Hybrid capacitors

ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

I was wondering whether there are

in order to preserve the low inductivity.

medical, etc.

What's the application? You can get 10-22 uF ceramics cheap, 100 uF affordable. A 10 uF 0603 has about the same ESL as a 10 nF part, so there's no good reason to mix caps if you need a lot of uFs... just use the big ones.

The best high frequency cap is the planes of a multilayer PCB.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com   

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME  analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

in order to preserve the low inductivity.

medical, etc.

Depends on what your board is made of. FR4 is a rotten dielectric. Some of the Rogers high frequency substrates are a lot better.

--
Bill Sloman, Nijmegen

ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

You don't need a good dielectric to make bypass caps out of power and ground planes. Dielectric losses are an asset here, not a liability.

In theory a higher dielectric constant is better, but most of the Rogers HF stuff has lower Er than FR4. There is some Er==10 stuff around, but it's exotic and expensive. You usually wouldn't want to use that for signal layers, and mixed-dielectric boards are an expensive hassle, and tend to warp. FR4 is fine in 98% of applications.

Bypassing is simple on a multilayer board: use close-spaced solid ground planes and power pours; sprinkle that structure with a modest number of ceramic caps here and there; add some aluminum, tantalum, or (best) polymer aluminum caps if needed for bulk/low frequency bypassing. Ignore most of the conventional "Black Magic" nonsense about staggering resonances.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Ignore most of the conventional "Black Magic" nonsense

Yeah, the trade mag articles with their synthetic graphs of these huge resonances always make me laugh. On a real board, with tons of small series inductances and lots of DC load (read parallel resistance) there will be no resonances like their simple SPICE models spit out.

Jon

Exactly. Most of those articles have at least one author who works for a capacitor company.

There are people who seriously recommend using hundreds of bypass caps per FPGA.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

Extra equivalent series resistance makes the capacitors lossy, which can be a good thing, but it's extra impedance, which means that you've got more ripple current running around your power connections when you'd prefer it to be flowing through the nearest capacitance.

Only if you are silly enough to make them asymmetrical. The only one I've actually used didn't warp at all, but all it had were two outer layers of polyimide (IIRR) bonded Teflon cloth, which was pretty soft. The inner four layers were all FR4 (epoxy bonded glass fibre). It was a triple extended Eurocard, so we would have seen any tendency to warp.

98% of your applications.

We threw in a few microwave capacitors. The clock frequency was 800MHz, and while the edges weren't all that square there was probably some

2.4GHz component floating around. The microwave capacitors still looked like capacitors up there.

--
Bill Sloman, Nijmegen

(1n-100n) ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

The losses in the FR4 don't add meaningful amounts of ESR. They do add some useful damping to the entire structure, but the skin losses of the copper foil are better, especially when they use black oxide treatment to aid adhesion; that stuff is really nasty, electrically.

Well, I am in the picosecond timing/laser driver/pulse generator/fiberoptics business.

How's this look to you?

That's the output of our little T240 pulse generator. It worked first time, as expected. Or better, actually. I didn't expect a 40 ps edge to be this clean.

And wasted money and real estate. The T240 above uses ordinary 330 nF

0603 bypass caps, and it's around 8 GHz equivalent. I did use some of those rotated 0306 caps in the signal path, just for fun.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

Including 'X' and 'A'.

(1n-100n) ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

Any idea what the dielectric was? IRRR our microwave parts used porcelain.

We were a bit more worried about interactions between the various components of our system than you would need to be with a simple pulse generator, even if it is about an order of magnitude faster than our stuff.

We were producing tolerably clean 500psec pulses (FWHM) to turn the electron beam on for just 500psec - basically letting a couple of electron through - and we aimed at having+/-7.5V across the beam when it was turned off. IIRR we had to settle for about +/-6V at 500psec. There were a couple of HP RF power transistors in the output stage, mounted on a solid Teflon daughter board, with a couple of off-the-shelf broad-band transistors.

We were claiming to be able to position the edges with a resolution of

10psec, though our clock oscillator was rubbish and seemed to have about 60psec of edge jitter. Fixing that was on the agenda, along with a lot of other minor stuff, when they canned the project.

--
Bill Sloman, Nijmegen

table 2-1 only goes to around 60

-Lasse

I've had this conversation with 'X' and they admit that they make their recommendations based on a lack of knowledge about their customer's designs because the designs vary across the map. Their recommendations are based on the worst case ever possible FPGA design with a huge dose of CYA thrown in.

On the other hand, I have taken a class in high speed digital design with someone who used theory, then simulation and finally testing on boards built to test power delivery system designs and came to the conclusion that mixed capacitor values and a much smaller number of caps than are typically used are the best approach for digital designs.

Specifically, he used an approach of actually calculating impedance at various frequencies compared to the required impedance calculated for the application. No rules of thumb, no guess work.

Rick

Sure, they admit that but it really is a huge dose of CYA in even the worst case. I generally try to add the capacitance recommended then just no-pop a good share of them. It's a lot easier to add caps if the footprints are already there. BTW, I've never had to.

I've seen people simulate just about every fool thing. I stopped blindly trusting simulations long ago. ...particularly other people's simulations.

I've seen that done, too. It works, but so does sprinkling a few caps around. They're cheap.

Did you read the part about building a board to test the issue? He has verified this every way you might want.

Depends on how many... 10-20 per board times X can be a lot of dollars. On the other side, if you use have too few on the board and your design is actually more sensitive than you expected... well, that's why engineering isn't just winging it.

Rick

I once tried the power supply impedance calculator from Altera, which does much the same thing. I had reasonable results after a little work.

Input rough dimensions of how much plane you're going to have, physical dimensions, vias, traces, that sort of thing, and it'll guess how many bypass caps you need.

Of course, its first guess is something like 4 x 0603s, 9 x 0402s and some

0201s if you can even see them, plus whatever tantalums it can find.

After poking around a bit, I found excellent results with a single, fairly bulky tantalum, and a small number of 0603 0.1's per I/O bank (this was a middle size Cyclone).

It's worth noting that results are almost always *WORSE* using a low-ESR bulk cap, like an aluminum polymer. This is because it short-circuits the series resonances, making things a lot worse. The ESR of a tantalum (or, if you can accept the variance in ESR, aluminum electrolytic) dampens it much better than staggered values or huge arrays of similar values.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

around.

A good opportunity for Muntzing, though, if you have the space. Lay out more decaps than you think you need, and then don't populate all of them.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510
845-480-2058

hobbs at electrooptical dot net
http://electrooptical.net

(1n-100n) ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

The board was FR4 and all the caps were X7R ceramics. I don't think porcelain caps have any advantage except in RF power apps, where ESR can cause solder joints to melt. Body geometry dominates ESL. The

0306's are cool, lower ESL and not outrageously espensive... we're paying 6 cents for a 1 uF part. They look neat.

That statement makes no sense to me.

Sounds very expensive. FR4 works at 40 ps, so why not at 500?

We did the arbitrary waveform generators for the NIF beam modulators,

16 bits at 4 G samples/second, jitter a couple of ps RMS. And the master timing system, 1 ps resolution and similar jitter. All FR4.

Oh.

--

John Larkin         Highland Technology, Inc

jlarkin at highlandtechnology dot com
http://www.highlandtechnology.com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom laser drivers and controllers
Photonics and fiberoptic TTL data links
VME thermocouple, LVDT, synchro   acquisition and simulation

He verified that it worked. He likely didn't verify that a lesser amount didn't. Hell, cover the thing with caps and it'll work.

Not *nearly* as much as too little and finding out about it a year later.

around.

That's essentially what I do. On my last board, the board stuffer did it for me though. ;-( ...still worked fine. I won't take them off this spin, though.

(1n-100n) ceramic and larger (say 100uF) tantalum/electrolyte capacitors.

manufacturability I was wondering whether there are

pattern in order to preserve the low inductivity.

devices, medical, etc.

Perhaps. We really didn't want to have to lay out any of the boards a second time. The drawing office had given us a splendid excuse to rework one of the boards, by telling the printed circuit shop that the order of the inner layers didn't matter. Since that meant that we had to get the board working by replacing all the fast connections with miniature coax, that prototype board didn't tell us as much as it might have

has the usual graphs of impedance versus frequency. Your caps would have looked very like inductors at anything over 100MHz, though - as you say

- that's mainly due to body geometry and dimensions.

So you do concede that they do have a lower ESR, but choose to think that ESR doesn't matter; presumably because ESL swamps it.

But the ESL won't be all that much lower.

It's called cross-talk. If an asynchronous trigger was going to fire earlier or later because it nearly coincided with a clock edge, we - and our customers - would have seen it. The very first electron beam testers were horrible like that, and I got to be very picky about grounding, shielding and having anything fast as a balanced differential pair. By

1989, I'd learned to be picky enough.

Not my choice. Management bought in a microwave expert, and he prescribed the substrate. It didn't do much better than the blanking boards on the previous generation of machines where we hadn't used anything more exotic that polyimide bonded glass-fibre, and that mainly because it didn't discolour when it got hot, which turned out to be the only detectable advantage.

I reworked some of the ideas for a fast programmable pulse generator for electron spin resonance around 1997 at Nijmegen University. The thing never got built, but if it had I would have use a 500MHz clock based on very thin - etched - crystal resonator.

That did promise 1psec jitter.

The original intention has been to use a Gigabit Logic part that should have run at 800MHz, but didn't, and couldn't be persuaded to run anywhere near that fast, so we grabbed an 800MHz VCO designed for use in mobile phones and went with that. It worked, even if the jitter was horrible - we were under absurd pressure to get things working at the time.

I did fancy a magnetically tuned YAG-based oscillators as an upgrade. They were big and expensive, but they offered something like a 4:1 tuning range. The manufacturer stopped making them sometime between 1989 and 1997.

--
Bill Sloman, Nijmegen