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From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: 50 ohm on FR4
Date: Mon, 3 Feb 2025 03:12:02 +1100
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On 3/02/2025 12:35 am, Cursitor Doom wrote:
> Grok reckons to make a 50 ohm tx line on FR4 I would need  a Trace
> Width of approximately 2.73 mm and a Trace Spacing of at least 2.73
> mm. Does that sound about right?

<snipped non-right-sounding advice

ECL data books tended to include an application note that spelled out 
the constraints more sensibly.

You need to distinguish between microstrip (on the surface of your 
printed circuit board above a - possibly buried - ground plane) and 
strip line where the conducting strip of copper is buried within the board.

The thickness and dielectric constant of the insulating layer (or 
layers) is a critical parameter.

The ECL data books have nice simple formula for 50R and 75R transmission 
lines. I've got a pair of texts which give longer and more accurate 
formulae which stay accurate further away from the 50R to 75R range.

"High Frequency Circuit Design and Measurements" by Peter C, L. Yip  - 
ISBN 0-412-34160-3

"Microwave Components, Devices and active Circuits"
by Peter F. Combes, Jaques Graffeuil and Jean-Francois Sautereau
ISBN 0-471-91277-8

I picked them both up at Heffers bookstore in Cambridge when I needed 
that kind of information. They aren't anything special.

Anybody who just gives you a trace width in mm isn't doing a proper job.

-- 
Bill Sloman, Sydney