Question : Fixed Cellular Terminal using TC35i GSM module

I am developing a fixed cellular terminal (similiar to Nokia 32) using TC35i module. For the purpose of generation of ring tones and for converting telephone two wire communication to four wire I am using SLIC STLC3055. The application has been developed and is running successfully. However, there is one problem for which I have not been able to find a solution, there is a constant huming noise in the speaker of the called party. We have tried number of hardware solutions but have come to the conclusion that humming noise seems to be generated by the TC35i module. can somebody help?

Answer : Fixed Cellular Terminal using TC35i GSM module

If it is the 200Hz interference from the GSM intermodulation then it is coupling in from the antenna to your audio circuit. Remember that you have got a 2W transmitter within a couple of centimeters from the audio circuitry and you need to take great care to get the audio noise-free by isolating it.

It is in a way thus originating from the module, but it is originating from the antenna.

If the antenna impedance is not accurately matched to the output impedance of the module or if there are bad connections (connectors) causing reflections this can cause the GSM signal to be modulated onto the power supply in the module due to reflections.

If you can move the active elements of the antenna via a matched cable to be further from the handset this may also help.

I assume you are using a standard POTS handset? Do you get the same noise if you are calling on the POTS system and you have a cellphone close to the handset?

These POTS handsets are known to be very badly isolated from GSM intermodulation noise, so the problem may lie in the handset being sensitive to the antenna and not in your circuit at all?