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

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From: Chris.H.Simon@gd-is.com
Date: Mon Nov 01 1999 - 08:49:29 PST


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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