I’m exploring the LF spectrum (< 500 kHz) using a USRP N210+LFRX+active
loop antenna, and GR 3.7. I’m pulling my hair out trying to down
convert the USB as I can’t get the LSB rejected. My GRC flow graph is
It is based on figure 5 of below; quadrature, direct down conversion
with Hilbert transform.
I’m taking the real output (i.e. the DAC hooked to the LFRX channel 1)
of the N210 at 25 Msps with 0 Hz DDS, then low-pass filtering at 550 kHz
and decimating to 1.25 Msps. This seems to provide better rejection of
the AM band than using the DDS tuning internal to the N210.
I then use the translating FIR filter to tune a 2 kHz passband,
decimated to 5 ksps.
Problem comes when I try to do the LSB rejection with the Hilbert. I
think I’m using it correctly; I have tried swapping the Re and Im
outputs of the Hilbert with no luck. Maybe there is something
conceptually I am missing since I am starting with a real signal. I
have done SSB demod with complex USRP signal with no problems.
Any ideas of what I am doing wrong? This should be basic stuff but I’m
not getting something.