RF coupling with multiple capacitors

The book "Experimental Methods in RF Design" Hayward, Campbell, Larkin, published by the ARRL has a section called "Bypassing and Decoupling". Their conclusions about putting two different values in parallel are not good. "The results are terrible!" ... "This behavior is a dramatic example of lore that is generally wrong."

They do recommend using a couple caps of the same value and that a single larger cap isn't bad.

That book changed my view on the subject. Good to see other measurements that reach the same conclusions.

Hi Xray,

(Looking at the book...) Hmm... I believe they calculated (simulated) figure

2.90 and what I'm finding is that, in actuality, the anti-resonance isn't as bad as a simulation would suggest. (Although I'm looking at caps as coupling elements, whereas they're --iniitally -- looking at bypassing, and they do say, "A blocking capacitor is not as critical as a bypass, for the impedance on either side will usually be higher than that of the block.") I'm going to try out some different values, though -- hopefully I can get results as 'bad' as simulation would suggest. :-) (Maybe I got lucky in initially finding two series inductors that very clearly didn't work well together?)

---Joel

Hi Joerg,

AVX gives you their 'SPIcap' programs that spits our ESR, ESL, after you select package size, dielectric, voltage, and capacitance,. Although their program varies ESR as a function of frequency, they fix the ESL based on the package size... I'd guess they took a bunch of data, computed a mean value of ESL to get the SRF correct, and then varied ESR as a function of frequency to fit the data as best as they could (the fact that they use the same ESL for

0402 and 0603 packages concerns me a little, however!)

Good to hear you had similar results!

For inter-stage coupling, I initially thought one ought to aim for something like 1/10th or 1/20th the design impedance (e.g., for a 50 ohm RF design, aim for 2.5-5 ohms), but having now played with it some, shooting for more like

1/2 or 1/4 seems more realistic, and will allow for smaller caps and hence better high frequency performance.

---Joel

The important thing is to have the cap width match the trace width, and to have a smallish gap in the trace, maybe not the ideal smt footprint. For a wider trace, like 110 mills (50 ohms on thick FR-4) you can bridge the gap with several identical 0603 or 0402 caps, which gives a very nice broadband DC block.

I tried the DiLabs super-wideband DC block caps, and they didn't seem any better than a couple of cheap ceramic caps.

For really wideband, make a z-cut in the trace and bridge it with a mess of ceramic and tantalum caps!

Most coupling and bypassing lore is plain wrong, especially if it's from a capacitor company. People insist that caps shouldn't be used above their SRF, which is nonsense; all that matters is their actual impedance.

The "opposite" problem, how to use multiple inductors to make a clean broadband DC block, is more difficult. We did disassemble a PSPL wideband bias tee, which turned out to be fairly weird.

John

Thanks for the comments, John -- I've been impressed by the various pictures of your equipment you occasionally post to ABSE, and I expect the designs perform as well as they look.

Sounds nice. I finally managed to see a couple dB dip in S21 by paralleling a

2.2uF 0805 cap with a 22pF 0603 cap... although its effect was actually not that much larger than a 'divet' I get due to the trace widening from ~55mils (this is a .031" board) to nearly triple that where the caps are mounting (the original idea was to try mounting up to three caps side by side).

I'm not quite sure I follow what shape a 'z-cut' looks like?

Yeah, so I've been 'discovering!'

Was it something like

?

100kHz-4.2GHz for the JEBT-4R2GW-- pretty nice!

---Joel

Phil,

I'm not quite THAT young, but almost. :-)

I'm liking the idea of a 100nF 0306 caps for pretty wideband bypassing... looks reaaalllly nice.

If you have a multilayer board with close power and ground planes, any trashy caps, placed most anywhere, will be fine as bypasses.

John

OK, I'll try to draw it....

A

----------------- --------------------------------------- | |__________ ---------- | | |

---------------------------- ---------------------------- B

(pretty bad, huh?)

So you put small ceramic caps horizontally at A and B, and fill in the middle, horizontal gap with vertically-oriented bigger stuff. Seems to work.

I wonder if long interdigitated fingers would be good. The fingers themselves would be the highest-frequency couplers. Too hard to draw.

If there's interest, I'll post pics of the guts of the PSPL tee, and the reverse-engineered schematic, to a.b.s.e. It's rated to 18 GHz or something.

John

It got the point across just fine; thanks!

I believe I can imagine what you're saying; interesting idea.

I'd love to see it!

---Joel

You said it. In my first engineering job, many many moons ago, we used a lot of 0.1 uF stacked film caps for bypasses. There was one school of thought that said that film caps were bad because they became inductive at high frequencies and led to lots of ringing. Being naive, I thought that I could settle a religious war using data--silly me. So I got a few of them, soldered them across the ends of short pieces of hardline, and put them on a nice HP vector network analyzer. Result? no noticeable self-resonance in any of them--Qs of maybe 0.3 or less--and around 100 MHz they went very slightly inductive, about j50 milliohms worth--probably indistinguishable from zero ohms on that analyzer. You couldn't ask for a nicer bypass cap.

I sent a memo round to this effect, with the analyzer trace, and was told by a senior engineer, "You see? It went inductive. That means it'll ring."

I went back to my cube and forgot about it. About half the boards in that system went out with useless 1000 pF ceramics in parallel with every one of those nice film caps.

Cheers,

Phil Hobbs

OK, posted to a.b.s.e.

John

Hello Joel,

Best is to measure the caps on an impedance analyzer and study their data sheet. The RF behavior is often in some kind of 'family spec'. However, that still leaves the chance that someone changes suppliers later.

I found cap resonances to be less of a problem than I initially thought. But at the same time I usually do not specify a cap that is much higher in capacitance than is needed for the job.

Regards, Joerg

Hello Joel,

Best is to measure the caps on an impedance analyzer and study their data sheet. The RF behavior is often in some kind of 'family spec'. However, that still leaves the chance that someone changes suppliers later.

I found cap resonances to be less of a problem than I initially thought. But at the same time I usually do not specify a cap that is much higher in capacitance than is needed for the job.

Regards, Joerg

Thanks for the nice hack.

In pic 348x3 the teflon looks pretty solid.

In 353x1 I guess we are looking at the blocking cap? Hard to figure how this relates to the other pic. There seems to be some slices to the left side of the pic. Wondering how the bias connects to the coax line. Is it shown (did I miss it).

I think I almost ordered one of these or something similar on eBay recently. Maybe you outbid me.

Hello Joel,

I usually go a bit lower than that in impedance. Four times Z is quite customary in inductive coupling though.

Regards, Joerg

Hello John,

When I open stuff like that I sometimes get upset that a $1000 device has next to nothing in there. Maybe a buck in parts, an aluminim case, a few screws and a couple of N-connectors. But when I have to make one I begin to appreciate it more, slaving an hour over it at the analyzer.

Regards, Joerg

The transmission line conductor is surrounded by plastic... the insulator is actually a number of tubular sections, some solid and some split. The fine wire, wound around the ferrite/foam (?) stuff, is just soldered to the line.

Yup. It's just roughly midway along the main line, with the dielectric split there to allow access.

I'll add a couple more pics.

I got this one on ebay, but haven't bid on any more lately.

I'm thinking you could make a pretty decent bias tee on a small pcb, without all this machining.

John

There's a neat chapter in Jim Williams' first "Analog Design" collection (1991 version, chapter 26) about the ultimately-failed design of a frequency synthesizer system, with lots of groveling around with nanovolt VCO pertubations, shot noise, microphonics, fun things like that. If you haven't seen it yet, I suspect you'd find it amusing. It's sort of like coming at DDS from the wrong direction.

John

Depends for what. This was a satellite telecom system, with a truly nasty spurious response spec--5 Hz RMS in a 5-100 Hz BW around a 14 GHz carrier, plus equally unpleasant stuff further out. The 14 GHz had to be tunable in 1 MHz steps, and was derived from a 120X multiplier on a

115 MHz reference, which was one of the boards I was designing. Made for interesting PLL work, especially since this was well before DDSes.

Gave me a useful postgraduate course in shielding and bypassing.

Cheers,

Phil Hobbs