Updated 10/30/2025 by W1DRF

Software Defined Radio or SDR is the use of digital signal processing and analog-to-digital sampling to detect radio signals. There are two broad categories of SDRs, Hybrid SDRs, which use classic analog heterodyning to tune and capture and recieve some amount of spectrum, and direct sample SDRs, which use ultra high speed Analog to Digital samplers and FPGAs to capture as much spectrum as possible all at once. Hybrid SDRs have the advantage of not being limited by sample rate, which allows them to sample incredibly high frequencies, and which make them resistant to being overloaded by a strong signal far away from the recieving frequencies, but they have a limited sample bandwidth, so they cant recieve too many bands at once and the analog mixing has the potential to introduce noise into the system. Direct sampling SDRs on the other hand have the advantage of being able to sample a large amount of spectrum simultaneously, while also being more compact than a hybrid SDR because of the lack of analog circuitry. The downside of Direct Sampling is that without bandpass filtering they can be too open, with strong out of band signals degrading reciever performance, and they are limited by their A/D sample rate, which currently makes them limited to the VHF and lower frequerncy bands, though they can recieve the entirety of their operating range simultaneously making them popular as digital skimmers. 

While you don’t have to know how it works to use it, these articles are a great resource for hams who enjoy learning about technology.

• 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
• 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

• 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

• 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. 
• All-Mode 1 kHz to 1.7 GHz SDR Receiver
  January 2016 QST pp 30-33, Feedback March 2016 QST p.46

• 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.
• f4dan.free.fr
  Christophe Bourguignat, F4DAN, maintains categorized and annotated links to different software defined radio technologies.