OK, I'm a dinosaur in the jet age, so somebody have pity on me please. My radios have a mixer, local oscillator, IF strip, detector, and so forth. I've HEARD great things about software defined radios but I'm woefully ignorant of how they work. I see lots of "USB dongles" converted to radios but I'm not quite sure how to do a stand-alone radio that doesn't require a dedicated computer to make it go. Any suggestions on where I can start with information that any competent history major could understand would be appreciated.
The wikipedia article isn't awful.
https://en.wikipedia.org/wiki/Software-defined_radio
Basic concept... take an A-D converter and digitize EVERYTHING received. Then apply math in a very fast processor to replace all of the old stuff you and I know from drawing block diagrams.
Want to filter? Apply a Fourier Transform to whack off whatever bandwidth you want. Want to demodulate FM? Apply math. Etc.
That's the "theory". In practice, a cheap SDR like these DVB dongles, can be completely swamped by nearby strong signal sources, etc... as they have zero or nearly zero protection on their front-end. A high quality SDR receiver will include "traditional" filters in front of the A-D converter to dump unwanted RF (out of band, etc.) just like any other "traditional" radio, but they'll often computer-control the band switching and filter switching.
They're also limited by Nyquist and how fast you can A-D and shove the bits across a bus into the general purpose processor (usually a laptop for these cheapie USB dongles, but doesn't have to be).
Depending on how much mathematical work the "radio" has to do, sometimes they need really beefy processors... but that's basically what happened to bring about their popularity... processing power got really cheap.
An example of a "high quality" SDR would be the Icom 7300. The guts of that radio is a hybrid of the traditional radio tech and an SDR. Icom made a user interface for it with a general purpose processor driving a screen and knobs on the front of the radio, to make the user feel like they're operating a traditional rig... but there aren't exactly any long shafts turning variable capacitors behind those knobs... they're just altering what the software filters are doing in the processor.
Still costs $1200 because it still needs high quality filtering (which can be augmented by software updates -- probably the niftiest thing about SDR -- if it doesn't perform right, just write new code to change the math, and update the radio), and a real power amplifier for the TX portion.
HF is also fairly "easy" in the SDR world. VHF+ is still a PITA... the processors and whatnot, simply won't go fast enough for the quality needed, so there's still traditional frequency tripler circuits to get up there, and the resulting frequency stability problems, etc.
There's no such thing as a free lunch, and SDR is cool, but it has places where it shines, and things it doesn't do well.
One of the best applications is in digital modes, where if a new digital mode comes out, you just download updated firmware for the rig, dump it in, reboot it, and now it knows how to demodulate that new mode. This is handy in the public safety world in that they didn't standardize on ONE digital mode, worldwide... so in the US you can sell an APCO P-25 (Phase X - insert whatever number you like for X) radio, and overseas the same rig can have different code in it and do TETRA, or DMR, or whatever digital system they decide is the digital system du jour.
