RE: [SI-LIST] : Comment on Johnson's article (Now Buried Capacitance)

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From: John D. Allen (jallen@vhfcom.com)
Date: Mon Nov 01 1999 - 09:26:48 PST


Speaking of buried capacitance, any comments on it's self resonant
frequency? Does it have one? Or, is the limiting factor the propagaion
delay due to the distance traveled along the plane?

John Allen  Sr. Consulting Engineer   jallen@IronBridgeNetworks.com
<mailto:jallen@IronBridgeNetworks.com>
IronBridge Networks / 55 Hayden Avenue / Lexington MA  02421 / USA
phone: 781-372-8151  fax:   781-372-8092  Main Number 781-372-8000
 
> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of
> Chris.H.Simon@gd-is.com
> Sent: Monday, November 01, 1999 11:49 AM
> To: si-list@silab.eng.sun.com
> Subject: Re: [SI-LIST] : Comment on Johnson's article
>
>
>
> Mark,
>
> I've found no capacitors (at least the types of component
> capacitors that I
> can afford to mount on my boards) that have self-resonant frequency above
> the frequencies of interest in my systems. If you know of a
> capacitor that
> I can purchase and mount for under $0.20 each that has SRF above 1.5 GHz,
> let me know :)
>
> If we assume that the above is true then I will have a system where the
> frequencies of interest are in the range where the bypass capacitors look
> like inductors. And the article from Dr. Johnson addresses this:
>
> "... most bypass capacitors will be operating far above their series
> resonance point. That is, they will be acting like little
> inductors, not
> little capacitors. Their impedance will be heading up (+20
> dB/decade) and
> their phase angle will be +90 degrees (current lags voltage). Please
> don't be alarmed. Operating above series resonance is perfectly OK, AS
> LONG AS THE INDUCTANCE IS SUFFICIENTLY LOW."
>
>
>
>
>
> If this isn't good enough for a particular system then there are other
> options to get higher SRF, such as capacitance embedded in the PWB as has
> been discussed here previously.
>
> Chris Simon
>
> Isn't it a problem once the capacitors SRF is reached? Sure, that's the
> lowest impedance point, but beyond that the impedance begins to rise
> again. Past this first resonance everything I've seen shows that the
> component's impedance is somewhat unpredictable/unreliable.
>
> If the idea is the largest value capacitor that should be used has a SRF
> at or above the max frequency of interest, I have no problem there. "Of
> course", larger values with lower SRFs can be used too, as long as the
> higher frequencies are provided for with smaller capacitors or other
> circuits/structures.
>
> --
> Mark Randol, RF Measurements Engineer
> Motorola SPS, Inc.
>
> *****************************************
>
> > I have a question on the article posted on Dr. Johnson's web site at
> >
> > http://www.sigcon.com/news/2_3.htm
> >
> > entitled `Bypass Capacitor Layout'. At the end of the article
> > Dr. Johnson's states that since the only parameter that affects the
> > performance of bypass capacitors at high frequency is their parasitic
> > inductance, there is no point in using many small capacitors, rather
> > the highest value capacitor in the chosen package should be used.
> > Isn't this in contraddiction with the principle, also stated in the
> > book, that many small capacitors are a better choice since the total
> > parasitic inductance is lower? After the series resonant frequency
> > of 1/sqrt(LC) the impedance goes up with 20 dB/decade slope so the
> > only way to decrease it is by lowering either L or C, so both L and C
> > are important.
> > Am I missing something?
> > Thanks in advance,
> > -Arrigo
>
>
>
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