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From: john larkin <jl@glen--canyon.com>
Newsgroups: sci.electronics.design
Subject: Re: Asymmetric Stripline / Microstrip online calculators for impedance and velocity
Date: Sun, 25 May 2025 09:48:54 -0700
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On Mon, 26 May 2025 01:55:40 +1000, Bill Sloman <bill.sloman@ieee.org>
wrote:

>On 24/05/2025 1:22 am, john larkin wrote:
>> On Fri, 23 May 2025 10:31:27 -0400, bitrex <user@example.net> wrote:
>> 
>>> On 5/20/2025 5:39 PM, john larkin wrote:
>>>> On Tue, 20 May 2025 14:09:46 -0400, bitrex <user@example.net> wrote:
>>>>
>>>>> On 5/20/2025 1:59 PM, Bill Sloman wrote:
>>>>>> On 20/05/2025 11:53 pm, Mike Perkins wrote:
>>>>>>>
>>>>>>> I can find numerous calculators that provide impedance for the above
>>>>>>> structures, but are there any that give propagation velocity too?
>>>>>>
>>>>>> This is sort of nuts. Microstrip is on the surface of a printed circuit
>>>>>> board. Half the field is located in the substrate and the other half in
>>>>>> the air above the board. It's consequently dispersive - different
>>>>>> frequency components propagate at different velocities.
>>>>
>>>> I can probe a microstrip on a PCB and clearly see the propagation of a
>>>> clean fast edge as it moves down the board. Dispersion is not an issue
>>>> on a reasonable-sized PCB with, say, 250 ps logic edges.
>>>>
>>>> On some extreme gadgets, like skinny traces on gen5 PCIe or something,
>>>> the signals at a receiver look like noisy hairballs, but adaptive
>>>> equalizers in the receivers clean them all up.
>>>>
>>>>
>>>>>>
>>>>>> Strip-line is buried inside a printed circuit board and propagates in
>>>>>> what can be a uniform environment. It's non-dispersive. A thicker layer
>>>>>> of the insulating substrate above the strip line than below it could
>>>>>> make it asymmetric, but I've no idea if this would mess up the
>>>>>> propagation velocity. A different insulating substrate above the strip-
>>>>>> line than below it presumably could make it dispersive.
>>>>>>
>>>>>
>>>>> Ya as I've been trying to explain, the propagation velocity has to be
>>>>> taken as a given to make finding either the symmetric or asymmetric
>>>>> stripline capacitance (and therefore Z_0) tractable to closed-form
>>>>> analysis. The simple online calculators don't do shit but take it as a
>>>>> constant for stripline, based on the relative permeability of the
>>>>> substrate, in either the symmetric or asymmetric case.
>>>>>
>>>>> I didn't think this required a PhD to explain but maybe you or Dr. Hobbs
>>>>> or someone can explain it better than I can..
>>>>
>>>> Saturn has an extensive list of the sources and references that they
>>>> use. And it warns you if your geometry is outside the range that it
>>>> likes.
>>>>
>>>> The simple equations, like from the Motorola ECL book, get stupid (as
>>>> in claim negative impedances) for some cases.
>>>>
>>>
>>> The main limiting factor in the accuracy of evaluating stripline
>>> impedance in closed-form is not knowing the actual surface charge
>>> density of the strip, but the relative permittivity is what it is as
>>> there's no field outside the substrate. It's pretty straightforward to
>>> develop the equations for an offset strip just by changing limits of
>>> integration in e.g section 3.7 of Pozar.
>>>
>>> Microstrip is less amenable to closed-form solutions since the field
>>> extends outside the substrate so you have to come up with an approximate
>>> effective permittivity depending on the particular geometry.
>> 
>> Pozar has equations that cover a full page, and then it turns out that
>> some if the terms occupy another page.
>> 
>> Simulation often works better than equations.
>
>Simulation is just a computer evaluating equations. It's certainly a lot 
>easier to let the computer do it for you, but both approaches rely on 
>essentially the same set of equations.

Sure, dumb programs just execute dumb equations. There's a lot of that
online.

A proper em simulation doesn't do that. It does the basic physics. We
do a real e/m simulation to handle cases that have no textbook
equations, like impedance matching an edge-launch SMA on a 6-layer
board. Pozar didn't do that one. 

We use ATLC. Works great.

Has anyone used Comsol Multiphysics for e/m simulation?