Software defined radio

All of a sudden there seems to be a big upsurge in software defined radio in our local ham community.

Software defined radio of course is any radio receiver or transmitter where the modulation in completely performed in software. This allows any modulation scheme that fits into the given bandwidth to be demodulated assuming enough processing resources are available.

Now there are a number of quite ambitious, and succesful, implementations on the net. One of the projects, GNU Radio - The GNU Software Radio, that was started quite some time ago peaked my interest. I never bothered following the project since I believed that the complexity of the problem and lack of available expertise would results in the project dying a rather quick death. Boy was I wrong. Not only did they manage a working system, its quite well designed (the bit that I had a look at) and can even decode HDTV.

The image above is an example of an HDTV snapshot. Very impressive indeed. In order to get the signal into the PC one of their contributors designed a UniversalSoftwareRadioPeripheral with quite astonishing results.

Designed with upgradability and modularity in mind they are able to quickly put together various daughter boards that mix the RF down to IF I and Q for sampling and processing with the on-board DSP and FPGA. Both transmit and receive is supported at full speed. There is room for 4 daughter boards. 4 AD and 4 DA converters generate the relevant IF signals.

However, for the rest of us who don't want to go to the expense of designing a $550 system (yet) and just want to play around with the technology there is a much cheaper option. The softrock is an ideal starting point. Mine arrived today in the post (I'm so anxious to get started - however, read my previous post. Central locking may be the more important task).

The image above shows the simple softrock receiver. Quite cheap, but apparently very effective and sensitive. The little receiver is connected to the stereo input to the sound card. The two channels represent the I and Q channels and allow pretty much any signal to be demodulated with the correct software.

The standard software for suggested use is shown above. The SARL software defined radio forum thread described some of the experiences of the local SA hams.

The next step would be to make a switchable input band-pass filter and a variable input frequency allowing a much wider receive band. That is where the DDS-60 daughter card kit comes in.

It uses an Analogue devices AD9851 DDS chip capable of synthesising any frequency 1 to 180 MHz with 40 dB harmonic supression. Not bad. When used in smaller bands or with switchable filters harmonics shouldn't be a problem.

Using the DDS-60 with the softrock should allow much greater band coverage than the 48 kHz (or 96 kHz) allowed by the usual soundcards that are available. I'll document some of my experiences (if I don't mangle my circuit board). I would just love building my own tranceiver for VHF or maybe even UHF capable of pretty much any mode. Using the softrock of course will not allow ATV (amateur TV) unless some interesting modifications are made, but its should still be possible if one makes an IF input/output available for direct AV connection. At least the normal speech and data modes will be done in software and should allow a tremendous amount of flexibility.