GPS Test board PCB design - Page 1
Forum section: electronics / projects

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 05, 2015, 02:04:19 am
content: Hi all,Sorry for all the posts as of late!After the previous post about my watch, I've been designing a test board for a cheapo GPS module I can get off aliexpress (VK1097 -http://goo.gl/ODkCzD) and I'm hoping to use a small ceramic GPS antenna.What I really want to ask about is whether my PCB layout is sufficient. I've never had to design for such high frequencies to I've pieced together the little knowledge I've got and what I could find on the internet. I'll be using standard FR4 with a thickness of 1mm and a copper thickness of 35um (1oz/sqft).The ceramic antenna I'll be using is:http://goo.gl/GQmnlYI've got solder mask keepout around the connecting trace too. The trace length is ~8mm.The whole PCB is 50x50mm and I'll be buying off iTead Studio as per usual.Cheers!

author: kizzap
author_rank: Unknown
timestamp: January 05, 2015, 02:56:33 am
content: I don't know a lot about the higher frequency stuff, but it seems like you could neaten a lot of the traces on that board, and remove the vias on the Tx/Rx lines too.If you throw vias around each ground pin on the GPS device to the bottom ground plane, you should be able to run traces under the device on the top layer, which would allow you to bring the resistors down to near the connector on the top layer, and run them straight to the pin on the device.I think that a lot of the traces can be made larger. If you have the space, I would be working with 16mil traces at the min, possibly up to even 24mil if you can afford it.I suspect the decoupling capacitors that you have selected are not going to be of adequate size. I would go check digikey/anycomponentprovider for package sizes. I also think C2 should be placed closer the the pin.being that the diode and RC are related to the battery (capacitor?) backup, I would be bringing those devices closer together, as they are part of the same system. I try keep related components close to each other, as it helps with fault finding later on.But that is all my opinion.

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 05, 2015, 04:23:35 am
content: Quote from: kizzap on January 05, 2015, 02:56:33 amHey kizzap,Cheers for the help! I've moved the components as recommended, increased the trace length to 0.5mm and my Rx and Tx lines now run under the GPS module (with the rest of the underside via'd). For all the capacitors and resistors, I'm using standard 0603 components as I've got a fair few and it saves me purchasing new parts just for this board!Would you say its looking better now?

author: pyrohaz
author_rank: Regular Contributor
timestamp: January 05, 2015, 04:24:58 am
content: Also, the Tx and Rx lines will be operating at 9600 baud so they won't be sending data into the massively high frequencies, unless of course the GPS module has some ridiculously fast logic on the Tx output though the resistors should help to cut the highs off a bit.

author: @rt
author_rank: Super Contributor
timestamp: January 05, 2015, 09:14:00 am
content: If you read the data sheet for the uBlox series of GPS modules.. for example LEA-6T (a similar module) they go into detail about the circuit board footprint.Actually much more so than any minimal circuit configuration which is non-existent.impedance matching of the PCB traces for the antenna for example.Cheers, Art.

author: kizzap
author_rank: Unknown
timestamp: January 05, 2015, 09:22:59 am
content: Quote from: pyrohaz on January 05, 2015, 04:23:35 amgetting there, still some things I'd change though, if you upload the Eagle files I can show you what I mean. As a hint, I would be running the power under the GPS Module, rather then the data lines.