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

From: Tom McDermott
Subject: Re: [Discuss-gnuradio] Trouble with SSB demod of real signal
Date: Sun, 16 Feb 2014 09:19:17 -0800 (PST)

Hi Lou - the diagram you sent isn't going to perform a single-signal conversion.
Taking just the real part of the received signal (and throwing away the imaginary part)
gets rid of information that distinguishes negative frequency components from
positive frequency components. After that point there is no possibility to separate out
the LSB components from the USB components.
You will need a bit more complicated design, I've used the following configuration for
implementing a phasing type receiver. There are other ways as well, the Weaver method,
and a filter method (using complex taps to a filter).
Look at about slide 14.
-- Tom, N5EG

From: Louis Brown <address@hidden>
To: GNURadio List Discussion <address@hidden>
Sent: Sunday, February 16, 2014 9:10 AM
Subject: [Discuss-gnuradio] Trouble with SSB demod of real signal


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:


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.


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