Re: [softrock40] SSB demod, was Re: carrirer oscillator
- Thanks,,, very fun following the list. I'm not likely to get involved with
this type project at the hardware level, my current interests are mostly
tube gear, but current interests do change over time ;-)
At 09:26 AM 9/29/2005, you wrote:
>--- In email@example.com, w keith griffith <kgriffit@w...>
> > I have,,, 'SomeWhere' a really good description of the Weaver ssb
> > method.
>There's a decent description of the Weaver method in the DSP section
>of all the recent ARRL Handbooks.
> > How, if at all, does that method compare with the
> > softrock?
>The SoftRock itself does no demodulation. All it does is sample the
>incoming signal in such a way as to mix down to 11250Hz. The
>downconverted signal is then dumped to the computer's soundcard.
>In the subsequent software DSP on the digitized signal, the SSB
>"demodulation" consists entirely of a bandpass filtering stage and
>another mixing stage to spin the signal centered at 11025Hz down to
>CW is the same, except the mixing stage spins the signal down to the
>CW offset frequency (700Hz or whatever) rather than 0Hz.
- When tuning an SDR 1000, if spur reduction is turned off, the DDS is tuner
11025 khz below the frequency of interest and the software (DttSP)
oscillator is set to -11025 khz to recover the frequency of interest. This
works well but for the spurs from the DDS.
If spur reduction is turned on (the normal case) the DDS is tuned to the
nearest low spur point (as documented by some of the AD docs) and then
DttSP oscillator is set to an appropriate value to recover the signal of
interest. This generally results in a DttSP oscillator value between -9.5
and -12.5 khz.
On the SoftRock all the tuning is done with the DttSP oscillator and it
ranges over +/- 24 khz - giving a tuning range of 48 khz.