Gumstix: GPS Stix Active Antenna Modification

The GPS Stix Daughter board for the Gumstix comes set up for passive antennas only. In order to get it to work with a passive antenna you need to make a small (perhaps a little too small) modification.

Step 1: Cut the Trace From the Antenna Plug to Pin 19

Take a sharp exacto knife or razor blade and cut througth the copper trace between teh antenna mount and pin 19. BE CAREFUL there is another trace that goes to pin 17 which should not be cut. Use a multimeter to test continuity between the antenna post and the pin. There should be no continuity if you did it correctly.

Step 2: Place Electrical Tape

Cut a small piece of electrical tape and place it so as to protect board from dropped solder and to protect against shorts once the modification is complete.

Step 3: Cut and Fit the Resistor

Cut the resistor's wires to the proper size and bend them so that it will be easy to place and solder. Try to fit it as well as possible. Once one end of the resistor is soldered down it is VERY HARD to bend the resistor to fit without snapping your soldering job and perhaps damaging the board.

Step 4: Practice Soldering!

I'd say this is VERY IMPORTANT unless you do a bunch of small surface mount soldering. The last thing you want to do is fry your new unit. If you are anything like me you have a bunch of old boards hanging around to practice on. I found a chip that looked similer and did a bunch of practice runs to get down my technique.

I used a cold heat soldering tool. It actually worked very well which suprised me. I never used it directly on the point of solder but rather used it to heat the resistor wire well away from the point of contact. Cold heat is neat but you need to be careful to not ever burn out your electronics by shorting the two halves of tool's tip through your board. Heating the resistor wire away from the soldering point lessens your risk of this happening.

Step 5: Tin the Tips of the Resistor

Use a tiny amount of solder to pre tin the two ends of the resistor. When tinned the ends of the resistor should only a slight swell where the solder is. You don't need much solder to get the resistor to hold and too much solder will increase the likelyhood of causing a short circut.

Step 6: Solder One End of the Resistor to Pin 19

Steady yourself, here is where it gets fun! Move the tip of the resistor to pin 19 and get everything lined up just right. Once it is soldered it is very hard to bend or move anything without breaking the solder and if you are unlucky tearing off the contact point. Put the tip of your soldering tool ont eh resistor wire, wellaway from the actual soldering point and let the wire heat up. Be patient, when the solder on your tinned tip melts the resistor should shift a little. Take the heat off and let it cool for a few seconds before gingerly testign teh solder with a wiggle. If it worked the resistor should not move. If it didn't work try again and hold the heat on for a little longer.

Step 7: Solder the Other End of the Resistor to VCC

Check that everything is lined up. If it isn't and you only need to make a small correction (1 mm) then VERY CAREFULLY bend teh resistor wire. I would suggest using to pairs fo pliers to do this. One pair to hold the resistor and the point of the bend and another to do the bending. This should protect the soldering point at pin 19 from damage. If you are off by more than 1mm or so I would suggest heating up the solder point at pin 19 to make the adjustment.

Once you are all set solder the other end of the resistor to VCC.

Step 8: Test It Out!

Screw on your antenna and fire up the gumstix! Do a cat /dev/ttyS2 to view the raw GPS data stream. How do you knwo if it worked? do a cat /dev/ttyS2 | grep GPGGA to just see the GPGGA sentences (They contain time, latitude, longitude, ebaring and some other stuff). If the GPS isn't seeing any satallites you will see something like:

$GPGGA_0,00,99.99*48

Don't dispair yet, make sure that the antenna has an unobstructed view of the sky, this is important! Let it stand for a few minutes. If nothing changes then go back and check for short circuts or broken connections. If it worked you should start seeing stuff like this:

$GPGGA,141812.00,4221.7816,N,07105.9727,W,1,03,2.45,00033,M,-033,M_*59

HUZZAH you have a lock and your location is 4221.7816 North longitude by 07105.9727 West latitude!