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PCB calculator: Impedance calculation for coupled microstrip lines inaccurate

Description

In the PCB calculator, most of the Transmission line calculations are quite good. But the "Coupled Microstrip Line" is not.

Coming from the Z0, the implemented impedance calculation Zdiff = 2 x Zodd is not realy good. A much better aproximation is shown on this web site:

https://www.everythingrf.com/rf-calculators/differential-microstrip-impedance-from-zo-calculator

image

The same formular is used in the well known "Saturn PCB toolkit:

https://saturnpcb.com/saturn-pcb-toolkit/

It delivers a value for Zdiff which is close to the polar fieldsolver output. In the example below:

  • KiCAD PCB calculator says 119,5 Ohm
  • with new formular is 106,7 Ohm
  • polar field solver calculates 101 Ohm

image

Steps to reproduce

KiCad Version

Application: KiCad PCB Editor (64-bit)

Version: (5.99.0-12157-g5a28a06597), release build

Libraries:
	wxWidgets 3.1.5

Platform: Windows 10 (build 19043), 64-bit edition, 64 bit, Little endian, wxMSW

	wxWidgets: 3.1.5 (wchar_t,STL containers)
	Boost: 1.76.0
	OCC: 7.5.0
	Curl: 7.74.0-DEV
	ngspice: 34
	Compiler: Visual C++ 1929 without C++ ABI

Build settings:
	KICAD_USE_OCC=ON
	KICAD_SPICE=ON