How is PCB Impedance Calculator: Microstrip and Stripline calculated?

This equation is for the surface microstrip approximation; other geometries use their own equations. Use Z0 ≈ 87/√(εr+1.41) × ln[5.98h/(0.8w+t)]. Estimate impedance for seven PCB geometries using analytical approximations. Compare width, dielectric height and spacing before fabricator validation.

Sources: polarinstruments.com [1]

How do I calculate this example: εr = 4.2, h = 0.794 mm, w = 1.5 mm, t = 0.035 mm?

Z0 ≈ 49.46 Ω. This is a calculated example under the model stated above; it is not a measured component specification.

Which units does PCB Impedance Calculator: Microstrip and Stripline use?

Ω, mm, mil; εr. Convert prefixes before substituting values; keep the input and output units attached to every number.

Sources: polarinstruments.com [1]

Is differential impedance always twice single-ended impedance?

No. Coupling changes odd-mode impedance. Pair spacing, geometry and reference planes must be included; widely separated traces approach the uncoupled limit.

Sources: polarinstruments.com [1]

Can I use a generic FR-4 dielectric constant for final design?

Use the actual laminate and process data. Permittivity depends on frequency, resin content and measurement method; solder mask and copper shape can also affect impedance.

Sources: polarinstruments.com [1]

Is this a fabrication or thermal guarantee?

No. Use the actual stackup, copper geometry and operating conditions. Confirm critical designs with the PCB fabricator, a field/thermal solver or measurements.

Sources: polarinstruments.com [1]