Demodulation of SSB LSB

I am working on a project to demodulate SSB signals using software defined
radio on a blackfin DSP. We are required to design and build a lower
sideband SSB communications-quality speech receiver which will tune the
band 7.040MHz�7.100MHz. We are using a softrock 40 board which multiplies
the signals by a sin and cosine wave at 7.055MHz to generate the I and Q
branches. I was just wondering if anyone can give me some pointers as to
where to start. I am a bit confused about how to do the fine tuning to
different carrier frequencies.

Thanks



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On May 2, 7:49 am, "benzini1985" <benzini...@yahoo.co.uk> wrote:
> I am working on a project to demodulate SSB signals using software defined
> radio on a blackfin DSP. We are required to design and build a lower
> sideband SSB communications-quality speech receiver which will tune the
> band 7.040MHz-7.100MHz. We are using a softrock 40 board which multiplies
> the signals by a sin and cosine wave at 7.055MHz to generate the I and Q
> branches. I was just wondering if anyone can give me some pointers as to
> where to start. I am a bit confused about how to do the fine tuning to
> different carrier frequencies.
>
> Thanks
>

Hello,

To do the fine tuning, just multiply your I,Q signal by another
quadrature oscillator whose frequency is the last little bit of
shifting you need. I.e., treat your radio's I,Q signal as a complex
number and treat the oscillator's output also as a complex number and
then do a complex multiply.  (A+jB)(C+jD) = (AC-BD)+j(AD+BC)

IHTH,
Clay

Thanks, Clay.

Just to clarify things i have my signals I and Q components and then i set
A = I, B = Q, to give me A+jB. And then C = D = sine waves at the frequency
of the last little shift?


_____________________________________
Do you know a company who employs DSP engineers?  
Is it already listed at http://dsprelated.com/employers.php ?

benzini1985 said the following on 06/05/2007 13:21:
> Thanks, Clay.
> 
> Just to clarify things i have my signals I and Q components and then i set
> A = I, B = Q, to give me A+jB. And then C = D = sine waves at the frequency
> of the last little shift?
> 

C and D represent the in-phase and quadrature components of 
exp{2j.pi.f}; therefore C will be a cosine, and D will be a sine.

-- 
Oli