Why SDR type?
Does it have to be an SDR type scanner/receiver?
My personal opinion is that you get far more "oppurtunity" digitising the IF outputs from a good analogue type receiver i.e. performance per bucks using something along the lines of a plug-in card in your desktop (e.g. PCI or similar), which acts as an input into a PC for a analogue IF signal, is in my opinion, still the way to go.
Why?
The average PC offers far more of an open platform in terms of software and processing oppurtunity than do most SDR's, and by that I mean SDR's that fall within the price scope of the average amature listener. Of course if you go Mil spec or to full on commercial/defence type SDR's then its another story, but for amatures the way to go currently has to be using an analogue front-end to feed a PC based A/D card, and then select software for whatever demod/process task you wish to tackle.
Modern PC CPU's are plenty fast enough today to tackle all but the most bandwidth intensive and complicated digital modulation/multiplexing techniques. The only restriction you have from analogue front-end's is the restricted bandwidth on IF outputs that come with most off-the-shelf receivers/scanners - but even then, all you have to do is intercept the IF at the first or second stage (from within the reciever), if you wish to get something much wider than the final IF output, or go buy yourself an old Watkins Johnson 8617 or similar - which can offer as much as 8Mhz final IF output bandwidth (with the correct IF bandwidth and Demod card installed, and on some old WJ boxes as much as 16Mhz or more IF output bandwidth).
Not to be forgotten as well is/are the restricted "computer performance/s" offered by most SDR's - your PC at home probably offers far more processing power than does the internal processing power of most SDR's on the market.
In short: for the time being I am inclined to stick to keeping the "RF sections" and "processing sections" as INDEPENDENT PLATFORMS. You'll be able to do more and will have more scope to upgrade and chop & change components than you will by way of investing in a single box SDR after all, what is an SDR? - a receiver, no different to the average off-the-shelf receiver, but with a computer added on to facilitate digitising and signal processing.
You'll get far far more performance for your bucks keeping the platforms independent.
Good luck
