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How I leared what a decoupling capacitor is for, the hard way

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Re: How I leared what a decoupling capacitor is for, the hard way

#81

Post by madaxe_again »

vintermann wrote: post #60
madaxe_again wrote: post #27

Earlier quoted context omitted.

I feel it’s a function of abstraction. You learned when analogue circuitry was the norm. I learned when digital circuitry was simple enough that you could readily take something apart and understand it. Now, EE courses often start with cad, simulations, digital electronics, and you end up with people building ziggurats atop an ocean of incomprehension. It’s exactly the same thing with software. I don’t scorn people f…

> you end up with people building ziggurats atop an ocean of incomprehension. Everyone does. There's probably a layer below for everyone but the most theoretical physicists. I don't know where the leaks in electronics engineering's abstractions are, but I'm pretty sure they exist.

This is why I went on to study physics - I never was someone who could stop asking “why?”

All it does is provide yet more profound questions.

Re: How I leared what a decoupling capacitor is for, the hard way

#82

Post by ErroneousBosh »

jacquesm wrote: post #44
dragontamer wrote: post #41

Earlier quoted context omitted.

That's because you weren't dealing with 20MHz noise. Hobbyists are not dealing with 20MHz noise issues. Period. And if you are actually crazy enough to deal with high frequency circuits like that, you would well know that the land of through hole designs is simply insufficient, and that you are probably somewhere with some 0402 capacitors and some tweezers right now.

> Hobbyists are not dealing with 20MHz noise issues. It happens. Not often, but it does happen and it depends on the hobbyist and what they're up to (but you won't be sticking that together on a breadboard). Also: if you start using HCT, AHC or even G parts where you don't really need them it can happen to you in places where you don't normally expect it. Those things have crazy fast rise times.

Or analogue. 20MHz is basically only just above audio. You can amplify it with a cheap jellybean opamp that requires no particular care to use.

Re: How I leared what a decoupling capacitor is for, the hard way

#83

Post by dragontamer »

ErroneousBosh wrote: post #82
jacquesm wrote: post #44

Earlier quoted context omitted.

> Hobbyists are not dealing with 20MHz noise issues. It happens. Not often, but it does happen and it depends on the hobbyist and what they're up to (but you won't be sticking that together on a breadboard). Also: if you start using HCT, AHC or even G parts where you don't really need them it can happen to you in places where you don't normally expect it. Those things have crazy fast rise times.

Or analogue. 20MHz is basically only just above audio. You can amplify it with a cheap jellybean opamp that requires no particular care to use.

Which one?

MCP6002 jellybean is GBP of 1MHz. Most "jellybean" OpAmps I know cap out at 10MHz GBWP (aka: a useless 10x gain at 1 MHz).

LM358B is 1.2 MHz GBWP or even 700kHz depending on the design. Magnitudes away from 20MHz especially when you want more than 1.0x gain.

If you want even 10x gain (aka around 10% error), you might suffice with 200MHz GBWP at 20MHz. Or maybe get a nice ADC and just go all digital given today's equipment...

Come on man. Typical "high speed" OpAmps are like 100MHz GBWP or less... correlating to only 5x gain at 20MHz. This sort of stuff is well outside "jellybean" amps. And I'm not even sure if a 100MHz amp is very effective at 20MHz.

Re: How I leared what a decoupling capacitor is for, the hard way

#84

Post by nielsbot »

bavell wrote: post #66
nielsbot wrote: post #28

Earlier quoted context omitted.

> questionable daycares If they don't find fraud, is it "questionable"?

Didn't the guy flee the country after posting bail? Doesn't exactly scream "innocent".

I didn’t see that. Is there a source? I saw Minnesota authorities investigated the business and didn’t find evidence of fraud.

I did read there was some amount of fraud in state sponsored care programs but nothing extensive.

Re: How I leared what a decoupling capacitor is for, the hard way

#85

Post by hulitu »

Lerc wrote: post #21
hilbert42 wrote: post #18

Ah how things have changed. When I was learning electronics we mainly dealt with radio and TV circuits and just about the first lesson one learned was to keep leads short (reduce unwanted inductance) and use decoupling capacitors everywhere. I recall some years later a young graduate engineer coming into my office with a rather involved circuit consisting of 30/40 TTL ICs and complaining that he'd double checked the…

I can see how that happens when people come at things from a conceptual digital side first. It probably doesn't help when you have a circuit diagram that while topologically correct doesn't show the relative positioning between components. The first time I saw all the decoupling caps rendered in a single chain on the side of the diagram I was mightily confused. It seemed like utter nonsense until I realised where the…

> It seemed like utter nonsense

This is utter nonsense. Just ask the layouter where they will be placed. (at the output of the voltage regulator or where he will find empty space on the board, completely missing their function). Where your schematics is bad, the layout will be also bad.

Re: How I leared what a decoupling capacitor is for, the hard way

#86

Post by ErroneousBosh »

dragontamer wrote: post #83
ErroneousBosh wrote: post #82

Earlier quoted context omitted.

Or analogue. 20MHz is basically only just above audio. You can amplify it with a cheap jellybean opamp that requires no particular care to use.

Which one? MCP6002 jellybean is GBP of 1MHz. Most "jellybean" OpAmps I know cap out at 10MHz GBWP (aka: a useless 10x gain at 1 MHz). LM358B is 1.2 MHz GBWP or even 700kHz depending on the design. Magnitudes away from 20MHz especially when you want more than 1.0x gain. If you want even 10x gain (aka around 10% error), you might suffice with 200MHz GBWP at 20MHz. Or maybe get a nice ADC and just go all digital given t…

LT1812 is my weapon of choice for ultra low RF stuff (think Tayloe mixer frontends). Readily available, pennies to buy, reasonably flat to about 20MHz although THD is getting a little rough up there, possibly because I'm using it wrong.

They cost pennies.

Re: How I leared what a decoupling capacitor is for, the hard way

#87

Post by Kirby64 »

jacquesm wrote: post #42
WarmWash wrote: post #16

Earlier quoted context omitted.

The capacitor doesn't have a concept of "fast enough", it's a passive component. The signal is what determines what it does when it encounters the capacitor. Non-linearities and capacitor species aside, a good ole x7r 100nF would clean this up. In general you can just liberally dump 100nF caps all over your pcb power traces and quash most problems like this before even knowing they exist. I joke that you make a circu…

The capacitor has a self inductance. That's why you use low self inductance capacitors with very short leads or traces in this role. 100 nF ceramics are fine, but you may actually need a 100 nF and a 10 nF side-by-side because of that inductance depending on how dirty your power line is. Fast clocked circuitry can be pretty nasty.

> 100 nF ceramics are fine, but you may actually need a 100 nF and a 10 nF side-by-side because of that inductance depending on how dirty your power line is.

Capacitance value is essentially irrelevant to inductance. The thing that dictates inductance of a decoupling capacitor is essentially only the package size. The way to fix that is to use smaller (physical) sized components. The only reason it may be related is that some larger values of capacitance physically can't fit into tiny package sizes. For 100nF, you have essentially no restrictions though. 0201(i) are easy to find in that value.

Re: How I leared what a decoupling capacitor is for, the hard way

#88

Post by dragontamer »

ErroneousBosh wrote: post #86
dragontamer wrote: post #83

Earlier quoted context omitted.

Which one? MCP6002 jellybean is GBP of 1MHz. Most "jellybean" OpAmps I know cap out at 10MHz GBWP (aka: a useless 10x gain at 1 MHz). LM358B is 1.2 MHz GBWP or even 700kHz depending on the design. Magnitudes away from 20MHz especially when you want more than 1.0x gain. If you want even 10x gain (aka around 10% error), you might suffice with 200MHz GBWP at 20MHz. Or maybe get a nice ADC and just go all digital given t…

LT1812 is my weapon of choice for ultra low RF stuff (think Tayloe mixer frontends). Readily available, pennies to buy, reasonably flat to about 20MHz although THD is getting a little rough up there, possibly because I'm using it wrong. They cost pennies.

https://www.digikey.com/en/products/detail/analog-devices-in...

Looks like they're closer to $3+? Like this is more expensive than a lot of microcontrollers for just one OpAmp.

I wouldn't call any Analog Devices / Linear Technologies parts to be a jellybean. Jellybean means matching common specs that any manufacturer can do.

But at $3 per chip, that's very far away from jellybean status. And AD / LT devices are known for high quality that no one else replicates.

> although THD is getting a little rough up there

Yeah, because a 100MHz high speed OpAmps only has 5x gain at 20MHz. So your error is now well in excess of like 20% (best case) or probably way way worse errors in practice.

As I said earlier: Im not even convinced that 100MHz OpAmps have the GBWP to effectively operate on 20MHz signals. You need more GBWP than that to be comfortable.

Re: How I leared what a decoupling capacitor is for, the hard way

#89

Post by hilbert42 »

madaxe_again wrote: post #81
vintermann wrote: post #60

Earlier quoted context omitted.

> you end up with people building ziggurats atop an ocean of incomprehension. Everyone does. There's probably a layer below for everyone but the most theoretical physicists. I don't know where the leaks in electronics engineering's abstractions are, but I'm pretty sure they exist.

This is why I went on to study physics - I never was someone who could stop asking “why?” All it does is provide yet more profound questions.

"…I never was someone who could stop asking “why?”"

When a kid of about four I found a pair of WWII headphones and took them to my father who pulled out the iron diaphragm and showed me magnetism at work—somehow some magical force was pulling the diaphragm back into the headphone with seemingly nothing in between. Absolutely fascinated, I wanted to know what this invisible 'magic' was. Many decades later every time I look at my fridge magnets I still ask the same question and I don't believe I'm much closer to the truth!

Sure, there are the simple answers everyone's taught, then there's QFT but even that doesn't tell me exactly what's going on. And why does alpha have the value it does, and why exactly does c = 1/(μ0ε0)^1/2? Not knowing and not being able to figure these questions out is, at times, infuriating.

For me, solace of sorts can be found in engineering—I can build an electronic circuit and end up with a tangible working device. On the way I'll curse my electrons for making so much noise that they sound like ball bearings rattling around in an empty oil drum but I'll eventually calm down and apply Johnson–Nyquist to shut them up (well, a little bit anyway).

Re: How I leared what a decoupling capacitor is for, the hard way

#90

Post by jacquesm »

Kirby64 wrote: post #87
jacquesm wrote: post #42

Earlier quoted context omitted.

The capacitor has a self inductance. That's why you use low self inductance capacitors with very short leads or traces in this role. 100 nF ceramics are fine, but you may actually need a 100 nF and a 10 nF side-by-side because of that inductance depending on how dirty your power line is. Fast clocked circuitry can be pretty nasty.

> 100 nF ceramics are fine, but you may actually need a 100 nF and a 10 nF side-by-side because of that inductance depending on how dirty your power line is. Capacitance value is essentially irrelevant to inductance. The thing that dictates inductance of a decoupling capacitor is essentially only the package size. The way to fix that is to use smaller (physical) sized components. The only reason it may be related is…

That's fair, I was thinking of hole-through components and I should probably stop doing that in 2026... But: if we're being pedantic: it isn't the physical size as much as it is the loop area that the current travels through, and that doesn't depend so much on the size of the package as on how the package is constructed. Low ESR caps such as IDC or LGA type caps will do better than other types even if the physical size is identical. Lead length also still matters, whether physical leads or length of the traces connecting the cap to the device and these should be kept as short as possible.
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