Software Defined Radio attempts to place much or most of the complex signal handling involved in communications receivers and transmitters into the digital (DSP) style.  In its purest form, and SDR receiver might consist simply of an analog-to-digital convert chip connected to an antenna.  All the filtering and signal detection can take place in the digital domain, perhaps in an ordinary personal computer.  While there are still good reasons to use some analog components in high-performance gear, the SDR approach is becoming more common in Amateur Radio.

• The DSP-10: An All-Mode 2-Meter Transceiver Using a DSP IF and PC-Controlled Front Panel by Bob Larkin, W7PUA
  What's neat about this 2-meter transceiver is that most of it is in software! Your PC is its front panel.
  Part 1 QST September 1999, pp. 33-41
  Part 2 QST October 1999, pp. 34-40
  Part 3 QST November 1999, pp. 42-45
  Note: additional Web link
• A High-Performance, Single-Signal, Direct-Conversion Receiver with DSP Filtering (283,604 bytes, PDF file)
  QST April 1998, pp. 40-43
  Articles referenced in the above article
  • A High-Performance, Single-Signal, Direct-Conversion Receiver
    QST January 1993, pp. 32-40
  • High-Performance Direct-Conversion Receivers
    QST August 1992, pp.19-28
• Signals, Samples and Stuff: A DSP Tutorial by Doug Smith, KF6DX/7
  Part 1 QEX, March 1998, pp. 3-16
  Part 2 QEX, May 1998, pp. 22-37
  Part 3 QEX, July 1998, pp. 13-27
  Part 4 QEX, September 1998, pp. 19-29

• A DSP-Based Audio Signal Processor
  QEX September 1996, pp. 8-13
• Linux, Software Radio and the Radio Amateur
  QST October 2002, pp. 33-35
  How software radio technology might revitalize experimentation in Amateur Radio
• A Software-Defined Radio for the Masses by Gerald Youngblood, AC5OG
  This series describes a complete PC-based, software-defined radio that uses a sound card and an innovative detector circuit.
  Part 1 QEX Jul/Aug 2002, pp. 13-21
  Part 2 QEX Sep/Oct 2002, pp. 10-18
  Part 3 QEX, Nov/Dec 2002, pp. 27-36
  Part 4 QEX, Mar/Apr 2003, pp. 20-31

• The Need for Standard Application-Programming Interfaces (APIs) in Amateur Radio
  QEX Jan/Feb 1999, pp. 19-21.
• Software-Defined Hardware for Software-Defined Radios
  QEX Sep/Oct 2002, pp. 41-50
  Using programmable logic in Amateur Radio applications
• A High-Performance Digital Transceiver Design by James Scarlett, KD7O
  Data-converter technology has made tremendous strides in the past several years. Lets take a look at how we can achieve high performance in an almost-all-digital radio design.
  Part 1
  Part 2
  Part 3

• Linrad: New Possibilities for the Communications Experimenter by Leif Åsbrink, SM5BSZ
  Discussion opens with analog versus digital RF-input techniques and attendant performance considerations.
  Part 1
  Part 2
  Part 3
  Part 4
  Part 5
• An All-Digital SSB Exciter for HF appeared in the May 2008 issue of QEX, pages 3-10. James Ahlstrom N2ADR’s transmitter uses an FPGA  and software. 

• High Performance Software Defined Radio--An Open Source Design
  A High Performance SDR (HPSDR) -- an open source (GNU type) hardware and software project intended as a "next generation" Software Defined Radio.
• Advanced Technology in Amateur Radio video - Talk given by Doug Smith, KF6DX, QEX Editor at Georgia Tech, March 10, 2003. Windows WMV format, 110 minutes.
• FCC News:
  FCC adopts rule changes to facilitate deployment of Software Defined Radio technology.
• Introduction to DSP
  The BORES Signal Processing DSP course - Introduction to DSP - is free of charge on line.
• f4dan.free.fr
  Christophe Bourguignat, F4DAN, maintains categorized and annotated links to different software defined radio technologies.