I again forgot and faced problems with SIM808 GSM/GPRS/GPS module GPS engine not being able to acquire location fix from the satellites.
This time the circuit i designed was an add on for a Alcohol sensor+GPS vehicle tracking system.
We were sending and logging device event data to a server along with the vehicle location details.
We used long wire active antenna and struggled to get the GPS signal fix for a long time. What we observed was that the device never acquired satellite fix when the antenna was connected to the coaxial connector. But when we removed the ground connection of the antenna the device acquired signal after sometime but it used to jump and was not accurate and reliable. We thought that there is a impedance mismatch between SIM808 antenna port and GPS active antenna. But we were wrong.
We contacted SIMCOM India support head Mr.Rajesh for help and their Field Application Engineer Mr. Siva Ganesh helped use to fix the issue.
This issue was the same as i faced before for the previous SIM908 project and looked like i didn't learn from my past mistake. Again i had forgot to provide bias for the GPS active antenna.This part is clearly mentioned in the SIM808 hardware design guide v2.02 (Later we came to know that we were referring to a older version of the document).
So, even though the antenna specs said that the antenna needs 3V to 5V for operation, we were worried that higher voltage might damage the SIM808 antenna pin so we gave the bias voltage connection from VDD_EXT output of SIM808 which is 2.8V through a 10Ohm resistor as shown in the images above. After this SIM808 was able to acquire GPS signal and location fix in no time.
Hope it helps someone :)
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