Summary of SARA presentation on Software Defined Radio (SDR) by Lyle Koehler, K0LR

An SDR system is defined as one in which transceiver hardware components (mixers, filters, amplifiers, modulators, demodulators, detectors, etc.) are replaced by software processes. In an "ideal" SDR receiver, the antenna is connected directly to an analog to digital (A/D) converter which then feeds a digital signal processing (DSP) unit. At the output of the DSP unit, the digitized audio signal is converted back to an analog signal and fed to an amplifier which drives the speaker. Until fairly recently, it was not practical to implement the ideal system because of the speed and precision requirements placed on the A/D converter and the DSP unit, so RF signals were mixed down to a much lower frequency, and the A/D conversion was often performed by a computer sound card. As digital electronics have continued to get faster, more complex and less expensive, it is now possible to go "direct to digital" and there are commercially available ham SDR transceivers based on this approach. Bandpass filtering and preamplification are still used, but beyond that, the components we traditionally expect to find inside a ham transceiver are replaced by digital circuits. Functions of the knobs, buttons and display on the front panel of a traditional radio are often performed instead by the keyboard, monitor and mouse of a computer.

Some of the unique features offered by SDR rigs include a waterfall and panoramic spectrum display, point and click tuning, completely configurable filters, multiple simultaneous receive channels and a digital recording feature in which you can save not only the audio output of the signal you're listening to, but all of the radio signals within a specified band of frequencies!

Ham SDR transceivers are available at prices from below $1,000 to over $7,000, and those at the high end of the price range have specifications that match or exceed the best of the "traditional" ham rigs. For those of us who just like to tinker with SDR, there are much less expensive options. At the bottom end of the price range is something called an "RTL-SDR dongle", a little device that looks like a fat USB memory stick and costs about $15.

The RTL-SDR dongle was originally developed as an inexpensive USB device that people could plug into their computers and watch digital TV. It probably works well for that in much of the world, but not here in the Americas because we use a different TV standard. Fortunately for us tinkerers, someone discovered that the RTL-SDR dongle puts out a digital data stream which can be processed in a computer, turning the dongle into an all-mode SDR receiver with a continuous frequency coverage from about 24 to 1700 MHz. There are several excellent (and free) software programs that work with the RTL-SDR dongle. Although it's not what I would call a high-performance receiver, it works surprisingly well on the frequencies that it covers. And with a slight modification to my old IC-706 (a capacitor and a phono jack to bring out the 69 MHz IF signal), I can enjoy the features of SDR -- panoramic spectrum display, digital filters, noise reduction -- over the entire range of frequencies that the IC-706 covers.

The presentation ended with a demonstration of the RTL-SDR dongle, including a "live" demo in which TeamViewer was used to access my desktop computer at home so that we could see and hear the SDR dongle and IC-706 in action.

As with just about everything else, there is a wealth of information about SDR on the Internet. For starters, I recommend:

QST product reviews, including the review of the FlexRadio 6000 SDR series in the April 2015 issue.
“Cheap and Easy SDR”, QST, January 2013.
http://en.wikipedia.org/wiki/List_of_software-defined_radios
http://www.rtl-sdr.com/big-list-rtl-sdr-supported-software/