Hi Pablo, short answer: Because radio is hard, you need to do corrections before you actually have your signal where you want it, e.g. at 0Hz.
long answer: The USRPs have mixers, yes. But you can't tell such a device just to "tune to that e.g. GSM channel"; this will not work due to frequency offsets. Two non-synchronized oscillators will never exhibit exactly the same frequency. Also, even with perfect oscillators, due to doppler, you might have to shift your input signal around until it ends up where you want it. There's a lot of more things affecting the characteristics of your signal, so phase and frequency synchronization is usually necessary for a working receiver. With broadband receivers, you might even want to pick up multiple senders, so you' might have to have a reception branch for each of them, each having a different frequency (and frequency error) and timing... By the way, many textbooks and papers on communication (especially on communication networks) just assume your receiver is synchronized, and then start of calculating error curves for arbitrarily low SNRs. I sometimes find this kind of funny, because synchronizing is often a very hard part of the implementation, and also because frequency and time synchronizing to a signal that has a very low SNR is not really easy, and can often only be done using an impressive amount of processing gain. Greetings, Marcus On 21.06.2014 19:17, Pablo Fernández Alonso wrote: > Hello, > > > > If UHD source acts as a mixer, why some examples (like usrp_rx_hrpt) use > baseband information as the input of a PLL? > > > > Thank you in advance > > > > > _______________________________________________ > Discuss-gnuradio mailing list > Discuss-gnuradio@gnu.org > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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