[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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