Differential pair routing - Page 1
Forum section: electronics / projects

author: chrischina
author_rank: Contributor
timestamp: June 13, 2016, 08:24:35 am
content: I am a bit lost about how to establish the parameters to establish differential pair routing. I am currently trying to route several differential pair and I am a bit new to this. I have searched online some tutorial but I don't find any really explaining **easily and out of the theory** with sipmple succession of steps about how to do itI have downloaded saturn pcb software to see if it can help but I don't understand well how the soft is working because what it ask me to input is typically what I am searching for (it looks it is reverted... !!!)Assuming the differential pair I am routing is requiring 90 ohm (this is what the hardware guideline say "Route the USB differential pair on the top layer with a trace width and differential spacing tuned to thePCB stack-up for 90? differential impedance ").So in Saturn PCB I have indicated in target zdiff : 90Conductor spacing: I don't know (Am I supposed to arbitrary settle this, then accomodate the other parameters ?)Conductor width: sameConductor height: this I suppose I need to check my layer stack up. In this case I suppose I have to decide the spacing between top track andfirst prepreg or core ?I would really appreciate if someone who have experience could explain their methodology via a "1./ 2./ 3./ 4./ points methodology" to be able to achieve any differential pair routing.Thank you in advance for your help.

author: ap
author_rank: Frequent Contributor
timestamp: June 13, 2016, 08:35:24 am
content: Eeweb.com has a microstrip calculator. This should give you what you want. There are certainly others.Enter the data that is fix (pcb er, thickness, trace thickness) and play arround with the other data until you get what you want. Keep in mind that there are quite some tolerances in pcb material anyhow.

author: chrischina
author_rank: Contributor
timestamp: June 13, 2016, 08:54:50 am
content: Hello,I have not allocated the trace width.Do I understand that I should first decide the trace width then to adapt the other parameters ?

author: tggzzz
author_rank: Super Contributor
timestamp: June 13, 2016, 09:00:53 am
content: Quote from: chrischina on June 13, 2016, 08:54:50 amIf you have simultaneous equations with N variables, then in order to solve them you must specify N-1 variables. It does not matter which N-1 variables.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 13, 2016, 06:03:38 pm
content: FYI: the coupling between edge-coupled traces is very small. The purpose of differential routing is not to preserve the differential mode (because there is hardly such a thing), but so that the trace lengths are equal, and each trace is routed through/over an environment which is similar to the other trace's. Thus, if one trace picks up a common mode noise voltage, the other one picks up the same signal at the same time.Broadside coupled traces (possible in multilayer PCBs) can have a large coupling amount, but these are less commonly used (many signaling systems are designed with the other case in mind), and 6+ layer boards are less commonly used, too.So for the edge-coupled case, do not worry very much about the trace-to-trace distance, but make sure that, if the traces must cross any interference source (such as a gap between inner layer power planes), they cross with equal lengths / time delays at that location, and that the total lengths be matched.Tim

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: June 14, 2016, 04:39:55 am
content: I'm new to the world of RF PCB design as well, but have found the following resources useful:1. Saturn PCB Design Toolkithttp://www.saturnpcb.com/pcb_toolkit.htmcan use this to calculate all sorts of parameters2. Layout Design Guide - Toradexhttp://docs.toradex.com/102492-layout-design-guide.pdfhave a look at section 4 on wards for some good tips

author: Rerouter
author_rank: Super Contributor
timestamp: June 14, 2016, 04:58:20 am
content: I would also like to ask if there is any public ways to calculate an edge coupled differential pair with ground immediately next to and underneath the pair,Been struggling to pull it off on a 0.8mm thick 2 sided fr4 board without going below the cheap PCB maker specs,

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 14, 2016, 05:02:49 am
content: That's CPW with ground plane.I don't see a differential calculator around (unless Saturn does it?), but you can use something like ATLC to calculate a given geometry.Tim

author: Rerouter
author_rank: Super Contributor
timestamp: June 14, 2016, 10:17:47 am
content: So just as a sanity check, I should make my usb traces on this board 0.65mm wide?I'll have a crack at ATLC, was just hoping i could use the edge coupling to reduce the trace sizes a little,

author: nctnico
author_rank: Super Contributor
timestamp: June 14, 2016, 11:40:46 am
content: It might be necessary to also take the Zo of the individual traces into account as well. A while ago I have found this very comprehensive guide with the specifications for routing many kinds of high speed signals:https://www.picmg.org/wp-content/uploads/PICMG_COMDG_2.0-RELEASED-2013-12-061.pdfThis guide lists the single ended (Zo) impedances of the differential pairs as well.