Re: [Discuss-gnuradio] NCO-CORDIC Problem

[Eric Blossom]

On Wed, Dec 07, 2005 at 02:51:58AM -0500, Blue Sky wrote:
> I am trying to build a AM receiver by using USRP. I did setup the IF
> frequency to 1.43 MHz by using set_rx_freq. I was sending a 1 Khz signal 90%
> modulated at 1.43 MHz carrier. I should get something from the USRP output
> just the 1 KHz sine wave. But for some strange reason I am not getting that.
> I am getting some extra 333 Hz freq signal with it. I dont' know where is
> the problem. So, I am guessing the NCO-CORDIC is not completely removing my
> carrier frequency. I saw the waveform just after the USRP it's the same with
> some extra frequency. Anybody's help and thought would be highly
> appreciated. Thanks.

The FPGA DDC has a finite frequency step.  If you're worried about the
333 Hz, you'll need to follow it up with a software DDC
(gr.freq_xlating_fir_filter_xxx: s/w DDC plus channel selection filter). 

Take a look at gnuradio-examples/python/usrp/usrp_nbfm_rcv.py
Skip all the GUI cruft, and look at the set_freq method. 
It uses the high level daughterboard independent method to control the
(possibly null) analog front end and the FPGA DDC, then follows it up with
a software DDC to handle any residual frequency offset.

Eric