What is meant by direct digital conversion as applied to software defined radios?

  1. Software is converted from source code to object code during operation of the receiver
  2. Incoming RF is converted to a control voltage for a voltage controlled oscillator
  3. Incoming RF is digitized by an analog-to-digital converter without being mixed with a local oscillator signal
  4. A switching mixer is used to generate I and Q signals directly from the RF input