Electronics > Projects, Designs, and Technical Stuff
power decoupling myths
OwO:
I did the simple experiment of soldering various 0603 parts directly to a SMA female connector (by sawing off all legs first) and measuring with a good VNA (not a cheapo nanovna). The results agreed with what the manufacturer's graphs say. So it's impossible that you can not get good decoupling out of a common X7R cap. How you lay out the board should also not be based on rules or simulation, but rather what you have tried and verified (with measurements) in the past. I recently designed a upright SMA to microstrip transition with almost 30dB return loss to 3GHz, and found that simulations are completely inaccurate and such results can only be achieved by experiments and a good VNA/cal kit.
T3sl4co1l:
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.
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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
OwO:
So how did he conclude that discrete capacitors are ineffective beyond a few hundred MHz (which is demonstrably false)? If your simulations differ that far from reality, then I'll say it's almost useless.
SiliconWizard:
Simulations are as good as your models are. I'm kind of questioning the models he used.
ejeffrey:
--- Quote from: OwO on July 21, 2020, 04:15:04 pm ---
--- Quote from: TheUnnamedNewbie on July 21, 2020, 03:08:48 pm ---Another thing I learned: Discrete components are pointless to do any decoupling past a few hundred MHz. Even if you ignore any layout effects, the interal ESL of a 008004 cap makes it's self-resonance frequency be around the 100 MHz range. You just can't get around that without using special bonded capacitors meant for RF/MMIC applications. And to be frank, even if you could, your bond wires will just ruin that too. Just trust that the IC guys know their stuff and take care of decap past 100 MHz.
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Or actually you know, make real life measurements that will tell you a typical 0402 1uF capacitor has good decoupling up to 6GHz. Or if you don't have the TE maybe manufacturer's graphs will convince you? http://weblib.samsungsem.com/mlcc/mlcc-ec-data-sheet.do?partNumber=CL05A105KA5NQN
You are already halfway to being enlightened. The first half is knowing that "best practices" are BS, the second half is knowing simulations are BS.
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Trusting manufacturer graphs over simulations is terrible advice. Even trusting your own measurements over simulation is dubious at best, but at least you know the measurement setup. In this case, those graphs are de-embedded to the pads, and don't reflect performance in any real circuit. They show the AC voltage across the capacitor for a given excitation current. But nobody actually cares about the voltage across the capacitor. These models are in fact best suited for embedding into field simulators and in that capacity they are quite useful. For directly predicting performance they are a guide to be used cautiously at best.
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