Re: [SI-LIST] : High Permittivity Board Level Decoupling and

alterra@adnc.com
Mon, 15 Jun 1998 15:34:30 -0700

Hi Elya,

In addition to the other comments on your approach, I want to point out the
risk of creating unintentional resonators and thus creating a new EMI
problem rather than solving one.

I guess that you are considering the use of Fodel 5530 and/or 5540 which can
be combined to produce dielectric constants in the range of 3000 - 7000
which includes your value of 5000. I also assume that since you are an EMC
engineer, you are interested more in keeping unwanted RF off your power and
ground nets than in providing high peak currents to your board.

As someone else already mentioned, the electrical size of your capacitors
(using k=5000) will be 70 times the physical size. (e.g. a 0.1 inch object
will be a half-wavelength at about 840 MHz or a quarter wavelength at 420
MHz). It would be reasonable to print a 100 mil x 100 mil, 7nF capacitor as
you suggest but it would resonante at about 840 MHz which could easily
correspond to a fundamental or harmonic in your high speed circuit. If so,
you might make an effective but unintentional transmitter. Or to state it
another way, the 7nF capacitor will only act like a true lumped element
capacitor for frequencies below about 100 MHz (including harmonics).

I suggest that you make your capacitors as small as possible (perhaps 10-20
mils on a side) and use several in parallel if you need additional capacitance.

Best regards,

Eric Wheatley

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Eric Wheatley Ph.D. Electromagnetic Modeling Services
Alterra Technology Co. (760) 942-9426 (phone)
435 Dunsmore Ct. (760) 942-2366 (fax)
Encinitas, CA 92024 US alterra@adnc.com
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At 09:58 PM 6/15/98 +2, you wrote:
>Dear Members,
>
>a) High Permittivity Dielectrics
>
>We are considering to use, for RF and ultra-high speed digital circuits,
very high permittivity dielectrics between the PCBs ("Thick Film FODEL").
>
>Assume a separation between GND and VCC of 1.6mil, and a dielectric
constant Epsilon(r) of 5,000.
>
>We have calculated that for a sq. Inch of PCB this is "worth" about 700nF
of decoupling.
>
>In this case, would discrete decoupling capacitors actually be necessary,
would they be of any avail or perhaps even the opposite? Any input on this
will be greatly appreciated.
>
>b) Analog Circuit Decoupling
>
>Does anyone know any "rule of thumb" for selecting the proper value of
decoupling capacitors for analog circuits? In digital circuits - the rise
time and switching currents of the devices play an important role... What
are the considerations in Analog circuits???
>
>Any reply to either or both questions will be greatly appreciated.
>
>Thanks alot,
>
> Elya B. Joffe - EMC Engineer
>
>Every success begins with the launching of a dream
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