GPS consideration for PCB with GSM (interference, setup etc)

Question

I am working on a project which has a GSM and GPS on the same PCB (fairly near each other in an enclosure). Since I am not familiar with GPS I was wondering if any of you could give me some tips on this matter since I am going to create a new PCB design.

In my current prototype, I have a SIMCOM GSM module and the gp-635t gps. They are both in an enclsoure including antennas (gps antenna is integrated). What I experience is sometimes poor GSM and GPS connectivity and sometimes GPS doesn't get any fix (in the 2 minutes of time i allocate for it) and the GSM fails to send data sometimes.

• Are the two modules interfering with each other?
• How should I approach the new pcb design? Ideally I would like to have both gsm and gps on the same layer, is that a bad idea?
• I read it is suggested the GPS to have its own ground plane?
• Are GPS integrated antennas worse than external ones?
• GSM have 2 types of antenna: internal and external . Is one better than the other? Ideally my application would require both to be hidden (inside the enclosure) but not if the quality will be awful.

I hope some of you guys who have experienced this before me can give me good tips in this regards.

Answer 1

Amtenna signals shouldn't interfere since the GSM and GPS are on different bands. However, the electronics might interfere with each other. Put GPS and GSM modules on their own ground planes such that all the parts are covered, then connect ground and supply in one place, plus bypass it with capacitors right there. Use at least three - some high quality 22pF in small package (0402), 1nF, and 100nF. At least the first two should be with NP0 dielectric. It is also good idea to bring the power from the other layer and to connect the two grounds (for GSM and GPS planes respectively) as close as possible to the input power connector (or battery).

Naturally, you want both modules as far as possible from each other, it doesnt matter whether it is on the same layer or not.

Connect to RF inputs through short microstrip. Make it thin, cover it from all sides with ground (including the other layer). Also use small high freq. parts for input filters (if its not in the module already).

Antenna performance depends on whether it is active or passive. Active ones are usualy external as they are bigger. Passive ones have low gain and it might take while for acquisition to identify enough satellites in case of GPS. The receiver correlates background noise with known pseudorandom transmission pattern of the GPS satellites since the signal is that weak. Its a wonder it works.