[NOTE: this originated as a Linux User Net discussion topic for Aug. 5, 2019]

The term “SDR” — Software Defined Radio — has been widely used and misused, both for descriptive and marketing purposes. As a result, it has can mean many things, depending on the context in which it is used, as well as the perspecive of the speaker.

If someone tells you, “I have an SDR”, you have to dig deeper to discover what that “SDR” actually is. Part of the confusion comes from the fact that “software-defined radio” originally described a process, not an object.

I have come up with a list of seven possibile meanings — there may be more.

People sometimes cofuse Digital Signal Processing with SDR. The difference is that DSP works on audio signals, not radio signals. Many analog radios use DSP in their audio stages. Note that a true SDR almost always will encorporate DSP. So, this is not actually SDR.

Again, not really SDR. I hear this most often applied to QRP rigs, such as the uBitx. The radio itself is fully analog, but it is controlled by a digital device, such as an Arduino or similar microcontroller board. This is nothing new, of course — virtually every ham rig produced in recent years has come with a digital interface. What is new is that the control device is open to be programmed by the user. I believe this is the source of confusion with SDR.

• Here we get into radios are true SDRs, with full analog-to-digital (and possibly vice-versa) at the RF stage.
• This includes a large range of devices, from inexpensive plug-in dongles to sophisticated radio development devices.
• Most commonly receive-only, but could be a transceiver
• Output might be an I/Q stream for processing by another device. E.G., RTL-SDR.
• Might include a FPGA (Field Programmable Gate Array) to process the I/Q from the ADC, so the output is audio, plus perhaps metadata such as spectrum data for display. In a development device, the FPGA could be open for programming by the user.

Here the radio is fully software-defined in the inside, but externally appears to be, and functions as, a traditional radio. These rigs are essentially designed to be drop-in replacements for older analog units, sporting the cutomary knob and button controls, along with legacy interfaces such as serial ports. It looks and feels so much like an analog radio that you wouldn't know it's digital inside without being told. Examples: Icom IC-7300, Yaesu FTDX-101D.

This is an all-in-one rig that includes a full MS Windows PC internally. That gives it the capability of running various ham software inside the box, rather than in a connected PC. Hook up monitor, keyboard, and mouse, and you have a complete shack-in-a-box. Example: Expert Electronics MB1.

• Again, a single box with traditional controls, but it also acts as a network server so that other digital boxes can connect to it, either to remotely control or be controlled.
• All signal conversion and processing is done onboard, with audio and metadata transferred over a computer network.
• Example: Elecraft K4 (I think with a Linux server inside).

• As above, all signal conversion and processing is done on board, with audio and metadata transferred over a computer network.
• No external user controls. All control is performed on networked computers.
• In essence, the network is the radio.
• Possible advantage is open API or software that allows anyone to program interfaces for them.
• Examples:
  • FlexRadio. Linux server inside. They produce a custom control unit, built around a Windows CE tablet. Also have grafted that unit to the front of servers to produce an all-in-one unit; still connected via Ethernet. Control software (SmartSDR) is proprietary, but the API (Application Programming Interface) it uses is open and well-documented.
  • ANAN transceivers: Fully open source software, inside and out. Apache Labs offers a Raspberry Pi-based control unit.

I'm always on the lookout for new examples of redundant acronyms, where people pronounce the acronym as a word and follow it with the word that the final letter of the acronym stands for. Examples are “PIN number” and “ATM machine”. I recently added “NIC card” to my list (thanks to KA7PLE).

“SDR radio” might be one — or it might not. It can be argued that if “SDR” describes the process of manipulating RF signals with software and the word “radio” describes an object that functions within the accepted meaning of “a radio”, then “radio” has two different meanings – and so there is no redundancy, even though it sounds that way.