to minimize it.
>
> My major point in writing this email is that ESL is not a very well
> defined parameter and the industry does not have good agreement on how
> it should be measured.
>
> regards,
> Larry Smith
> Sun Microsystems
>
> Virendra wrote:
> > Hello All,
> > I am trying to measure the esl of a ceramic capacitor.
> >  I have a test board of 2"x 2" for this.  I have SMA
> > connectors connected to two sides of the test board. 
> > Now to extract the esl of the capacitor, is it best to
> > measure the S11 or S21. 
> > Given that S21 gives lesser measurement errors does it
> > still make sense to measure S21 when the two
> > connectors are 2" apart?
> > 
> > thanks in advance,
> > virendra
> > 
> > 
> >             
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