Dear Forum members i am not sure this question directly relates to Osmocom, but I don’t know where I can post it else …. If you know some active / alive technical forum which relates to GSM side (frequencies, modulations, GMSK etc etc) it would be very great if you could advise me ….. Meanwhile I ll continue with my question…. 51 TDMA multiframe starts with a FCCH which carries frequency correction burst on TS0. Due to nature of GMSK modulation and initial zeros of frequency correction burst the aired signal (signal that GSM modulator emits into air) as I understand is nothing but sine wave of an approximately 67Khz on top of the carrier frequency. Once mobile finds the frequency correction burst on some frequency it subtracts 67 Khz and precisely gets / sits on beacon frequency. So when it is said that upon switching the mobile phone on - it is looking for frequency correction bursts means in different words that it is looking for sine wave in GSM frequency interval it can work. Let’s say if my phone can work only in E-GSM900 interval (925 - 960 Mhz) it is looking for sine wave (frequency correction burst) and once finds it will subtract 67 Khz and it will be precisely the carrier frequency of beacon channel ? So if I use my rtl-sdr and software that analyzes RF spectrum (something like only oscilloscope) in 925 960 Mhz interval and I notice sine waves with periodic occurrence - as many times as FCCH appears in 51 multiframe - can I say that got frequency correction burst and subtracting 67 Khz I got beacon frequency ? -- Mario Lucas
