[Discuss-gnuradio] Trouble with SSB demod of real signal

[Louis Brown]

Hi,

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 here:

https://dl.dropboxusercontent.com/u/49570443/test.grc.png

It is based on figure 5 of below; quadrature, direct down conversion with 
Hilbert transform.

http://www.arrl.org/files/file/Technology/tis/info/pdf/98qex003.pdf


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.

Thanks,
Lou