Introduction to SDR

A typical receiver collects RF as a voltage on an antenna feed line, amplifies it, mixes it with a locally generated reference signal which brings the frequency to some standard range, filters it to provide just the signal of interest, demodulates the signal to bring out the voice or music, and finally amplifies it to your headphones or speakers. All of this takes lots of analog electronics, and once wired up, is not easily changed.

A Software Defined Radio (SDR) does the same functional steps, but with most of the steps performed digitally with software in a computer. An antenna still supplies an analog signal but very soon after that the signal is converted into a digital form using an Analog to Digital Converter (ADC). Think of this as sampling the analog signal very often and creating a number that represents the strength of this signal at each moment. If you constantly stream lots of these numbers, you in effect capture everything that was in the original analog signal from the antenna. Once things are in numeric form, now they can be shifted, filtered and demodulated all by the computer software. Only at the very end is it converted back to analog with a Digital to Analog Converter (DAC) so it can feed your phones or speakers.

There are several advantages to doing it this way. One is far fewer parts, which can mean cheaper and definitely means more reliable. In essence the entire radio is the ADC, a fast computer and a DAC with some glue logic and power. Another advantage is enormous changes in functionality can be affected by just loading new software, there is no need to change any of the physical electronics. For example, suppose you have a traditional AM broadcast band radio. It would take quite a lot of rework to make it also receive FM stations. But if the AM radio was built as an SDR, adding FM would just be a change of software. And yet another advantage is the radio works almost perfectly. Analog components have tolerances, change with age and other wise act less than ideally. Not to mention they must obey the laws of physics! In a software radio, you can create functions that are better than you could ever do with physical components and they are not subject to wear and tear.

As with many things that seem new in the modern world, the idea of an SDR is not new, the basic ideas were set down way back in the 1950s. But it wasn't until the past several years that ADCs and computers have become fast enough to build real SDRs. As these devices inevitably become even faster in the neat future, the domain of applications for SDR technology will continue to spread until soon they will probably replace most, if not all, analog radios.

73, Elwood, WB0OEW

Some tutorial and reference material

Richard B Langley comments: For a very basic introduction to the mathematics of handling the I and  Q components of a radio signal, here is a link to a brief article I  wrote as a sidebar to my GPS World column in the February 2006 issue of GPS World magazine. Although it specifically references GPS (bi-phase modulated) signal tracking, there is a discussion on more  general aspects.

http://gge.unb.ca/Resources/minding_your_is_and_qs.pdf

Sigi (Siegfried Jackstien) suggests:

http://www.sdradio.eu/doc/Understanding_digital_signal_processing.pdf

http://www-sigproc.eng.cam.ac.uk/~op205/3F3_1_Introduction_to_DSP.pdf

http://www.DSPguide.com (free downloadable book in pdf form ... BIG but very informative!!)

http://dsp-book.narod.ru/InDSPrcs.pdf