Choice of DC blocking capacitors in Microstrip Design

Aug 11, 2008 28 Replies
J

Do you mean microstrip, a trace on the surface?

Below 1 GHz or so, anything reasonable works. If it's F connectors, the impedances are all crap already.

Better is edge-launch BNC or SMA connectors.

The best I've ever done is to get a gold-plated flange SMA, trim off any exposed teflon, cut the center pin short, and solder it broadside onto the edge of a pcb, center pin right on a CPW trace. That TDRs better than any edge-launch I've tried.

You can solder the coax right onto the board, too.

ftp://66.117.156.8/Sampler1.JPG

ftp://66.117.156.8/BB_fast.JPG

John

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J

In the Sampler1 photo, how did you cut the strip... etched or milled?

...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 | I\'m really surprised by John Edward\'s behavior... I expected him to be caught in bed with a guy ;-)
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J

That was home etched. I don't do that any more; too messy, waste of time, and proto pcb's are cheap nowadays.

John

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R

I had completely forgotten the old semi-rigid coax trick. With a milled slot for the coax body I actually got away with that trick at 13.3 with very decent results.

Jim

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M

If you're really good at it, you can upconvert it, feed it back to your distribution amplifier, downconvert it and send it back to every outlet on the system. A simpler version of the reverse RF channel in a CATV system.

http://improve-usenet.org/index.html aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages. If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you\'re crazy.
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J

That's an interesting thought. My Channels Plus box has two modulator inputs.

...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 | Common Values Common Purpose Common Ignorance
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S

d

ery

I use it all the time. In fact, I do layouts where the mask is already cleared off the ground where I know I'll probably want to tap in. (And making sure there are plenty of gnd vias there.)

I've been using 47 mil od with sma, up to 8G, for now.

----------- Regarding the DC blocking, I have to echo what JL sez. I've tried to measure 0402 L, and estimate it to be around 0.2 nH. If the application is wideband, then just put the biggest cap possible, as

0.2 nH is ignorable to fairly high frequencies. If narrowband, then just pick for the SRF, as it will be very slightly less series Z.

Also, in some cases and at higher frequencies the dissipation R can become as significant as the inductance X. So a jelly-bean X7R might not be so good in that respect. I think ATC has some better Q wideband caps.

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M

You know that I only like interesting projects. :)

http://improve-usenet.org/index.html aioe.org, Goggle Groups, and Web TV users must request to be white listed, or I will not see your messages. If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm There are two kinds of people on this earth: The crazy, and the insane. The first sign of insanity is denying that you\'re crazy.
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S

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Whether it is 50 or 75 doesn't really matter. If you are at a high frequency that it matters, then the launch onto the board will probably be an edge mount 3.5 (or similar) to either microstrip or grounded co-planer waveguide. Then transitioning to stripline is yet another discontinuity.

There should be no annular rings on the inner layer pads for the "via" down to the stripline layer. (That layer has the annular ring, of course.) Also, back-drilling the via is not too expensive from some manufacturers. (Cheaper than blind vias.) Instead of, say 30 mil of "hanging via," you can get it down to maybe 2-7 mil with the back- drill tolerance. To state the obvious, what you do depends upon you're requirements.

To really get an idea of the transition's performance before you build it, you'd need a full 3D simulator. HFSS or CST, that is.

THis was amusing at a quick scan:

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from googling:

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