KD5TFD Station: SDR-1000, Software Defined Radio

The radio I've been playing with most for the last few months is an SDR-1000 from AC5OG's Flex-Radio Systems. This radio is unlike any other on the amateur market, in that it is software defined, the vast majority of the demodulation and filtering is done by computer software instead of the radio hardware. Basically, the SDR-1000 hardware down converts a 40 khz swath of RF and sends it to a PC sound card for filtering and demodulation. This makes for a very flexible high performance radio by putting most of the work on a PC where it is easily changed.

This radio is ideal for someone that wants to experiment and learn about software defined radio. The design is well documented in "A Software Defined Radio For the Masses" published in QEX and available here. Additionally, the software side of the radio is licensed under the GPL, so it is totally open for experimentation. This has allowed lots of smart people to enhance and modify the radio, and a community is building around the radio.

My SDR-1000

My SDR-1000 setup consists of the SDR 1000 hardware, an HF Packer amplifier, WM-2 QRP Wattmeter, LDG Z11 Autotuner, and an Athlon 2100+ based PC. The PC has a USB attached Creative Labs Extigy sound card that is used with the PowerSDR software. I've also got an internal SoundBlaster PCI card that is used for digital modes connected to the radio via a homebrew attenuator that is seen at the lower right corner tacked to the workbench. I've been running with this setup for about 3 months now and have had great results with it. For digital modes, I run about 500 mW out of the SDR-1000 and get about 20 watts out of the HF Packer amplifier. I've added a fan mounted on top of the HF Packer amplifier to keep it cool when running digital modes. With the fan the heatsink barely gets warm when I get long winded on PSK!

The screenshot to the left shows the PowerSDR console, the software used with the SDR 1000. This particular screenshot shows about 20 hkz of spectrum during a CW contest on 40 meters. Each of the peaks is a signal so clearly the band is crowded. The green area in the spectrum is the portion of the spectrum that is being listened to. This spectrum display is very useful to see what activity is around. I work mostly digital modes (PSK, MFSK, Hellschreiber) etc. and have started to recognize different modes by the shape of the signal in the spectrum display.

Digital Modes on the SDR-1000

I'm very fond of the the digital modes (PSK, MFSK, etc) so have the SDR-1000 setup to run digital modes. To do this I have a second sound card in the computer that the digital mode program uses and the inputs and outputs of that card are routed to the SDR-1000.

To get transmit audio from the digital mode program to the SDR-1000 line out of the digital mode sound card is routed to mic in on the SDR via an attenuator.

The attenuator is needed to match the line output to the mic input on the SDR-1000 sound card. The capacitor is for DC blocking as the mic input provides phantom power. None of the component values are critical.

To get received audio from the SDR-1000 to the digital mode program, the line out of the SDR-1000 sound card is split and sent to the SDR 1000 hardware, as well as to line in of the SDR-1000. This allows the speakers attached to the SDR-1000 to be muted and still allow audio to go to the digital mode program to be decoded. The diagram below shows the connections.

When running digital modes my computer screen looks like:

The screen shot above is from a SDR-SDR QSO I had with KC2LFI in NY. In this QSO I had the SDR using a 70 Hz passband. The narrow filters you can get with an SDR are terrific for PSK and CW.

The folks working on the PowerSDR software are working on presenting a virtual sound card to the computer so that a second sound card is not needed to run digital modes. One will simply be able to tell MixW to use the SDR virtual sound card. Looking forward to that work being done.

DRM with SDR-1000 and Dream

DRM stands for Digital Radio Mondiale and is an emerging standard for digital shortwave broadcast transmissions. Using the SDR-1000 and the open source Dream DRM Receiver (binary availble here).

As can be seen from the spectrum above the DRM signal is about a 10 khz wide signal. The Dream program works by taking that signal at a 12khz IF and then demodulates and decodes that. This is a great application for the SDR-1000 as it's trivial to produce the 12 khz IF by simply changing the software. By comparison, to receive DRM using my Kenwood TS-2000 will require a soldering iron!

The DRM audio sounds pretty good to me. Some samples: