Software-Defined Radios

There's a lot of buzz about software-defined radios (SDRs) in the ham world these days. There were at least 3 on display at the Hamvention.  Basically, a SDR is a radio in which only minimal analog functionality is in hardware; a PC, possibly with the assistance of a DSP, does all the receiver demodulation and transmitter modulation.  This generally means that the receiver is a direct-conversion type, which may not be as sensitive as the superheterodyne types we have grown accustomed to.

The most well-known of the SDRs is the FlexRadio. This machine actually includes a great deal more analog circuitry than we might expect, but it achieves greater receive sensitivity, which is a big advantage.  It uses a very old-style interface to the PC, requiring a parallel port, and only supporting a small number of particular sound cards.  The software is open-source, so it may be possible to add new functionality or fix problems.

Alinco was demonstrating what they call an SDR, which will be available around the end of this year. It appears that this not actually a true SDR, but a conventional radio with the user controls implemented in software for Windows.  Only Windows Vista, 7 or 8 is supported.  A hardware front panel is available for when you want to use it without a computer.

I also saw a unit with only a tiny circuit board, using a single USB connection to the computer.  Unfortunately, I lost the information on it. Today's PCs are more powerful than yesterday's DSPs, so this seems like a good approach to me, altho I wonder about receiver sensitivity and transmitter power.  If you're going to operate QRP, you need a sensitive receiver. A drawback of that unit was that you had to assemble it yourself, and it was all surface-mount parts, including resistors and capacitors that look like specks of dust.

There are also some receive-only SDRs, which are of more interest to the shortwave-listener community.

On the software side, there are several projects underway, including GnuRadio (formerly OpenSDR), which like all Gnu projects  seeks to be all things to everyone, and may never be completed.

The SDR field is in a lot of flux right now.  I expect what will emerge in the long run is small units that connect to USB on one end, and an antenna on the other, but may have restricted bandwidths wherein they achieve good receive sensitivity.  These will be produced by vendors with real radio experience, and perform on a par with today's massively complex transceivers. They may not be open-source, though, since the main "good stuff" will be in software.