[SI-LIST] Re: Power Integrity measurement equipment
- From: Istvan Novak <istvan.novak@xxxxxxx>
- To: "Heinrich.Smith" <heinrich.smith2005@xxxxxxxxx>
- Date: Mon, 27 Mar 2006 21:01:53 -0500
Heinrich,
The first question is not really about measurements, it is about design
procedures.
Achieving 1mohm impedance on boards is not really practical at 100MHz.
As you say you would need many capacitors to do that, but there is a bigger
problem with that. You cannot put all of those capacitors very close to
wherever you need to put them. As soon as you spread them out, the
inductance
in the horizontal connection has to be considered. 1 milliohm at 100MHz
corresponds
to about 1.5pH inductance. To get a horizontal connection which would not
limit the overall performance you would really need a thin film connection.
In practical board applications, you have to live with higher
inductances, and you
need packages and chips, which can tolerate the higher inductance on the
supply
rail. As many repeatedly pointed out on this list, this also includes
the proper
assignment of return planes in the signal connection path.
Regarding the measurements with or without the DC-DC converter turned
on, you
can do it either way, but somehow you have to figure out the performance
with
the converter turned on, because that is the normal operating condition. If
you do that, you may be surprised to see how badly some converters may
interact
with your bulk capacitors. Mainstrem converters may change your board
impedance
up to about 1MHz at most, higher-frequency resonances or just the impedance
profile above 1MHz will be hardly changing.
Best regards,
Istvan Novak
SUN Microsystems, Inc.
Heinrich.Smith wrote:
> Thank you for your valuable introduction.
> I still have some questions need your advice.
> For modern IC design like CPU or FBDIMM architecture, the IO-buffer
> power rail is different from core power rail,right?
> Vio power rail has lower current driving but bringing higher
> frequency SSN into PKG,PCB and , However,Vcore power rail has
> higher current driving but the switching delta-I noise are enclosed
> inside of IC(die).
> From your experience,
> PCB's pi decap solution should cover to ?MHz for Vio power rail.(for
> example,data rate 800MHz)
> PCB's pi decap solution should cover to ?MHz for Vcore power rail.
> In my experiments,the ESR of deacp above 100MHz will be large
> increasingly,For example ESR:100mohm,that mean we need 100 capacitors
> to drop the impedance to 1mohm,
> My meaning is it is a hard work to drop the Z-impedance easily at
> hundred MHz range,Who can cover this range's pi issue?
> PKG's decap could solve it? or die-cap could solve it? or make an
> embeded capacitor on PCB?
>
> The second question,
> When i measure Z-impedance of the power delivery network,Should i turn
> on VRM?
> If not necessary, The resonant frequency will be shifted before and
> after turning on VRM,right? which result is important for me?
>
> Regards,
>
> -Heinrich.Smith
>
>
>
> 2006/3/24, Istvan Novak <istvan.novak@xxxxxxx
> <mailto:istvan.novak@xxxxxxx>>:
>
> Heinrich,
>
> You need to start with the BW. The required instrument BW for
> power plane
> measurement depends on the excitation BW and maximum modal resonance
> frequency you want to care. If you know the signal rise/fall times
> hitting your
> planes, a simple BW=0.35/trise gives a good indication. The lowest
> parallel
> modal resonance frequency of a rectangular plane shape is F=1/(2*tpd)
> where tpd is the propagation delay along the longer side. Unless
> you have
> a very skinny narrow plane shape, the modal resonances usually
> blend into
> an inductive slope after the first few harmonics, so looking out
> to the
> tenth
> harmonic of the lowest modal parallel resonance is usually
> enough. For any
> power plane, if you are sure that you dont excite particular modal
> resonances,
> you dont need to worry about them, so you can take the smaller BW
> number
> of these two. You can plug in your numbers for each power rail,
> take the
> highest numbers you get and apply some safety cushion (at least an
> octave, 2x)
> to get the BW necessary for testing.
>
> In terms of VNA features, you need to make sure that the dynamic
> range
> is large
> enough if you plan on measuring mid to high-power systems, where
> the supply
> rail impedance is supposed to be low. At very low frequencies,
> when you
> want to measure the output impedance of DC-DC converters and/or
> large-capacitance and low-ESR bulk capacitors, you need to isolate the
> source and receive port returns, something that most VNAs dont
> offer today.
> Also, if you consider the entire frequency range that you need,
> which could
> range from Hz to GHz, you dont find any single instrument that would
> cover it.
> The same applies to cables and probes; those which are convenient
> for low
> frequency measurements are not optimum for GHz frequencies and
> vica versa.
>
> In terms of VNA model, you can check out the major VNA manufacturers:
> Agilent, Anritsu, Rohde Schwartz. You may also want to look at
> the VNA from
> Ultimetrix, which offers the DC blocking between input and output
> ports and
> an easy way to cascade VNAs to cover a wide frequency range.
>
> For more details you can check some of the posted papers on
> http://home.att.net/~istvan.novak/papers.html
> <http://home.att.net/%7Eistvan.novak/papers.html> You can start
> with the
> two papers
> from DesignCon East 2003.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
>
>
>
> Heinrich.Smith wrote:
>
> >I would like to measure the power plane/capacitor impedance,Z.
> >I need which kind of VNA? how high BW?
> >which kind of the probe type? coaxial cable?
> >How to do the calibration?
> >Anyone could give me pictures or any documents ?
> >Thank you very much.
> >- Heinrich.smith
> >
> >
> >
> >
> >
>
>
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