Choice of DC blocking capacitors in Microstrip Design

There's no reason for that rule. Most of the time a reasonable, like

0.33 uF, 0603 maybe, surface-mount cap has a very low RF impedance, essentially the same ESL as most any other 0603 cap. SRF doesn't matter, impedance does.

A 0.33 uF 0603 cap is a very low z well into the GHz. Ideally, match the cap body width and the trace width.

John

Hi,

Is it the same for 0805 case or is there a difference.

Thanks

[snip]

Whose 0.33uF do you use that is not well above its resonance at 1GHz?

Bob

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AVX have capacitors whose SRF =950MHz at 47pF for 0805. 33pF --> srf =

880MHz for 1210

Consider a 1 uF 0603 cap. Its ESL may be ballpark 600 pH, so its SRF is around 6.5 MHz. So what? At UHF frequencies, it's still 600 pH. If you bought a 10 uF 0603, or a 0.1 uF, it would still look like 600 pH at high frequencies.

The SRF *doesn't matter*. The working impedance does.

If you want a really low impedance DC block, make the microstrip trace wide, break it with a tiny slit, and pave it over with a row of 1 uF

0402 caps as wide as the trace. They're short, so have even lower ESL, and in a 50 ohm trace, depending on width, you may be able to get 2 to 5 in parallel.

John

Let's look at Alec's figures (a different post in this thread) for an

0805 47pF having self resonance at 950 MHz.

I work that out to .6 nanohenry.

Impedance of .6 nanohenry at a gigahertz is a little less than 4 ohms.

Suppose that .33 uF 0603 also has .6 nanohenry of inductance. Self resonance would be a mere 11.3 MHz. Does that sound like what you expect? Impedance at a gigahertz is still a little under 4 ohms.

Just to explore further - characteristic impedance of a microstrip .03 inch wide, .015 inch thick, .063 inch over a ground plane.

Page 6

I figure 82 ohms for unity dielectric factor. That works out to 273 nH per meter (If I did that right for unity permeability and dielectric factor), .416 nanohenry for a piece of such conductor .06 inch long. Does this sound close to the inductance of an 0603 capacitor? If the capacitor's ESL is close to that of a same-length same-width trace, then the capacitor is just a capacitor or effectively just a piece of trace at frequencies even well past the self-resonance frequency figure.

One more thing - such a capacitor will have a litle more inductance and a little lower self-resonant frequency in free air than it will close to a ground plane.

- Don Klipstein ( snipped-for-privacy@misty.com)

John,

Yes, of course the resonance point is of no consequence on its own -- it's the impedance at frequency of interest (assuming you don't care about phase angle).

However, from my experience, the smaller caps (much smaller than 0.33uF) are much lower impedance at 1GHz. We used to use some specialy high-freq caps (0.01uF iirc) to couple XFI (10Gbps Ethernet) 50ohm (100ohm around the loop) signals. They were expensive and they weren't available as a RoHS part.

I guess the OP has to tell us what the spectrum of the signal he is trying to couple, and what the loop impedance is, in order to suggest a good solution.

Bob

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I'll second that, with a bit more explanation: smt caps have inductance that's determined by their physical size. A piece of wire has inductance, after all; a length even .06 inches or .08 inches of microstrip has inductance. What it also has, though, is capacitance to a ground plane. The distributed inductance and the distributed capacitance cause it to have a particular characteristic impedance, sqrt(L/C) (neglecting the reactive part of the impedance caused by resistance, which is generally quite small in the GHz region). If you model the MLCC smt part as a conductor the length, width and thickness of the part, you'll be really close to the way it actually performs in the circuit. That is, make its inductance part of the transmission line you mount it in. If the capacitor is slightly narrower than the microstrip trace, the impedance should remain very close to constant through the capacitor. If you want to get fancy about it, use the freeware ATLC program to calculate impedance for that cross-section; or use Agilent's ADS to accurately predict performance to well beyond

1GHz, if you put in an accurate model of your system. -- The reason I say that ideally the capacitor will have a width slightly less than the microstrip trace is that the added height of the part adds capacitance to the ground plane and slightly decreases the series inductance compared with a thin trace the same width. But...the propagation velocity of the line is such that .06 inches of line represents only about 3 electrical degrees at 1GHz using an FR4 substrate, less for lower dielectric constant substrates. One way to think about what that means is that a reflection off the "leading" edge is very nearly canceled by the corresponding reflection off the "trailing" edge, where you have an impedance discontinuity only that long. If you try to think about this stuff in terms of self-resonant frequencies, you'll get yourself all needlessly worried. The model I suggest above is far closer to what's really going on. (This assumes the series reactance of your capacitor is very small compared with the transmission line impedance, of course; otherwise, just include that as a series impedance at that point along the line.)

Cheers, Tom

Nice explanation! Thanks, Tom! Comforting enough for me to re-pursue my thoughts of putting a remote repeater right down the same coax as the cable TV ;-)

...Jim Thompson

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| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
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| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
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=A0 =A0 ...Jim Thompson

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quoted text -

I am designing a phase shifter hybrids(branchline) reflection type using pin diodes. I presumed according to the datasheet in

_requestid=3D314218 having a graph with series resonance and freq plot . for my center freq 910MHz this capacitor has SRF of 940Mhz... But theoretically, xc =3D 1/cw ... will give a reactance of 3 ohm but this is where where the parasitic inductance would be best compensated is it not...

The row of 0402's spanning the trace width seems to work pretty well. A wide trace bridged by one narrow cap tends to make an inductive blip. The expensive DiLabs parts don't TDR any better than cheap surfmount caps.

One thing I've done is make a z-shaped slit in a fairly wide trace

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And bridge it with a bunch of caps, two physically small ones on the vertical parts of the slits and several bigger ones, like tantalums even, on the long horizontal part. Seems to work well for a really wideband DC block. If the caps aren't too tall, the lumped-C-to-the-universe effect seems small.

John

text -

Narrowband, there is some cap value where Xc and Xl pretty much cancel, so that may be better than a bigger cap. But you'd probably have to experiment, as Xl is layout and geometry and solder dependant to some extent.

The pin diode wire bonds are maybe a bigger concern.

John

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=A0 =A0 =A0 ...Jim Thompson

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wat is the minimum length of a transmission line that would not hinder soldering of capacitors(0805 case).., less than 0805 would be difficult to solder for me.. :)

for the input port to the hybrid is an extra section of line of small length necessary to feed in thru coaxial.... cause even a small addition of a section line will result in phase change...?

Thanks

Yes, since 50 ohm microstrip on 1.6mm (1/16 inch) FR4 with no internal planes is over 2.5mm wide (over 0.1 inch wide), you'd want to do something like what John suggests for that, if you really want to keep things constant RF impedance. I tend to deal with microstrip on multilayer boards where the trace width is very similar to the width of an 0603 part. It becomes one of those things you think about in system design: the board stackup is not independent of the passive part selection. But again, until you get to pretty high frequencies (and especially if you are trying to keep things very wideband) it tends to not be a problem. If you are dealing with narrow-band, like I suspect the OP was ("center frequency 910MHz..."), you can pretty easily tune out the small effect of an 0.08" long capacitor which doesn't exactly maintain the microstrip's impedance. If I model an

0.08" long 80 ohm section (typical microstrip velocity factor) inserted into a 50 ohm line, at 1GHz, I see a return loss degradation to -28dB, and a whopping 6 millidegree insertion loss due to mismatch: both probably not worth worrying about outside of precision measurement systems. I expect that a conductor of the same cross- section as an 0805 cap, as a microstrip trace on a 1/16" board with ground on the other side, would be a lower impedance than 80 ohms, probably about 70-75 ohms.

Cheers, Tom

Now THAT's clever.

Jim

Don't get me wrong here: I have absolutely no idea why it actually works.

John

Having never done stripline myself, what's the best way to transition from 75ohm coax to the stripline?

...Jim Thompson

--
| James E.Thompson, P.E.                           |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

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Leave the "why" to the theoretical physicists; I'm good with the "that" and some observed results.

(I only degreed in physics because those clowns over in the engineering building were still using 6V6 and 12AX7s in their labs and I'd been dicking around with CK722s and their ilk for about five years. A "theoretical" physicist I ain't.)

Jim

First of all, "stripline" is a sandwich of two pc boards with the "circuit" in the middle of the sandwich between two ground planes. What John has drawn is more properly called "microstrip".

Having said that, the transition is completely dependent upon frequency. Up to about 300 MHz. you can simply strip the coax into a pigtail, solder the center conductor directly to the circuit and the twisted braid to a very closely constructed ground point. Up to about a gig, you can buy BNC connectors that will solder center pin to circuit and four thick wire leads to solder to the ground plane. Above a gig, you can get osm/sma connectors that have flattened center pins and formed ground connections and are good to ten gig or so. Above that, it gets pretty serious.

Jim