[SI-LIST] Re: Power Integrity Measurments
- From: steve weir <weirsi@xxxxxxxxxx>
- To: rula.bakleh@xxxxxxxxxx
- Date: Thu, 06 Aug 2009 18:58:28 -0700
Istvan Novak really pioneered getting the word out more than a decade
ago on ways to leverage two port VNA measurements to look at low
impedance systems. His web page has many useful papers on the subject.
But be careful how you select your measurement ports and how you
interpret your data. It is easy to go down a rabbit hole and obtain
completely wrong results measuring power systems.
Insertion loss results from the combination of series and shunt
impedances. Some amount of insertion loss at some frequency only tells
you how well the PDN will isolate those two ports when driven and loaded
with the 50 Ohms of the analyzer head. Where the impedance of the
interconnects between ports is small compared to the bypass capacitor
network, you can assume the loss is almost all due to the shunt
impedance seen at each port, and easily transform the loss versus
frequency into an equivalent shunt impedance versus frequency that a
load will see looking into either port. However if the impedance of the
interconnects is significant, the assumption is invalid, as will be any
derivation of shunt impedance versus frequency. This can be very bad if
you are relying on such a derivation to predict the impedance that a
load will see when connected at the probe port.
Steve.
rula.bakleh@xxxxxxxxxx wrote:
> Hi Everyone,
>
>
> I'm trying to validate our power integrity tools/simulations and I would
> like to make some power integrity measurements of our planes.
>
>
>
> With respect to performing these measurements, I plan to use a Vector
> Network Analyzer (VNA) to measure the plane impedances of a bare PWB and
> a PWB with a voltage plane that is populated with only caps in order to
> correlate these measurements to the simulations. I also plan to measure
> the plane impedance of a fully populated PWB, just to see the effects
> that other parts' impedance/mounting would have on the plane impedance.
>
>
>
> I also plan to use a spectrum analyzer to measure the plane noise
> spectrum of a powered up module and use an oscilloscope to measure
> absolute levels of this noise. The purpose of using a spectrum
> analyzer/oscilloscope is to measure the simultaneous switching effects
> on the plane, hoping to isolate the PWB effects from internal SSO
> effects of a component.
>
>
>
> My dilemma is the following:
>
> The VNAs are optimized for 50 ohms and plane impedances of PWBs are
> designed for very low impedance (most of the time much <1.0 ohm).
>
> 1) Due to the impedance discontinuities, what is the expected accuracy
> of such a measurement?
>
>
>
> All the research I have conducted concerning PWB impedance measurements
> points to using a VNA. During my discussions with an Agilent FAE, he
> recommended using an Impedance Analyzer vs. VNA to measure the PDS of a
> plane. He also recommended using a high impedance active solderable
> probe with the impedance analyzer with it being calibrated out to make
> this measurement.
>
>
>
> 2) What is the best equipment to use to measure the PDS of a PWB (VNA or
> Impedance Analyzer)?
>
> 3) If the answer is an Impedance Analyzer, is the approach described
> above correct?
>
> 4) Can somebody validated my approach for this type of
> analysis/measurements?
>
>
>
>
> Thank You,
>
> Rula
>
>
>
>
>
>
>
>
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--
Steve Weir
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