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Note: the word "greentape" refers to low-temperature co-fired ceramic, in its unfired (green) state. It comes in rolls, like tape, but it's usually white in color. Please see our other microwave slang words... Jim wrote this sometime in the early 1990s, when he was trying to explain to mechanical engineers why you need all of those zillions of ground vias in a complex circuit. The capitalization of the word "greentape" is his, not ours.
Ode to GREENTAPE
by James "Tennyson" Lampen
To help you with LTCC
I have composed this poetry
Because red tape you know so well,
GREENTAPE, you think, should work just swell.
RF expert, you are (you think)-
Be careful or your work could stink.
GREENTAPE really is great stuff,
But use it wrongly, life gets rough.
Remember this to make life pleasant:
Ground is not; ground just isn't.
Beware of RF grounds and vias-
They may be hotter than you please.
The ground design you generate
Can really, truly, resonate.
Scattering a few vias around
Should, its seems, suffice for ground.
You must take care or you will see
You may create a cavity.
These lines were written here to spare
You the curse, the shout, the swear.
The rules really are not profound;
You only need brains of a hound.
Basic structures are but few;
Look at Figures One and Two.
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FIGURE ONE: Half-Wave Resonator
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FIGURE TWO: Quarter-Wave Resonator
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Pads with vias at each end
Is a good place to begin.
Vias half wavelength apart
May not be so very smart.
Couple to this simple structure
And your schedule won't recover.
Pad with via at one end,
(I cannot help begin to grin)...
Open circuit at the other,
You may as well have sold your mother.
If it's a quarter wavelength long,
You've very truly done it wrong.
Couple to this structure and
Your house is built on sinking sand.
Observe Figures Three and Four;
There really is no need for more.
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FIGURE THREE: Resonant Cavity with Short Circuited Sidewalls
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FIGURE FOUR: Resonant Cavity with Open and Short Circuited Sidewalls
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These grounds, they look so good, so swell,
The fact is that they really smell.
Copy these and seal your fate,
For you will find they resonate.
RF "suck-outs" you'll see galore;
Look more closely and see some more.
Copy this design in haste,
And $30K of fab you'll waste.
Go ahead, make my day,
Do it, do it, anyway.
Copy these, you'll iterate,
And your program will be late.
The sketches shown here were inspired
By a program that expired.
Look at these sketches, study well;
Your testing will be spared much Hell.
Study them; know why they're wrong;
Your testing time won't take so long.
Heed not this, my finest work,
And you, my friend, will be the jerk.
Again, this fact I here present:
Ground is not; ground just isn't.