Welcome to our site! EDAboard.com is an international Electronic Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.
So you showed that the problem is related to power supply.
Without seeing power supply waveforms we can just guess, probably the 3 A supply isn't prepared for the current pulses of a GSM transceiver, or the voltage is beyond specified range. GSM modems usually need supply filter capacitors to absorb the current pulses, at least several 100 µF.
SIM900 throws an unsolicited text "UNDERVOLTAGE" or something like that, you can check this in the technical documents provided by SIMCOM. There is a big possibility not to get this text, if the voltage sink is very quick. So, you can adjust your software to perform an action if this text is received (light a LED or something). Or you can connect GSM to the PC and obseve communication history, via a terminal software.
Sir, am using 2A power supply, what happens is when i use 2A power supply there is no prob, but when i use 3A power supply am getting this issue.