Controlled Impedance Traces - Page 1
Forum section: electronics / general

author: Northy
author_rank: Regular Contributor
timestamp: October 16, 2023, 03:49:02 pm
content: Hi all,As my boards get more complicated, I find myself struggling with controlled impedance traces and reaching the limit of my understanding.Does anyone know of any good reference materials where I can learn more about doing it correctly?Up until now I've relied on the board house to tweak my designs to achieve the correct impedance on certain traces, I'd like to understand things better to try to reduce the back and forth with them.Many thanks,G

author: Northy
author_rank: Regular Contributor
timestamp: October 17, 2023, 09:27:32 am
content: I'm trying to do:100 ohm differential50 ohm single endedand 75 ohm single endedAll on the same internal layer. Is that a good idea? Or not physically possible with the same stack-up?G

author: asmi
author_rank: Super Contributor
timestamp: October 19, 2023, 11:48:15 pm
content: Quote from: Northy on October 17, 2023, 09:27:32 amTheoretically it's possible. In practice depending on stackup and manufacturing limits it may or may not be possible. Two rules you should memorize for single-ended traces:1) As trace width increases, it's impedance decreases2) As dielectric thickness increases, impedance increases.Since you can not vary thickness of dielectric in different parts of the same layer pair, the only "knob" you have to control impedance is trace width. So if you use a typical high-speed stackup with thin dielectrics designed so that you can achieve impedances of 40-45-50 Ohms (which are most commonly used) with reasonably narrow traces, achieving 75 Ohms on the same layer might require traces too narrow for your PCB manufacturer. If you design your stackup such that you can reach 75 Ohm, achieving 50 Ohm on the same layer will require unreasonably wide traces, and so while technically possible, is not very practical.For differential impedance situation is a bit more complicated as you now have two "knobs" instead of one:1. If you increase trace width, differential impedance decreases because single-ended impedance of individual traces decreases.2. If you decrease spacing between traces on a pair, differential impedance decreases due to coupling between traces.

author: tridac
author_rank: Regular Contributor
timestamp: October 20, 2023, 12:01:02 am
content: A book from Motorola, "Strip Line Design", or similar, used to be the goto book years ago. Probably find a copy on the abe books site...

author: rfclown
author_rank: Frequent Contributor
timestamp: October 20, 2023, 01:57:03 am
content: A number of free calculators are available. Some I use:txline 2003: no installation, just exe and dll file. came from AWRappCAD: was free HP tool. still availableSaturn PCB Design.Rogers has one.I also put some equations into a spreadsheet that I use. Don't have that with me right now or I'd attach it. I got the formulas from an online calculator (that published the formulas used).Even when the board house gives their calculated widths, I double check with the free tools I have. You want to download the PCB material datasheets to get the correct properties. The 3 examples you give an be done on the same layer. 50 ohm single ended stripline and 100 ohm differential stripline will have about the same line width. 75 ohm single ended will be narrower.

author: Northy
author_rank: Regular Contributor
timestamp: October 20, 2023, 10:48:48 am
content: Thank you all very much.I'm starting to make sense of it all now.G