[linrad] RE: DSP Question

[Leif Åsbrink <leif.asbrink@xxxxxxxxxxxxxxxx>]

Hi Mark,


> I'm starting a DSP receiver project. At this point, it's just a learning
> exercise for me.
OK. This is taken into account below.

> From what I've read, one of the ways of demodulating SSB is 
> through the use of an analytic pair, or I and Q signals. 
You do not demodulate an SSB signal. The SSB signal is not
modulated on a carrier. It is the desired signal shifted
up in frequency by use of a frequency mixer.

The only thing you have to do is to shift it back to the correct
frequency and to incorporate a filter that prevents signals outside
the range of the desired sideband to reach the loudspeaker.

If you do a mixer in quadrature and phase shift the outputs
you can suppress the undesired sideband. Then you can apply
the band pass filter on the sum of the two signals as a series
connected low pass and high pass filter at something like
300 Hz and 2kHz respectively.

You can also apply a filter with a bandwidth of 2.4 kHz on
the real valued input from tha A/D. Then you will only have
the desired signal so it will be enough to shift it to the correct
frequency by use of a single mixer.

A better method is to produce an I/Q pair and shift it so the passband
center comes at frequency=zero. You then apply a low pass filter
with a corner frequency of 1.2 kHz to both I and Q.
(you can resample at a lower rate after this filter)

Finally you shift the frequency of the I/Q pair by 1.2 kHz to
place the frequency where you want it. Route the new I (or Q)
to the loudspeaker. 

73

Leif / SM5BSZ


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