CPW impedance

I don't see how that can be. The capacitance per unit length has to be higher in your second example.

I'd say that dog don't hunt.

Bob

--
== All google group posts are automatically deleted due to spam ==

"John Larkin" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...

Hello John,

The formulas behind der Coplanar Waveguide must be wrong or it's at least only useful for very narrow gaps(a very few mils?).

There is another program containg an impedance calculator for the coplanar structure. It calculates reasonable values for Zo. Download and install the program LENA from the link below.

Run the program Select from the menu: LenaCalc -> Lines -> Config set your units(mil) Select the structure: CPWG

I know of one more program for microstrip and some other structures, but not for coplanar. It's named "mwi.exe".

Unfortunately this link doesn't work since a year.

Helmut

Yeah, that's what I'd assume as well. John, the 20 mils above the plane versus the 16 mil trace width sound like they might exceed the "sane" limits for whatever approximation they are using. There should be something in the help file about limits. Those approximations only go so far, they usually don't fire up ye olde Maxwell ;-)

Or maybe some value accidentally flipped to metric. I just found out that the US carpet industry is going metric, at least for tiles. Instead of 22" they are now 50cm square. Conveniently a little smaller ... and I had to re-calc the stuff. Luckily that was before we poured the Pinkus Mueller Hefeweizen.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

A 16 mil trace above a 20 mil ground plane seems very normal to me... it's just about 75 ohms as a microstrip. I just don't see how sneaking in the CPW topside grounds will make the impedance go *up*.

At least Appcad now has the impedance continually increasing as the gap increases. In an older rev, as G went up, at some point Z started going down again!

One cool test of an impedance calculator is to do a microstrip and keep increasing W. If the impedance goes through zero and turns negative, it's using the old crude Motorola equation.

Our house beer is the Widmer Hefeweizen.

Say, have you tried the "new" Pabst Blue Ribbon draft? Tasty with food.

John

[snip]

Same here.

Really? I'll have to try it.

...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

| 1962 |

Usenet posts via gmail, yahoo, hotmail, aioe, uar or googlegroups are now automatically kill-filed using Agent v5.0

To be white-listed, send request via the E-mail icon on my website

Probably an approximation bug. Many such routines run into trouble when the trace width isn't substantially wider than the distance. The recommended limits should be mentioned somewhere.

We have that in stock all the time as well. The Pinkus is from Muenster in Germany. Unfiltered, really good stuff, comes in the classic 1/2 liter bottles.

I had that a while ago but can't remember. I am more a fan of the heavier ales, the darker the better. However, tonight it'll be mostly Rolling Rock.

Hey, Sacramento now has their own dropping ball. S.F. doesn't :-)

--
Happy New Year, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.

John Larkin wrote: : But if I go to coplanar waveguide, 16 mil trace with 60 mil gaps, : again 20 mils above ground, the trace impedance goes *UP*, to 82 or 78 : ohms for the two programs. I have to crank the gap down to about 20 : mils to get back down to 74 ohms.

Like others have suggested, such a wide gap is probably outside the valid approximation range of the equations used. The analytic formula from Waddell's book (involving elliptic integrals) gives about 81 ohms for your geometry and er=4.9, and Zo does not approach the microstrip value when the gap gets infinitely wide. Perhaps this is the formula used by your programs as well.

When fiddling with the formula in Excel I got a feeling that the formula would ignore the microstrip stray capacitance and only the plate capacitance to the lower groundplane is accounted for. Then the formula would work reasonably well in the case that the main capacitance contribution near the trace edges is due to the top side groundplanes.

Anyway, I'm getting from the Waddell's eq. for your trace width, er=1 and infinitely wide gap Zo = 192 ohms, which would correspond to 17.4 pF/m. The microstrip approximation gives Zo = 139 ohms for er=1 (indeed 75 ohms for er = 4.9) corresponding to 24 pF/m. Both are much more than the parallel plate contribution of 7.1 pF/m. So the formula must fail in a more subtle way.

It's curious that Waddel does not comment on the limits of the validity of the formula, especially because it looks like it might be an exact analytical result. The original paper by Ghione and Naldi however says that they used approximate conformal mapping and assumed a perfect magnetic boundary at the gap, which fails for wide gaps.

Regards, Mikko