[LAD] Smallest, simplest, silliest SDR?

[Gordon JC Pearce]

On Sun, 2010-02-28 at 20:42 +0000, Victor Lazzarini wrote:
> Pardon the ignorance, but are the RF signals brought down to AF before  
> AD conversion? Otherwise, it sounds like a silly SR will be needed. Is  
> this the flowchart?
> 
> antenna -> RF demodulation -> ADC -> your software -> DAC -> audio

Not quite...

> So what exactly does your plugin do?

The incoming RF is downconverted to the audio range, but not
demodulated.  Effectively you've mixed a chunk of RF (7MHz band, in the
case of my sample file) down to an audio range but not yet demodulated.

At this point you may have a signal at 7066kHz which has been downmixed
to 10kHz, but that's outside the receiver passband - you can't hear it.
By mixing a further 10kHz signal into the audio you bring that down to
0kHz, and if it's an LSB speech signal it now occupies a range of
frequencies up to about 3kHz with an "image" at 20kHz.  Now we've got it
in the speech range and we can demodulate it by summing the imaginary
and real components so that the image cancels out.

Think of the SDR hardware as being a pair of direct-conversion receivers
with the local oscillators 90 degrees out of phase so the output is a
vector rather than a scalar.

Gordon MM0YEQ