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

[Louis Brown]

Tom,

Thanks for that link.  The LFRX board has two separate inputs.  I'm only using 
the one hooked to the I ADC, so the Q ADC is just noise, thus I discard the 
imaginary output of the UHD source.  The UHD source center frequency is set to 
0 Hz, so I'm direct sampling with no downconversion in the FPGA.  I'm moving 
what's done in the FPGA into GNU Radio to get better rejection, thus I'm really 
not discarding the imaginary part since there isn't one until after the 
translating FIR.

I could drive both the I and Q ADC by splitting my received signal, but then I 
have to deal with the imbalance between LFRX channels.

Thanks,

Lou, KD4HSO

On Feb 16, 2014, at 11:19 AM, Tom McDermott <address@hidden> wrote:

> 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).
>  
> http://www.tapr.org/~n5eg/index_files/DCC_2013_Gnuradio_Presentation%20-%20Rev%205.pdf
>  
> 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
> 
> 
> 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
> 
> _______________________________________________
> Discuss-gnuradio mailing list
> address@hidden
> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
> 
>