RF section GSM compatible

[AL_GATEL]

RF section which is GSM compatible

Could someone design a simple RF section, which I can connect to AVR microprocessor.

I want it to help me have understanding with GSM TX/RX system.

Thank you
AL GATEL

 

[tsvetelin_velkov]

GSM Tx/Rx is digital multi-channel communication protocol with frequent / semi-random change of the frequency / channel. I really doubt that this can be designed easy.

Please, have a look here, this is just basic explanation about the frequency / bandwidth / principle :

GSM - Wikipedia, the free encyclopedia

The modulation principle is explained here :

Minimum-shift keying - Wikipedia, the free encyclopedia

TDMA :

Time division multiple access - Wikipedia, the free encyclopedia

As you will see it is a high performance system, but it is not simple at all. Some time ago I tried to play a bit with the GSM network at low level ( only operation over the frequency band was required ). I took an old GSM, I think it was Siemens A35 or similar, I took the service manual and the schematics. It is easy to trace the main components on the PCB, like VCO, amplifier, crystal/reference oscillator, PLL. Then it appear that all digitally controlled functions are embedded in the baseband processor. I just removed the connections between the baseband chip and the RF part and I was able to control it from external micro. But this is very different than having it operational as GSM .

 

[AL_GATEL]

But this is very different than having it operational as GSM .

Tell me how you feel, or you find it different from using onboard button.

There are some chip which can do QUAD BAND RF section in a single chip, I just want to play safe by asking some expert to design single 800 MHZ system hopefully using only 1 IC.

 

[tsvetelin_velkov]

Yes, I exactly agree. Even this old GSM that I disasembled was implemented as single chip, baseband controller and application controller were integrated in a single big chip. Well, this chip is running it software.

By saying that this is complicated, I certainly don't mean that it is not possible .

The whole system will be certainly more than one chip, you have 5W 900 Mhz / 1800 MHz amplifier, band pass filters for Tx / Rx which are relatively big ( so called diplexer, this is needed because the system transmit and receive simultaneously over a single antenna, GSM is a full-duplex ), circulator to insulate internal from external antenna. This single chip also require some external components, at least reference temperature compensated oscillator ...

Do you want to share what is your application or what is your goal to have a GSM front-end connected to a AVR micro ?

 

[AL_GATEL]

Do you want to share what is your application or what is your goal to have a GSM front-end connected to a AVR micro ?

I can only good @ working with AVR. I want to build good knowledge base about writing software for SMS and baseband control using AVR. a nice flowchart of baseband software complete with polyphonic ringtone software would be nice decorating my office.

 

[tsvetelin_velkov]

Did you consider to buy a GSM module? You can easily interface it to any micro controller, you can control the basic functions like SMS, GPRS, phone calls, phonebook, etc. You can try one of those :

**broken link removed**

I know this is not a single chip solution, but is single module but will bring you to air immediately and there is no need to develop the GSM baseband functionality.

 

[AL_GATEL]

but is single module

hmmmm close enough to start with :razz:

this is the module people used to make public phone isn't it.

BGS2 is interesting

 

[tsvetelin_velkov]

Well, this is a ready GSM. Everything you have in the GSM, you have it in the module, of course there is no keyboard and display, but as far as I understand your application - this is what you want to make yourself There are different modules, try Comet Electronics and search for "gsm" - there you will found modules and external antennas

Wish you good luck with your application!

 

[AL_GATEL]

thank you for your pointer.

perhaps you have more knowledge in GSM you could share.