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Oli Glaser gave a fantastic answer to a question regarding termination resistors: Termination resistors: are they needed?

I am trying to replicate his wCalc and SPICE simulation calculations for learning purposes.

I've got about everything figured out except how exactly he came to his wCalc results.

Here is a screenshot at my failed attempt of coming to the same result as Oli did using wCalc.

wCalc screenshot

Oli gets:

Zo = 177.6Ω
L = 642.9 pH/mm
C = 0.0465 pF/mm
R = 34.46 mΩ/mm
Delay = 530.4 ps

I get:

Zo = 117.6Ω (considerably off)
L = 642.8 pH/mm (only 0.1 ph/mm off)
C = 0.0465 pF/mm (same)
R = 9.072 mΩ/mm (considerably off) 
Delay = 530.3 ps (only 0.1 ps off)

Since some numbers, L, C, and Delay are equivalent or off by just a fraction, but Zo and R are considerably off, I'm a bit perplexed.

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2 Answers 2

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I suspect it's a typo on Oli's part. When I put the parameters into the web-based calculator, my values match Oli's, except for the \$Z_0\$. You can see how 177 is a plausible typo for 117.

Your R is considerably lower because you 1) used a more accurate value for the resistivity and 2) analyzed at 100 MHz. You'll find that the characteristic impedance is not so sensitive to some of these other parameters (i.e. ones other than the dimensions and dielectric constant). The other parameters are more for calculating losses. Of course you still need to be careful because e.g. Er is generally not constant with frequency and this calculator assumes you take that into account "by hand".

Wcalc result

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  • \$\begingroup\$ Thanks. It didn't occur to me Zo could have been a typo. That does seem plausible. Also thanks for the tip about Er not being constant with frequency. \$\endgroup\$
    – acker9
    Jun 8, 2015 at 16:00
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This reference for microstrip design can be used as a 3rd opinion to resolve who may be in error. The calculation is left as an exercise for the reader. https://www.jlab.org/accel/eecad/pdf/050rfdesign.pdf

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