Newbie level 5
When my sim 900 modem turns ON , the network led starts to blink & status led turns ON. After few seconds both leds will turns OFF & starts again...this same process will repeats every time...pls any one help me
With two boards of same circuit one is not working means it could be your module problem. Can you interchange and test the GSM module with the working one and not working one?
Cannot interchange the module, because desoldring may cause the pad to damage& also overheatening the module. And i found a new thing that without sim card , module will works good, AT commands all are ok...but with sim card is being the problem.i checked the connections between sim socket & module, all are perfect & no short circuits & also i replace the sim socket... the result is same...but some times it works as good for a short duration
Then try with replacing other network SIM cards may be it is due to signal problem
If it works fine without sim card and restart with sim card then 99% your problems occur when the module tries to register into the network and it consumes high current from the power supply. If your power supply is capable to deliver 2A continuously that does not mean it is capable to withstand the 2A spikes when it registers.
Check with an oscilloscope the power supply pin when it registers. Most probably the first spike when it registers will drop the voltage below its minimum working voltage and that's when your modem dies. You will need a digital oscilloscope to clearly see it, with an analog oscilloscope it will be very hard. Two possible causes: first, bad capacitor at power supply (what capacitor do you use and what is the voltage?), second, bad antenna circuit (high vswr), and high vswr usually means high power consumption. There is also one small chance that your module is defective, in my experience, sim900 is one of the worst modems. sim800h is much better.
Is there is any important things to make the antenna circuit to avoid vswr problems??
Make sure the strip line and antenna is 50 ohms.
A good free tool for strip line calculations is Agilent Technologies AppCAD (now owned by Avago Technologies), download link:
I presume you don't know what transmission line impedance actually means and how it is determined. If you are talking about a short microstrip line (e.g. a few cm) between SIM900 and antenna connector forget the point for the time being.It is less than 50 ohms...how can i make 50 ohm???
You don't want to do that.can i apply some lead on it or add 47ohm resistor
I presume you don't know what transmission line impedance actually means and how it is determined. If you are talking about a short microstrip line (e.g. a few cm) between SIM900 and antenna connector forget the point for the time being.
You don't want to do that.