[SI-LIST] : Re: MEASURING POWER GROUND IMPEDANCE

Istvan NOVAK (istvan.novak@worldnet.att.net)
Mon, 1 Jun 1998 08:08:56 -0400

Dr. Johnson,

I am following with great interest the On-line Newsletters of High-Speed
Digital Design. Inj one of your recent responses regarding the measurement
of power/ground planes, you are suggesting not to connect the probes to
nearby points.
Can you elaborate somewhat further your reasoning for that ('local crosstalk
will pollute your measurement')?
Also, can you give some more reasoning behind your suggestion of using the
resistance divider theorem in this case? Do you mean here the magnitudes of
impedances?

Thank you

Istvan Novak, SI Engineer, SUN Microsystems

-----Original Message-----
From: High-Speed Digital Design Mailing List <hsdd@accessone.com>
To: High-Speed Digital Design Newsletter Subscriber(4) : ; <High-Speed
Digital Design Newsletter Subscriber(4) : ;>
Date: Tuesday, May 26, 1998 4:28 PM
Subject: MEASURING POWER GROUND IMPEDANCE

>*---------------------------------------------------------------*
> H i g h - S p e e d D i g i t a l D e s i g n
>
> *On-Line Newsletter*
>
> Dr. Howard Johnson, Vol. 2 Issue 14
>*---------------------------------------------------------------*
>
>*------------------------(ANNOUNCEMENTS)------------------------*
>
>Next Public High-Speed Digital Design Seminars:
>
> U. of Oxford, UK June 22-23, 1998
> San Jose, CA September 21-22, 1998
>
>Registration & information at:
>http://signalintegrity.com/seminar.htm
>
>Tell Your Co-Workers!
>
>*--------------------------(QUESTION)---------------------------*
>
>MEASURING POWER GROUND IMPEDANCE
>Larry Smith of Sun Microsystems writes:
>
> Dr. Johnson - thanks for your newsletter, I have just
> subscribed. I completely agree with the comments pertaining
> to power plane impedance and the 'single node' assumption
> below the frequency where the board resonates (Volume 2 Issue
> 14). We have checked the impedance of power planes with a
> network analyzer. With no capacitors present, you can see
> interesting resonances that depend on the 1/4 wavelength from
> the probes to the card edge (valleys) and half wavelengths
> that fit into the card dimensions. But these measurements
> always come in dB and my spice simulations come out in ohms
> (after I force 1 Amp). Do you have any ideas on how to
> correlate between them? I would like to be able to measure
> the plane impedance in Ohms!
>
> A minor point... We are looking at using HIGHer dielectric
> constants for the material between power planes in order to
> gain more decoupling capacitance. That is going to lower the
> resonant frequencies of the power planes, possibly into
> frequencies of interest (near the clock).
>
> Best Regards,
>
>*-------------------(REPLY FROM DR. JOHNSON)--------------------*
>
> Thanks for your interest in High-Speed Digital Design.
>
> When using a network analyzer to measure power and ground
> impedance, we drive the power and ground planes at one point
> on the board with a sine wave, and the measure how much
> voltage appears at a different point.
>
> Don't set the IN and OUT cable attachment points on the board
> too close together or else their local crosstalk will pollute
> your measurement.
>
> In this setup, we can relate ohms to dB if (1) we know the
> driving point impedance of the network analyzer test setup
> and (2) the network analyzer is calibrated in terms of dB
> gain, where 0 dB means there is no device under test
> connected (that is, the IN cable is directly connected to the
> OUT cable).
>
> To establish this relation, we just use the resistance
> divider theorem:
>
> dB gain = 20*log(Vmeasured / Vreference)
>
> dB gain = 20*log( Rpwr-gnd / (Rpwr-gnd + Rsource) )
>
> where Rsource is the driving point impedance of the test
> setup.
>
> If your network analyzer has a 50-ohm output, and a 50-ohm
> input, then the driving point impedance at the device-under-
> test point is 25 ohms. Rsource = 25 ohms.
>
> Now we can simplify things a little if we use the fact that,
> even when resonating, the power and ground planes have an
> impedance much less than 25 ohms (if they didn't the whole
> system wouldn't even come close to working). Therefore we can
> ignore the term Rpwr-gnd in the denominator, and just use the
> approximation:
>
> dB gain = 20*log( Rpwr-gnd / Rsource )
>
> Converting this formula to express Rpwr-gnd in terms of dB,
> we get:
>
> Rpwr-gnd = Rsource *{10 <raised to the power of> [(dB
> gain)/20]}
>
>
>Best regards,
>Dr. Howard Johnson
>
>*---------------------------------------------------------------*
>
> Comments welcome! hsdd@signalintegrity.com
>
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