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

Howard Johnson ([email protected])
Tue, 16 Jun 1998 07:38:42 -0700

Dear Elya,

I agree with Larry about the time-of-flight issue. It's quite

important. This method may not be as effective as it first sounds.

As you scale up the dielectric constant, the capacitance per square

goes up proportional to Er, but the radius of the "disk" of

effective capacitance surrounding your component shrinks.

The interplane capacitance has to near the driver, to within a

small fraction of a risetime, in order to be effective.

At very high speeds, only a small disk of

interplane capacitance is doing most of the work for each chip.

In a high-speed design, the radius of the effective

capacitive disk shrinks as 1/sqrt(Er),

therefore the area of the effective capacitance shrinks

as 1/Er, therefore the total amount of capacitance (C per square

inch times the area of the effective disk) remains constant.

Once you are going fast enough that the board no longer

acts as a lumped circuit, scaling up the dielectric

constant doesn't help. (For boards small enough to be

considered a lumped-element system, increasing the dielectric

certainly helps).

On another issue brought up by Eric, you may run into some

peculiar resonance effects with this design. The whole power

and ground system will support several resonant modes.

Viewing the power-ground planes themselves as a huge, fat

transmission line, the first mode of resonance will

occur at a cycle time corresponding to one round-trip

propagation delay from one side of the board to the other

and back.

If the board is, say, 6 inches x 6 inches, then

this first resonant mode will occur at time:

Tresonance = 2 * W * DelayPerInch = 36,062 ps

Where W is the board width (6 inches)

and DelayPerInch = 85 ps/inch * sqrt(Er=5000) = 6010 ps/inch

The resonant frequeny, Fresonance = 1/Tresonance = 28 MHz.

If your circuits demand significant currents from the power

system at at repetitive frequency of 28 MHz (or a multiple

thereof), the bypassing system will prove to be ineffective.

Regarding Eric Wheatley's comments, I don't see how dividing

the planes into multiple patches of capacitance will

really help very much. You'll still have a coupled network

of capacitors, which will still support a rather low-frequency

resonant mode. With normal bypass capacitors, the (rather substantial)

lead inductance of the parts prevents such wierd resonance

effects from taking hold. Here that effect doesn't help.

It's probably important to take a look at the Q of the power

system resonance. For ordinary dielectrics (FR-4), the first

mode is up in the 500 MHz range, and the Q is only 4 to 8, so

it doesn't cause that much of a problem. The primary loss factor

in a normal board (as far as I can tell) is the skin effect

resistance of the copper planes, with a little help from

dielectric loss.

In Elya's case, the skin effect isn't going to help as much

(because the first resonance is a lot lower). Elya, can

you tell us what is the loss tangent of your material?

Has anybody got some real-life measurements of this stuff?

Best regards,

Dr. Howard Johnson

At 09:58 PM 6/15/98 +2, you wrote:


<excerpt>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

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 <underline>analog circuits</underline>? In
digital circuits - the rise time and switching currents of the devices
play an important role... What are the considerations in Analog

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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Dr. Howard Johnson, Signal Consulting, Inc.

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