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

About this list Date view Thread view Subject view Author view

From: Bill Dempsey (bdempsey@dnaent.com)
Date: Mon Nov 01 1999 - 11:02:48 PST


Mark Randol wrote:

> 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,

  When you say "most 0603/402's can have SRF around 3-4GHz" you can NOT be referring to the typical ceramic X7R type decoupling caps used by "most" on this list. 0.1uF ceramics (0805) SRF around 30 MHz (depends...) and .001uFs
~150-200 MHz~. Add a long squirrley trace to this and watch the capacitor's effectiveness drop.... Todd Hubing published a good paper in the IEEE a few years back on the effectiveness of decoupling caps vs. board capacitance.
It would behoove all to review this before making leaps of faith on decoupling strategies.

Regards,
Bill

>
>
> --
> 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
>
> **** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****

**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu/si-list ****


About this list Date view Thread view Subject view Author view

This archive was generated by hypermail 2b29 : Tue Feb 29 2000 - 11:39:29 PST