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power decoupling myths

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OwO:
For a beginner wanting to start with RF design, start with pen and paper. Start with deriving the transmission line equations (telegrapher's equations) to understand 1D wave behavior, and work your way up to wave equation in 2D/3D and maxwell's equations. You need to be able to visualize how a changing electric or magnetic field give rise to each other, and how fields are related to currents and charges. None of this insight can ever be gained by playing in an expensive EM simulator. I recommend just going to your local university and auditing the high frequency electronics and electromagnetism classes up to 4th year level. None of these classes will involve a 3D EM simulator, maybe RFSim99 at most. RFSim99 is useful for checking basic circuits, but again ONLY if you already understood the underlying transmission line theory beforehand. Same story with full EM simulators, don't even touch them until you can fully visualize EM wave behavior and know maxwell's equations by heart.

Alex Eisenhut:
I'm a big fan of the Smith Chart. In any case you'll need to know about it to interpret VNA displays.

TheUnnamedNewbie:

--- Quote from: OwO on July 22, 2020, 03:57:24 pm ---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.


--- End quote ---

So, just some comments on this:

- I figured that someone at this level of engineering, where they start to really ask questions of how power supply decoupling and filtering works, is no longer a beginner. Hence I stand by my statement that they could gain a lot of knowledge from simulating this kind of structure. At least a lot more than just trying to make sense of the 200 contradicting guidelines online.

- Simulations with PCBs at frequencies up to a few GHz, in my experience, never show nonphysical behaviour unless you are just purposefully messing up the meshing settings. Just runing through the few tutorial examples should teach you all you need to know to simulate some decap networks. This isn't simulating some 10-pole comb-line filter or SIW or multi-band stacked patch antenna. Just a capacitor and a few pieces of copper wire.


--- Quote from: OwO on July 22, 2020, 03:57:24 pm ---
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.

--- End quote ---

My latest work involves a 100-190 GHz PCB vivaldi antenna, and a 110-170 GHz 4-layer orthomode waveguide coupler. All on PCBs. Not even that extremely fancy PCBs, regular HDI boards. Heck, I don't even have frigin blind vias! The 600 GHz transmitter was just an example.

Just because I work in a team that does RFIC design does not mean that I only do RFIC design. And if you really think making 600 GHz RFICs in CMOS is that trivial, be my guest, and let me know when you pull it off.



--- Quote from: OwO on July 22, 2020, 04:07:18 pm ---For a beginner wanting to start with RF design, start with pen and paper. Start with deriving the transmission line equations (telegrapher's equations) to understand 1D wave behavior, and work your way up to wave equation in 2D/3D and maxwell's equations. You need to be able to visualize how a changing electric or magnetic field give rise to each other, and how fields are related to currents and charges. None of this insight can ever be gained by playing in an expensive EM simulator. I recommend just going to your local university and auditing the high frequency electronics and electromagnetism classes up to 4th year level. None of these classes will involve a 3D EM simulator, maybe RFSim99 at most. RFSim99 is useful for checking basic circuits, but again ONLY if you already understood the underlying transmission line theory beforehand. Same story with full EM simulators, don't even touch them until you can fully visualize EM wave behavior and know maxwell's equations by heart.

--- End quote ---

I fully agree that people starting with RF design should start there. That is also what we teach. But this was never fully on RF performance - this was power supply decoupling. Most cases, the caps are actually close enough to the IC's you are decoupling that you can ignore transmission line effects up to a few GHz. So all I said still stands. You are trying to make this an RF discussion, but it is not - I merely used my RF background to illustrate that I do actually have some knowledge when I talk about simulations and their merits.

Also: We do actually use ADS/Momentum to teach students in I think their 4th year here. With these tools being so accessible and common-place now, and frequencies going up in the industry, more and more companies are asking for students with better understanding of the simulation workflow.

T3sl4co1l:
I enjoyed the implication that one cannot learn anything about a system by poking at it blindly. :)

Tim

exe:
Question: are capacitors with reverse geometry any better for decoupling? I concluded that, unless it helps to minimize track lengths, it doesn't matter. So, it should help if supply pins close to each other, but it is pointless if supply pins are far. But I've never checked this. Any opinions?

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