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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: transmission line z
Date: Sat, 21 Jun 2025 16:21:46 -0700
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On Sat, 21 Jun 2025 21:31:11 +0100, JM
<sunaecoNoChoppedPork@gmail.com> wrote:

>On Tue, 10 Jun 2025 20:06:13 -0700, john larkin <jl@glen--canyon.com>
>wrote:
>
>>On Tue, 10 Jun 2025 21:00:37 +0100, JM
>><sunaecoNoChoppedPork@gmail.com> wrote:
>>
>>>On Tue, 10 Jun 2025 07:53:38 -0700, john larkin <jl@glen--canyon.com>
>>>wrote:
>>>
>>>>On Tue, 10 Jun 2025 13:33:57 -0000 (UTC), "Don" <g@crcomp.net> wrote:
>>>>
>>>>>JM wrote:
>>>>>> john larkin wrote:
>>>>>>
>>>>>>>Suppose you have a slab of FR4 with copper on both sides, standard
>>>>>>>ebay stuff. Now shear off a long thin slice. That's a balanced
>>>>>>>transmission line.
>>>>>>>
>>>>>>>air
>>>>>>>__________________ copper
>>>>>>>.........................................fr4
>>>>>>>__________________ copper
>>>>>>>
>>>>>>>air
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>>What's that called?
>>>>>>>
>>>>>>>Does anyone know of a calculator that handles this case?
>>>>>>
>>>>>> It's just a parallel plate waveguide.
>>>>>>
>>>>>> For the TEM mode Z0 = 377*sqrt(ur/er)*(d/w).
>>>>>>
>>>>>> ur/er - substrate permeability/permittivity (relative)
>>>>>> d - dist between copper
>>>>>> w - width copper strip
>>>>>>
>>>>>> Formulas for the higher modes also exist.
>>>>>>
>>>>>> But I'd have to look them up.
>>>>>
>>>>>Your free space impedance is easier for me to comprehend than the one
>>>>>contained in Chemandy's calculator:
>>>>>
>>>>><https://chemandy.com/calculators/microstrip-transmission-line-calculator.htm>
>>>>>
>>>>>Danke,
>>>>
>>>>And that one assumes an infinite ground plane.
>>>>
>>>>I suspect that all such formulas are wrong, except in a few rare cases
>>>>like a coax. EM simulation is better.
>>>
>>>Get Cadence to come along and demonstrate Alllegro + Clarity to you.  
>>>
>>>The days of using formulas to calculate these things are long gone.
>>
>>We have lots of programs and web sites that use the formulas!
>
>I'm having to do some EM simulations at the moment for some fast stuff
>(13ps rise time) so thought I'd also check the accuracy of that
>equation I gave.  
>
>Although it agrees with the one posted by Leo Baumann from his
>reference book (for sensible trace widths and heights) the impedance
>it calculates is nowhere near that given by simulation.  If I do a
>parametric sweep on the width of the return conductor, by the time
>it's x10 or so that of the trace conductor the resulting impedance
>agrees with that given by the likes of the Saturn toolkit, or the
>website linked to by Don, so my simulation is probably correct.
>
>If I have time I'll bend some copper tape over some Kapton tape and
>cut off a few slices of varing widths to do a TDR measurement on as a
>reality check.

Where do you get a 13 ps rise time?