I have been working with SIM908 GSM/GPRS/GPS combo module for some time and faced problems with the SIM908 GPS engine not being able to acquire location fix from the satellites.
The circuit i designed was for a vehicle tracking system product. We chose SIM908 because it was the most economical option which suited the requirement.
The idea was to use SIM908 to send data to server through TCP at set intervals to server.
We used long wire active antenna and designed according to the reference circuit but were not able to get the signal even when the device was kept in the open sky. The board image is shown below,
We contacted Excelpoint systems for help and their Field Application Engineer Mr. Sivakumar B was able to figure out the mistake we did.
In the hurry, we forgot that the reference circuit was for passive antenna and we didn't take care of the bias voltage required for the operation of active antenna. This part is clearly mentioned in the SIM908 hardware design guide as shown below.
So, all we needed to do was to provide a connection between pins GPS-VANT-OUT and GPS-VANT-IN. I did that with just a short circuit using soldering lead as both were adjacent pins as shown below, and SIM908 was able to acquire GPS signal and location fix in less than a minute.
Hope it helps someone :)
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