Fiew questions about impedance control - Page 1
Forum section: electronics / projects

author: hitech95
author_rank: Regular Contributor
timestamp: August 08, 2018, 03:11:21 pm
content: Hi,I never worked with impedance control so I'mhaving a couple of questions.I'm working on a SBC as a hobbyist. I'm not an EE so I'm just tring to understand all the possible by myself.I have to route out DDR3, Diff Audio, USB, SDIO. All of those have different impedance. Just to simplify we can take the USB as example.If I have a 2 layer board I can route the USB signal ONLY on one layer becaouse I need a ground or a supply plane. To calculate the width and the spacing I can use one of the online tools. Is this correct?If I have a board with 4 layers I can use both TOP and BOTTOM to route it because the inner layer must be bground and VCC planes. Now if I have a 6 layer board can I use the TOP, BOTTOM, Mid 1, Mid 2?6 layer stackup example (inner 1 and inner 4 are planes):Now the tricky question, I have seen a fiew fedevel videos he have some documents created by the pcb manuf. with the proper spacing and width for the traces of different impedance. What I don't understand is: If I have the pcb stackup from the pcb manuf why they have the "Impedance Control" section on the quote form? If I don't check that they don't give me the document? Are they changing my design to make the board with the right impedance without making me spend lot of time to calculate the right values?This last part is making me really confused! I have seen people using JLCPCB to make pcbs with USB or ethernet. if you ask to the jlcpcb helpdesk about the controlled impedance they replay with the standard link to the help page. So I don't understand how can those design works!Thanks to everyone that can answer my questions.If you're interested on what I'm doing:https://hackaday.io/project/159654-fcthermostat-another-overengeneered-iot-device

author: robertferanec
author_rank: Regular Contributor
timestamp: August 09, 2018, 02:58:04 pm
content: Quote from: hitech95 on August 08, 2018, 03:11:21 pm- If you do not ask for impedance control (if you do not check it), PCB manufacturer doesn't care if your real impedance meets the required values - so they are not going to make adjustments in your stackup or tracks and they will not measure the final PCB. If you ask for impedance control (you check it), the PCB will be a little bit more expensive, but PCB manufacturer will make sure, that the tracks on your PCB really meet the required impedance (e.g. they may help you to tweak your stackup, advice correct track geometry width/gap and what is important, after they manufacture your PCB, they will measure the impedance, to be sure it is correct).- Even if you do not meet impedance, interface still can work fine. That is not the best approach, but sometimes there is no other option e.g. if you have to use 2 layer PCB. In these cases, try to make the tracks as short as possible.

author: hitech95
author_rank: Regular Contributor
timestamp: August 13, 2018, 12:30:14 pm
content: The only sensible interface that I have is the ddr3 with both single ended and differential signals.Unfortunately I have checked for online quotation but the PCB prices goes up to more than 50% when you ask for impedance control.please reply if you know a good and cheap manufacturing company that make impedance controlled PCB