Re: [SI-LIST] : Comment on Johnson's article

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From: Ron Miller (rmiller@Brocade.COM)
Date: Mon Nov 01 1999 - 14:01:10 PST


Try ATC, and Murata/Erie.

ron miller

Chris.H.Simon@gd-is.com wrote:

> 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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--
Ronald B. Miller  _\\|//_  Signal Integrity Engineer
(408)487-8017    (' 0-0 ') fax(408)487-8017
     ==========0000-(_)0000===========
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