Time_recov and freq_recov in generic_mod_demod

GNU Radio
1

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

2

And also, if I want to test the frequency at right after the usrp source
and after time_recov block at the receiver, what can I do?
Thanks!

From: [email protected]
To: [email protected]
Date: Wed, 5 Jun 2013 14:47:26 +0000
Subject: [Discuss-gnuradio] time_recov and freq_recov in
generic_mod_demod

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