Re: Re: Bypass thoughts

Peter Hallenbeck wrote:
>   I think we're in the same ballpark here.  Next time I have a small test
> board for some chip, I'll play with the "two of the same size in parallel"
> vs. "one cap, and another at 1/100 it's value" approach.  I'll also take a
> board with a HCS08 on it (which has only 3 bypass caps) and change out the 1
> nF to a 100nF so all three are the same and make some measurements on it.

Yes, please try it and see - I make things up as I go along, based on 
what I've read, what I've heard from others, and what I've learned from 
experience - my formal education is in maths and theoretical computing 
(I'm mainly a software developer).  My methods work for me, but do some 
tests before committing your designs!

>   I know the "spin frequencies" for caps (the self resonant point) has got
> much better (higher) with time, and there's less lot to lot variations.  My
> production house does much better with 0805 than 0603, and so it's possible
> that when they get more accustom to 0603 that the smaller size will change

In 0805 packages, the inductance is a bit higher, so the relevant 
frequencies are lower.  You can help by using more than one via to the 
power/ground planes.  And as always, two identical caps in parallel have 
twice the capacitance, half the resistance, and half the inductance.

And if your production people complain about you using 0603, point out 
to them that fewer cap sizes means fewer reels to mount on the 
pick-and-place machine, and smaller inventories.

> things too.  I started this practice back in the 0612 .1 uF days, and am
> probably guilty of not re-thinking / re-verifying the approach.  So I
> appreciate your email.  Bypass design is part of every PCB design, and it's
> easy to forget that and get stuck in a rut.

That's not exactly unique to bypassing!

>   The key here is knowing about the impedance 'V' graph and the self
> resonant point (if working with frequencies above 500 MHz).  That and some

The self resonant point is a lot lower than 500 MHz - expect below 100 
MHz in practice (depending on the vias and wires).  Chip pins and balls 
are major inductance sources already at that frequency.  Of course, it 
doesn't really matter even if your bypass caps resonate below some of 
the frequencies you use on the card - they still do their job.  They 
just don't do it as well as for lower frequencies, as current peaks are 
reduced by their inductances.

If you are talking about 500 MHz frequencies (for a seriously 
overclocked HCS08...), there are different things to consider.  One is 
to look for X2Y capacitors, which have much lower inductances 
(especially with the right via layout).  You also want to consider plane 
to plane capacitance - it's small in capacitance value, but works at 
very high frequencies.  Get chips with onboard caps and power planes 
(serious high-speed chips have them), and use differential signals 
everywhere (differential drives reduce the current peaks, and are less 
susceptible to ground and power bounce.  I've never gone above about 200 
MHz myself, but that's the theory :-)

muRata have some useful tools which generate impedance-frequency graphs 
for different capacitors:

http://www.murata.com/designlib/mcsil/index.html
(windows only, I'm afraid!)

mvh.,

David


> knowledge about PCB layout and you're good to go.  I might also be overly
> concerned about emissions because the product I'm working on has 120
> unbalanced, unshielded, single ended wire coming out of it that are as
> little as 1 foot and as long as 180 feet long.  It's a consumer produce, so
> it must pass (and did pass) the tougher emissions requirements.  The
> daughter board the 5282 is on has more $$s in the EMI suppression than in
> the micro, and it's a multi-layer PCB.
> 
>   All this said, tacking on some more caps is a small time hit that can
> eliminate a possible design problem on a new PCB layout.  If it makes a
> difference, then all these issues being discussed here can be worked
> through.
> 
> Regarding development platforms:
> 
>   Everything you've said makes sense, and I've suspected that Linux is the
> way to go.  I was curious if anyone has had luck on a MAC now that it runs a
> variant of Unix underneath it all.  It is clear that for debugging it's a PC
> or linux.  The FPGA tools I use are only for PC's (Altera Quartus II free
> Web Edition), it just seems odd to run something under cygwin that was
> clearly written for a unix like system.  I suspect there will always be a PC
> around somewhere, I just want to reduce my exposure, so to speak.
> 

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