Introduction: SIM5320E Shield- GPS Antenna Fix
I bought 3 of these 3G GPS combo shields for making car trackers but that is another story (Future Instructable).
When testing these I found they were not getting a GPS lock at all. They did find a few satellites after an hour. Not Happy Jan.
After some investigation and searching the datasheets I found that the active GPS antenna circuit on the top right had a capacitor at R1 where there probably should be an inductor. I luckily had some parts given to me by a friend who bought up some junk at an auction and viola, there was the exact part I needed.
After replacing the capacitor with a 30nH 0603 surface mount inductor, the GPS acquired a fix in less than 30 seconds, cold start. I call that a win.
What follows is what I wanted to know a few days ago.
Step 1: Locate the R1 Component
Firstly, to check my theory that the bias voltage isnt supplied to the GPS connector, I powered the shield and measured the voltage on either side of R1. 2.7V on the right side and 0V on the antenna side. So no bias voltage to the active GPS antenna.
I unsoldered the capacitor ar R1 right beside the GPS connector.
A fine tip and desoldering wick, heat one side and wick up any solder.
Rinse and repeat for the other side.
Then swap the tip back and forth and apply a little up force to the capacitor. Careful not to lift the pads under the cap.
Once the capacitor is off, a little heat and wick to smooth the pads.
Step 2: Solder in the Inductor
Thanks to Adafruit, I had a look at their design to see what might work. 30nH seems to be about right and that is the one I have in the bucket of spares so that will be the one to use...
RS has this part that I was going to buy if I didn't have anything to use.
I like to use a pair of reverse tweezers to hold the part in position as I solder both sides of it.
Step 3: Happiness
Now when I test the shield, the time to get a GPS fix, the TTFF is less than the time for the ESP8266 micro processor to get connected to my wifi and power up the SIM5320E and check it has a 3G connection. I guess less than 30s from cold start.