At times like this, I like to start out with a definition of what we’re talking about just to make sure everyone is on the same page. Several groups have suggested definitions, but I like the definition used by one of the biggest companies in the ham radio SDR world, Flex Radio Systems, who says an SDR is one:
“where components that have typically been implemented in hardware (e.g. mixers, filters, amplifiers, oscillators, modulators/demodulators, detectors. etc.) are instead implemented using software running on a personal computer or other embedded computing devices.” (Note: that last phrase could mean internal to the radio and not your computer)Those of you with some familiarity with SDRs might think of something like the RTL-SDRs, the seemingly magic little receivers that tune from around 24 MHz up to 1.8 Gigahertz and use software on your computer to do the many functions, “typically … implemented in hardware … modulators/demodulators, detectors...”
Calling those little RTL-SDRs software defined is a distraction, though. What makes those little SDRs capable of tuning 24 to 1800 MHz in something the size of a USB drive isn’t the software, it’s the hardware. Dedicated hobbyist-software hackers (in the good sense) took a chip designed for TV reception and realized that they could control it and get outputs from it that they could use for whatever they thought of. There are plenty of software packages to try to see what you like, things like SDR # (SDR Sharp) or HDSDR. That previous link is your source for RTL-SDR information.
In the professional world I retired from, we had radios that met the basic SDR definitions 30 years ago. These generally ran the Analog/Digital Converter (ADC) with a much lower clock frequency than the IF it was converting, called undersampling. The first ones I worked on were in the early ‘90s, and that advanced with the hardware capabilities until the last radios I worked on (starting in about 2008) were close to having the ADC attached to the antenna. These are called band samplers; they convert the entire desired band RF spectrum to digital and process the bits. There's simply not much analog hardware at all.
The main radio in my ham shack has met the Flex Radio definition of an SDR since the early part of the ‘00s; I honestly don’t remember the year exactly but it was an Icom 746Pro. In analog radios it was common to need to buy additional IF filters and you ordinarily only had a choice of only two filters for both voice (SSB) and Morse code (CW). With the IC-746Pro, with filtering and demodulation done in software, I could have my choice of three filters for every mode and I could change the filter bandwidth while I was operating if I felt the need.
This is typical of the choice to buy an SDR over an “all analog” radio. It may cost more but the cost buys you tons of convenience features; a choice of 3 IF widths for every mode (3SSB, 3CW, 3AM, 3FM and any other mode on the front panel) versus a couple of filters bought after the radio. I believe the number talked about is at any given moment you have the equivalent of 15 IF filters in the radio and by changing them on the fly you have 41 IF filters available. Plus, any other audio filtering or audio processing that can be done in Digital Signal Processing (DSP) software comes along with it. Correlation cancelers (eliminate the annoying sound of guys tuning up on frequency; some systems remove more than one station tuning at a time), correlation enhancers (noise reduction), adjustable dual peak audio filters (for RTTY for example) and more operating conveniences.
It’s not that 15 filters couldn’t be done in hardware, but it’s terribly impractical; 15 filters would massively increase the size of the radio and add at least 14 more of the most expensive parts in the radio. Additionally, the software allows the manufacturer to load the same software on successive models and have them behave the same way.
To paraphrase something I said back in the Radio Sunday series,
The most important thing to point out is that an SDR isn't doing things that an analog radio can't do, but the SDR does them in more repeatable ways, and in ways that can be miniaturized more easily.Today, the emphasis is on band sampling receivers, which buys you a tremendous advantage over most other radios: better linearity. In a linear system, the only signals out of an amplifier stage are the ones you put into it. A nonlinear stage adds signals because of mixing, a multiplication process. The only intentionally nonlinear stages in an analog receiver are the mixers. The incoming RF is multiplied by the Local Oscillator (LO) giving rise to more signals on the output than the input.
In a band sampling receiver more linearity is relatively easy to obtain. Run more current in the RF amplifiers to increase their intercept points. In an analog receiver, there’s always a mixer (sometimes more than one) and those are intentionally nonlinear. Making a more linear mixer is a more difficult design; there are off the shelf solutions for different levels of linearity, but since the intent of the part is to be nonlinear, it’s quite a balancing act.
A band sampling HF receiver looks like this. It uses some filters to remove possible sources of signals that would combine into interference products, has separate gain reduction circuits (Automatic Gain Control or AGC) for on channel and off channel signals, then samples them directly in 16 bit, high linearity analog to digital converter.
Naturally, you can't have a software defined radio without the software. Flex Radio made software their emphasis, thinking that at some point within a few years anybody can do the hardware. None of their radios had a front panel - until the advent of the Icom IC-7300 which took the ham radio world by storm. There was a shocked reaction from the industry almost saying, "you mean people want an SDR with a front panel?" Flex introduced their Maestro series of tablet computers that mount to the front of the radio and implement some knobs that can be assigned a function in software. The major ham radio brands all have radios on the market which are SDRs with their software built in behind a front panel with tuning knobs, so that in some ways they behave much like radios have for decades but are already enhanced and can be enhanced more so with external software. Icom, Yaesu, Kenwood. I believe only the Icom is a band sampler.