On Mon, 1 Dec 2014, Ross Archer wrote:

On Monday, December 1, 2014 6:32:45 AM UTC-8, dave wrote:
Direct conversion is all the rage these days. Or Direct conversion to
I/Q quadrature directly into a DSP. A crystal radio is direct conversion
so I guess we are back where we started (except now we have the DSP).

How would you do quadrature at baseband? Two VCOs both digitally
controlled and 90 degrees out of phase, or some witchcraft on the
baseband signal itself?

I think his wording is throwing you off.

If they just convert to audio, there's now way to get rid of the audio
image. All the digital filtering in the world can't get rid of it,
because it's in the same area as the wanted signal.

If you convert with two mixers and an oscillator with two outputs, 90
degrees apart, you get two audio channels. The digital processing can
make use of that, and knock out the unwanted image.

It's just a more modern version of the phasing method of sideband
rejection (once upon a time common in SSB transmitters, and often used as
external devices to improve sideband reception on existing receivers when
SSB was new.

But instead of an audio phasing network after the two mixers (which is
what was used in all those sideband slicers in the fifties), digital audio
processing takes care of the audio phase network and can do a lot more.

What sometimes happens is they use the same scheme, but convert to a very
low IF. There, the two channels are used to get rid of the RF image, which
is relatively close to the signal since the IF is often below 100KHz, but
somewhere above audio.

Michael