Andy --snip--
> > Now, for the SDR related question : > Will this work? > > It is easy to get at the Rx and Tx mixer IF ports on their own, with > some > PCB surgery using little more than scalpel and small soldering iron. So > I can bring both of these out to the external world, then connect to > the I/Q > inputs of an SDR soundcard. > But, the LO has to be fixed - tunable LOs at 24GHz with the low > phase noise > needed for SSB/CW is not an option, Why not? There are lots of low noise VCOs operating in the low microwave range that can be multiplied up to 24GHz; for example the CRO2010A has superb phase noise at 2GHz. Using one of the new Fractional-N PLL ICs it would be fairly easy to build a 2006MHz PLL, which when multiplied to 24GHz would have a close-in phase noise of approx -78dB or so, which equates to a resisudal FM of approx 18Hz in a 3kHz bandwidth, which too all intents and purposes would be a T9 tone - and probably sound better than some of the current 24GHz beacons! I'm laying out a PCB to do just this at the moment (high perforamnce 2GHz PLL) - the multiplier is a seperate project. --snip-- > So, how about a double conversion I/Q route? Take the quadrature IF and > with no additional filtering apply to another pair of mixers fed > with equal > phase local oscillator signals for a parallel, coherent, non phase > shifted > second conversion. This second IF can be anything low and feasible, > I was > thinking in the region of 1 - 3MHz to allow a DDS based second LO. That would work, but might be stretching the idea of 'simple' to the limit! We now > have a linear translation of the first IF down to audio, and the > fine tuning > can be done by altering this second LO. As the 90 degree phase shift > generated at the original RF input is preserved through both frequency > conversion instead of the usual one - is this the same as a > conventional SDR > front end? Yes, but somewhat more complicated. > AND... > If, instead, of a non-phase shifting second conversion, a quadrature > second > LO is applied, we don't even need an SDR, a single audio channel will > suffice. Seems too good to be true! Yes, that should work. That's actually quite a good idea, although it's getting less simple all the time! > ...must have missed something obvious, > ...it can't be this easy. 'Simple' and 'easy' are relative terms. > ...does there have to be some physical RF filtering around the second > conversion to remove image sidebands? Yes. You have to filter out the image noise of the second mixers; if you don't you will lose 3dB of S/N, regardless of how much front end gain you have. > ...Am I missing the wood for the trees? > Forget practicalities, gain control etc, that is hardware and can be > dealt > with. Is the concept OK? Concept looks fine to me. As always, the devil is in the detail. > -- > Andy G4JNT > www.scrbg.org/g4jnt regards Grant G8UBN
