[Discuss-gnuradio] time_recov and freq_recov in generic_mod_demod

[yeran]

Hi everyone,

I have a question in understanding the generic_mod_demod.py. 

As I looked up from the documents, it was said that the signal after the usrp source at the receiver, is baseband signal, whose central frequency is around 0 Hz, and already take off the carrier frequency. But in the demodulation, there are still freq_recov and time_recov block. Are they at the symbol level? Since when I add a file_sink after the time_recov, and try to do channel estimation, the plot I get looks like sine wave, making me think if the carrier frequency is still there. 

I'm quite confused with how the benchmark, actually the demodulation in generic_mod_demod.py works. When does the carrier frequency is taken off? What's the function of the channel_filter, time_recov (including the rrc filter) and freq_recov? Can anyone give me some suggestions on how the blocks work? And any possible answers why the channel estimation I get in narrowband looks like sine wave?

Thanks a lot!

Ada