Micro strip line questions

Propagation delay time.

But that's not right for microstrip. The correct epsilon is the effective dielectric constant, which is less than the PCB dielectric constant, because one side of the microstrip trace is in air.

Appcad does the math.

T_pd is the propagation delay time, i.e. the length divided by the speed.

Those formulas are only good for traces much shorter than a wavelength.

Cheers

Phil Hobbs

I highly recommend downloading these free software tools, femm 4.2 [finite lelement analysis program] and LTspice

Using femm you can draw your stucture, and even include the expected manufacturing variations/tolerances, then using the electrostatic analysis, calculate the Capacitance per length. Then using the mangetics analysis, calculate the inductance per length, including VERY accurate skin effects [even determine the coupling between traces, etc] each calculation is done at a frequency, so you get a very accurate transmission line model vs frequency. If you're after accuracy, don't forget to include any GNDed shielding above your microstrip. Its presence will lower the Zo a bit.

Then insert these components per length into LTspice at some conveniently small enough length to make a 'lumped model' segment. Using the 'ARRAY capability of LTspice you can then string together 1000's of these segments to make a VERY accurate model for your PCB microstrip and see what that transmission line does to your circuit performance.

After doing all the above, you can trust your results more than using some equation with poorly defined assumptions, or using some online calculator that runs off doing 'who knows what' with your inserted data.

At least with the above process you can include the effects of any potting compound, GND shielding above the PCB, bad PCB trace overetching undercuts, etc etc.

For practical engineering, Appcad is handy. You can get the numbers that you need in seconds.

Thanks for the recommendations, right now I am using my own C code, that includes high frequency dispersive effects as filling factor, skin depth, effective relative dielectric constant at xero and non-zero(GHz) frequency, loss tangent drawing on the material of the classic papers on this topic by Wheeler, Schneider, Hammerstad and Jansen, Kircshning and Jansen, Yamashita, Kobayashi, Getsinger, Diebele and Bayer etc., I have also an old text by Terry Edwards, for quick look-ups from time to time. My whole purpose was to get a good feel/understanding for the underlying physics before using the tool, so that later when I use the tool, I can correctly interpret the results. There is another tool Microwave Office, which quickly generates results, but it is not often clear how the result was arrived at. I will definitely get Femm some time soon.

Yes these are approximate formulas, because my "much shorter" may be "much longer" for you.

Your OWN code?! Kudoes. What OS will it run on? If just the code, what interface?

I'm doing some work right now where greater understanding of dielectrics and high frequency is what I need. We can take this conversation offline to continue. I can trade you some 'tutorial' models in femm so you can hit the ground with your feet running and I can gain some insight into what's happening in dielectrics. Some of these dielectrics are a bit 'lossy'.