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

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From: Mark Randol (ryvw50@email.sps.mot.com)
Date: Mon Nov 01 1999 - 10:12:52 PST


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

1st, a suggestion to all. When replying, please make your quotes
so that they are clearly quotes. A couple of messages recently
are "quoted inline" <hopefully that's the correct term>, and
after a little judicious editing it's difficult to tell who said
what. Thanks. (not intending to single anyone out, it's just
something I noticed lately)

2nd, most ceramic 0603's and 0402's can have SRFs up around
3-4GHz. Mounted parasitic inductances are another matter left
for the user :) The values top out at about 33pF for 1.5GHz, but
they are usable. Of course, I'm using them in a different
application than most on the list. I'm predominately concerned
with analog circuits, i.e. "discrete" frequency and relatively
low amplitude swings), so small capacitances are generally
sufficient. With digital circuits, I can understand where a
single small capacitance may not supply sufficient
"instantaneous" current. I don't know how much it costs to buy
and mount one of these, but from what I've been told by people
that should, 20 cents would be quite high unless you are using
porcelains or another special device.

-- 
Mark Randol, RF Measurements Engineer
Motorola SPS, Inc.
M/S EL381
2100 E. Elliot Road
Tempe, AZ 85284
(480)413-8052 Voice
(480)413-3455 FAX
M.Randol@motorola.com

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