In short: an SDR is a radio with no IF, Modulator, or Demodulator stages as we generally understand those terms: a receiving RF preamp feeds directly into a (very fast) A-to-D converter, which is connected to a computer DSP/CPU to tune a signal and extract the modulated audio or data; on the transmit side, the CPU and DSP generate the modulation directly, and feed it to an RF power amp.
In the HF and ham world, it has appeared in 2 distinct phases; in use in high end amateur transceivers and receivers, and utilizing a PC as a controller, letting external software define how the SDR is addressed and utilized.
Please add any appropriate links for SDR here. The forum for this wiki can be found here
An additional review of the Perseus can be found in the September 2009 MT article by Larry Van Horn, 'Perseus Software Defined Radio - A Whole Universe of Listening Possibilities' pp.66-68
