Please critique my GPS PCB - Page 1
Forum section: electronics / general

author: Illusionist
author_rank: Regular Contributor
timestamp: June 10, 2019, 02:19:19 pm
content: This is the first design I've done in many years, the first using predominatly SMD, and the first with KiCad. Previously I made my PCBs with Photoshop. I'm only a hobbyist.Please would someone have a look at the design before I send it off to JLCPCB to be made? I've attached some images and the ZIP of gerbers I made for JLCPCB.It's a basic NEO-M8T board, intended as a development board to prototype my GPSDO, and to eventually be incorporated into that once I design it. I can only afford one Neo-M8T to play with.The PCB is two-layer, 0.8mm FR4 (to get the 50Ω trace for the antenna down to 1.6mm wide) and will be hand soldered. I might try using a hotplate to pre-heat or reflow paste but I need to practice that first.Strike the following - I had missed the .drl files in the zip. D'oh!I noticed when I uploaded the gerbers to JLCPCB that their gerber viewer doesn't show actual holes in the PCB... it looks like it's just solid with copper 'pads' where the holes are. Is that just the way it's viewed? The drill files (and others) were generated according to their instructions.

author: Smallsmt
author_rank: Frequent Contributor
timestamp: June 10, 2019, 02:23:34 pm
content: As I remember you need to care ground planes below antenna and module to prevent cross talk.Please check the datasheet.

author: Illusionist
author_rank: Regular Contributor
timestamp: June 10, 2019, 03:50:20 pm
content: Could you elaborate on that or point me to where in the datasheet please? I'vereadskimmed through, to be honest, the datasheet and hardware integration guide and did not see anything similar.Note that the antenna is external and plugs in to the SMA socket. I'm not using a patch antenna on the board.

author: Smallsmt
author_rank: Frequent Contributor
timestamp: June 10, 2019, 04:24:13 pm
content: I check tomorrow in office.In our design we used a linux som module on the same board connecting LCD screen too.This design caused interference on ground plane which directly coupled over to GPS module and caused it to reduce the amplification because of the ground noise.This will maybe not your problem.A good idea is to add noise filters to the data lines if you connect to external controller to prevent ground noise too.

author: Illusionist
author_rank: Regular Contributor
timestamp: June 10, 2019, 04:33:11 pm
content: Thank you; it's worth checking.Quote from: Smallsmt on June 10, 2019, 04:24:13 pmI've used ferrite beads on the off-board data lines.

author: Gribo
author_rank: Frequent Contributor
timestamp: June 10, 2019, 05:53:31 pm
content: Assuming you will use the USB functionality, you should route the D+ and D- as differential pair with 90Ohm differential impedance, and keep them on the same layer.The GPS antenna trace seems a bit thick for 1.575GHz.Check if your module requires an active antenna, I don't see any LNA, it might not work so well with a passive antenna.If you plan to mass produce (more than 20 units), you want some fiducial marks on the top layer.Ublox offers an evaluation kit, even for a hobbyist it is most likely cheaper to buy it than to build your own.

author: Illusionist
author_rank: Regular Contributor
timestamp: June 10, 2019, 06:33:45 pm
content: Thank you Gribo. I'm only making one, just for myself. Ublox's evaluation kit is over £250, and I can't find a cheaper board that's suitable. I wish I could; I'm only building this as a requirement of my GPDSO project. I'm hoping to keep the total cost of this 'part' below £50.I don't have the feintest idea of how to do that with the USB, although I was aware of it. I was hoping it would be just 'good enough' to work. It's only to give me the possibility of using the ublox software for some testing and initial set up.The antenna trace was from an online impedance calculator. That's why I'm using 0.8mm FR4. Using 1.6mm FR4 needed a 3.2mm wide trace for 50Ω. If I could use a 4-layer board it would be better, but that's an order of magnitude jump in cost that I can't afford.The NEO-M8T has an extra, internal LNA on the input in addition to the usual SAW filter and following LNA the others have. It misses out on the built-in Bias-T so I've included an optional one on the board for an external, active antenna.

author: Ribster
author_rank: Frequent Contributor
timestamp: June 11, 2019, 09:07:53 am
content: Quote from: Illusionist on June 10, 2019, 06:33:45 pmCheck it out with JLCPCB, you would be suprised how cheap 4 layer boards are.

author: Illusionist
author_rank: Regular Contributor
timestamp: June 11, 2019, 12:07:18 pm
content: I did... I would have to add VAT and handling charges onto the price because it is over the UK's £15 limit. So it would take the cost up to about £36. The two layer is about £5 delivered.---------------------------I'm currently re-routing the PCB, with much improved USB routing, a couple of schematic changes (single buffer ic instead of the five individual buffers) and trace impedances recalcuated because I didn't realise that PCB thickness was stated including the top/bottom copper. It makes quite a difference when the substrate is thinner in the calcuations.