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power decoupling myths

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OwO:
If you have the luxury of high layer count boards with very close planes, then going to the plane is usually what you want to do. But if not, then this is probably the best decoupling you can get:


You can not make a blanket statement that capacitor "sequencing" is ineffective, or that you should always sequence capacitors. There is no substitute for simply having experience by doing lots of experiments, measurements, and trial and error. If you are new to high speed design, there is no better way to shoot yourself in the foot and lull yourself into a false sense of security than by playing around in a 3d EM simulator (and I learned this by experience too). Do some shitty designs, make mistakes, understand why they fail, make bodges on PCBs and do measurements to understand how the physical world really behaves.

Siwastaja:

--- Quote from: imo on July 21, 2020, 06:08:13 pm ---When you look at the "V" shaped frequency characteristics of the capacitors - smds of any shapes and values almost all manufacturers show it in their datasheets, it is clear, at least for me, that paralleling more capacitors makes sense. Not in 1:10 ratio, as the "V" shapes are too close each other and it really makes no sense - but 1:100 makes sense.. Talking RF here, not the power supplies, however..

--- End quote ---

Yes but your circuit likely does not draw currents at those exact frequencies defined by the "V" valleys. Your circuits want low impedance between those V dips as well. If you are able to provide low enough impedance through the whole spectrum, then adding a few additional even-lower dips is completely meaningless.

mawyatt:

--- Quote from: T3sl4co1l on July 21, 2020, 04:35:43 pm ---FYI, 3GHz is DC for @TheUnnamedNewbie.  Not sure how familiar you are with his work.



--- Quote from: OwO on July 21, 2020, 04:15:04 pm ---the second half is knowing simulations are BS.

--- End quote ---

Ah yes, simulations can never be correct so why even bother!  It's not like anyone has ever built anything of significance, let alone had it work perfectly the first time, using a filthy simulator! :-DD :-DD

Tim

--- End quote ---

Tim,

Well I also got a good laugh :-DD

I'm sure Maxwell didn't know anything, neither did Larry Nagal (SPICE), or Ken Kundert (Cadence) |O

Best,

mawyatt:

--- Quote from: TheUnnamedNewbie on July 21, 2020, 03:08:48 pm ---Ever since I put a number of layouts into actual EM solvers I realized that everyone on the internet who says 'you have to do it this way' is wrong.


Just trust that the IC guys know their stuff and take care of decap past 100 MHz.

--- End quote ---

On the IC side of things where I just retired from we always tried to simulate the effects of poor off-chip supply decoupling and included as much decoupling as possible on the chip. Circuits were used that were less supply sensitive like differential circuits if practical. We often used multiple regulators (Point of Load Regulation) because of decoupling, many times just getting to a chip pad wasn't practical, so a local regulator was utilized. This isn't restricted to analog/RF/MW circuits, with a massive array of digital gates all working in lock-step with the clock, very large di/dt are created and tax the decoupling schemes, both on and off chip.

Best,

OwO:
That was in response to this:


--- Quote from: TheUnnamedNewbie on July 21, 2020, 03:08:48 pm ---If you really want to understand and know, do yourself a favor: Stop going by some online forum post telling you what to do, and go rent/buy a copy of an EM package for a few weeks, and actually play around in that.

--- End quote ---

Simulations will not help you "understand and know". Maybe if you are already experienced and already know how to improve a layout, sure. But to suggest trying to *learn* by playing in a simulator is bad advice and will lead a beginner the wrong way. Someone experienced like you know instantly when a simulation is unphysical, either due to setting up the wrong conditions or bad models. Someone inexperienced with RF layout needs to do everything EXCEPT go buy an expensive 3d EM simulator and spend hours going down rabbit holes of unphysical and unrealizable designs, without even realizing why their design that performs perfectly in simulation is impossible to realize in real life.

I also don't get the point of saying 3GHz is DC to you, because a RFIC has geometries at least 1000x smaller than a PCB, so a 600GHz RFIC circuit is like laying out a 600MHz circuit on a PCB. Not the same thing of course, the basic circuit elements you have available to work with are completely different, but layout wise your elements at 600GHz are electrically small, while at 6GHz the simple via distance or the inter-plane distance on a PCB is getting significant relative to a quarter wavelength.

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