EEVblog Electronics Community Forum

EEVblog => EEVblog Specific => Topic started by: EEVblog on May 18, 2018, 10:12:01 am

Title: EEVblog #1085 - Bypass Capacitors Visualised
Post by: EEVblog on May 18, 2018, 10:12:01 am
An in-depth practical visualisation of how bypass capacitors work at both high and low frequencies.
Bulk decoupling capacitors vs bypass capacitors.
Capacitor placement and types are tested and the results examined.
How package inductance can have a large effect.
Loop area and what is means, it's impact on EMC emissions, and how currents flow in ground planes is demonstrated

Links: http://web.mst.edu/~jfan/slides/Archambeault1.pdf (http://web.mst.edu/~jfan/slides/Archambeault1.pdf)

https://www.youtube.com/watch?v=1xicZF9glH0 (https://www.youtube.com/watch?v=1xicZF9glH0)
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: SeanB on May 19, 2018, 03:43:57 am
Use that HCMOS oscillator to drive all the gates of a 74F00 in parallel, and use one output to drive the transmission line. 74F was the fastest standard TTL, and had massive power current needs. They were quickly replaced with S and then LS, as they were almost as fast but more importantly did not run ar so high a current.LSTTL in many cases was happy with little decoupling, but F series would make a very nice oscillator.

Did make a stack of 3 54S00 chips to make a clock driver, using all 12 gates in parallel, soldered the decoupling capacitors directly on the pins top and bottom of the stack along with a 10uf 25v wet tantalum package there. It ran hot, but the clock edges were really fast, you got to love the CERDIP for being able to handle heat well.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: glarsson on May 19, 2018, 04:56:01 am
74F was the fastest standard TTL, and had massive power current needs. They were quickly replaced with S and then LS, as they were almost as fast but more importantly did not run ar so high a current.LSTTL in many cases was happy with little decoupling, but F series would make a very nice oscillator.
You are writing about the 74H or 74S series, the initial fast TTL designs that were replaced by 74LS.
The 74F and 74AS was later designs introduced in 1985.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: GerryBags on May 19, 2018, 05:46:16 am
Great stuff, Mr. jones! Even when I have gotten an intellectual grip on these things I find it really handy having the practical demo to hang it on. I have a very visual memory and I find the demos help with remembering the maths involved. When they are purely abstract I have to keep looking stuff up, but once it's related to a physical setup it seems to stick.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: tautech on May 19, 2018, 06:31:36 am
Dave, is there a reason why you didn't look at showing how a tantalum cap can offer both bulk and local decoupling ?
We often see them used on longer supply rails to clean up both HF and LF.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: DutchGert on May 19, 2018, 07:53:28 am
Very nice video! :-+
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: Red Squirrel on May 19, 2018, 10:23:10 am
That was neat.  I never really considered much how the physical layout of something can make such a difference until I started watching your videos, and this particular one really is cool as you can easily visualize the difference.

About visualizing the path of electrons, I wonder if it could be done using a higher voltage, like mains, and then you could probe different parts of the board with a multimeter.  Could use something less conductive than copper as well, maybe lead.  Would make for a fun experiment. :D

Or solder a bunch of small LEDs and they should glow at different intensities. 
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: JustMeHere on May 20, 2018, 08:31:21 am
Thanks Dave
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: mcinque on May 21, 2018, 12:00:51 am
Well done and explained!

Just wonder why did you use that switching psu instead dp832 behind it, there is a specific reason? I guess to have on the screen of the scope some ripple that the linear one does not have, is this right?
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: JustMeHere on May 22, 2018, 08:46:05 am
I was wondering if you could take that test jig and do another experiment:  take a second scope and place a probe with its ground lead attached inside of the "loop area"  It would be cool to see how much noise the ground wire picks up.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: EEVblog on May 22, 2018, 11:27:13 am
Dave, is there a reason why you didn't look at showing how a tantalum cap can offer both bulk and local decoupling ?

Because this wasn't supposed to be a video on bypass capacitor types, the video was already longer than I wanted.
Title: Re: EEVblog #1085 - Bypass Capacitors Visualised
Post by: EEVblog on May 22, 2018, 11:28:09 am
Well done and explained!
Just wonder why did you use that switching psu instead dp832 behind it, there is a specific reason? I guess to have on the screen of the scope some ripple that the linear one does not have, is this right?

Because it was sitting there.