GPS Board design
sylvie369
Posts: 1,622
I put together a device that takes output from a GPS and sends it out an XBee, eventually for rocket tracking. It works fine on a breadboard, but when I put it onto a pc board, I can't get a consistence satellite lock. I've gone back a few times to verify that I didn't miss something in the circuit. That doesn't seem a likely culprit anyway, as I DO get data, and usually eventually get a satellite lock good enough to provide some valid data, for a minute or two, anyway.
I'm pretty certain that the problem is that I don't know anything about good pc board design, and have designed something that violates some important rules for working with RF. I did get advice that the traces should not run under the GPS/antenna, and have redesigned the board with the GPS the other way around. But when I ran wires from the GPS connector traces to the GPS unit off-board, I had the same difficulty getting a lock, so I don't think that's going to fix it. I have since put the GPS unit back onto the protoboard, and it gets a quick and solid lock there just like before. In fact I can consistently move it back and forth between the protoboard, where it works fine, and the pc board, where it doesn't, so I haven't screwed up the GPS unit itself.
Here's the board design. Yeah, I know, I've probably made every mistake in the book. Any specific suggestions?
It's a 3.3V LDO regulator that should be able to deliver plenty of current. I've switched out the XBee Pro for a lower power XBee with no improvement.
I'm pretty certain that the problem is that I don't know anything about good pc board design, and have designed something that violates some important rules for working with RF. I did get advice that the traces should not run under the GPS/antenna, and have redesigned the board with the GPS the other way around. But when I ran wires from the GPS connector traces to the GPS unit off-board, I had the same difficulty getting a lock, so I don't think that's going to fix it. I have since put the GPS unit back onto the protoboard, and it gets a quick and solid lock there just like before. In fact I can consistently move it back and forth between the protoboard, where it works fine, and the pc board, where it doesn't, so I haven't screwed up the GPS unit itself.
Here's the board design. Yeah, I know, I've probably made every mistake in the book. Any specific suggestions?
It's a 3.3V LDO regulator that should be able to deliver plenty of current. I've switched out the XBee Pro for a lower power XBee with no improvement.
Comments
▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
Leon Heller
Amateur radio callsign: G1HSM
Post Edited (Leon) : 7/13/2010 4:37:28 PM GMT
-Phil
There is obviously some difference between how I have it on the protoboard and how I have it on the pc board. I'm also going to try to run the protoboard's power supply to the pads where the voltage regulator currently goes, to see if the pc board's power supply is doing something.
Post Edited (sylvie369) : 7/15/2010 2:29:09 AM GMT
When the PMB-688 was the daily deal a few days ago, I ordered one. Just now I attached it to my board, powered it up, and it works just as I had planned, even with the antenna right next to the board. Very quick satellite lock even here indoors, and it seems quite stable. I do only get 6 satellites when it's sitting on my living room floor, and I don't get good altitude data, but out on the balcony that does seem to improve pretty quickly. With the external antenna attached, it's a little better still.
As a bonus, I learned how to use the SiRF binary mode, after I accidentally switched it into that mode when I was adjusting the baud rate, and had to figure out how to switch it back to NMEA mode. Yikes. Don't do that.
Conclusion:
- The SUP500F GPS that I got from Sparkfun is not reliable. I see on their forum that one of the Sparkfun techs agrees, and says he recommends that people avoid it.
- My board works. Woo-hoo! Of course now I have to figure out how to mount the PMB-688 in a rocket.