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

Mark Randol ([email protected])
Mon, 15 Jun 1998 13:16:54 -0700

Hello Elya,

Elya B. Joffe wrote:
> 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.

The differences between us squiggly line types and people that
only count to "1" are: <hey, it's a weak attempt at humor, ok?>

- RF/Microwave is typically frequency domain based. That means
a cap can be selected to optimally bypass a certain frequency.
- Typically the dynamic ranges are much greater than
for digital.
- Broad band matching usually doesn't mean DC to day light.
- Short paths are key. Keep decoupling close to the
part to minimize trace inductance. By the time you've
crossed circuit board layers, it's too late.
- There REALLY isn't a pure R, L, C, open, or short.

All these change dependent on what frequency is being
operated at. Some tricks work at microwave (>1 or 3 GHz
depending on who you talk with) but are impractical at
lower frequencies, and some things that are done at
lower frequencies don't look anything like "what they
should" at higher ones.

Regarding your in^2 PCB cap, it's going to depend on
the frequency of operation and shape of the cap as to
whether it will work. The shape will change the resonance
of the cap. Likely you'll still need the small values
to bypass the high frequencies, but the PCB will probably
take up for some of the larger, higher value caps.

There are other things that can get you too, but I'm
not planning on writing an online book anytime soon :)

I'll stress it again tho', think in FREQUENCY, not rise
and fall times, and it should help alot.

Mark Randol, RF Measurements Engineer   | Motorola SPS, Inc.
(602)413-8052 Voice                     | M/S EL379
(602)413-4150 FAX                       | 2100 E. Elliot Road
[email protected]                | Tempe, AZ 85284
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