Software Defined Radio

Use panoramic displays with point-and-click tuning to find your next DX catch!

Something very exciting is transforming radio. It is transforming how we communicate throughout the spectrum, from VLF to she shortest microwaves. Instead of using equipment specialized for specific modes, with their inherent limitations, radio designs are emerging that do most of the work in the digital realm. In essence, hardware captures huge chunks of spectrum, digitizes it, and passes it to a digital processing core. The DSP then carries out functions of tuning, filtering, amplification, noise reduction, and demodulation. Multiple operators can use multiple virtual transceivers.

SOFTWARE DEFINITION REPRESENTS A QUANTUM LEAP IN CAPABILITIES! Here is an example - a mere tip of the iceberg:

Before reading more, point-and-click on these WebSDRs for a while, and apreciate that each change of mode, bandwidth, and frequency was once a carefully orchestrated dance of capacitors, crystals, inductors, and resistors. Excellent performance was difficult to achieve and maintain. In the digital realm, these dances are done in less time than it takes for the sound waves to leave your headphones and reach your eardrums! Now, have a look at the RF components of a basic quadrature sampling (QSD) software defined receiver:

These QSD type software defined receivers are capable of very high performance due in part to superior "H mode" mixers, in which solid state switches are used instead of common diodes or MOSFET transistors. The H mode mixer effectively removes a limitation on the dynamic range and linearity of front end components of a receiver, leaving the roofing filter and ADC as limiting factors. Consider the importance of this evolution: preamplification is unnecessary from HF well into the lower VHF range, with the advantage that the receiver stays sensitive and linear despite the presence of extremely strong signals. These mixers can withstand VERY high signal levels! More information on H mode mixer design and applications are given here:

• Sergio Cartoceti's article on building a practical high performance H mode mixer.
• High performance up-conversion front end for a DSP receiver.
• AR7070 receiver dynamic range measurements.
• H mode mixer application in the Softrock SDR

After converting the receiver bandpass to a "zero IF," QSD type SDRs use a high quality ADC to digitize the signal and send it for processing in the digital domain. Modern wideband audio interfaces can digitize a 192 kHz chunk of spectrum at 24 bit resolution, and pass it on for processing in a desktop, laptop, or computing cluster. From there, the processed and detected signal goes to the end user.

The remote controlled SDRs listed above are all use rather basic RF components and still provide very clean, stable, and sensitive reception. University of Twente's receiver started as hardware built "dead bug" style on a piece of copper clad PC board!! Are you impressed? Now, get thee not to a nunnery, but to the gatherings of creative people building the next generation of radio. Most noteworthy are the people of Open High Performance Software Defined Radio project. They are designing sophisticated SDR transceivers, signal processors, and other equipment with an open, free (as in freedom), modular, "from the ground up" philosophy.

Watching two HF aeronautical frequencies at once on a webSDR.

Imagine a team of radio operators, scattered in different locations all connecting to one "antenna server" near a place with interesting radio traffic. One device digitizes the RF spectrum and a computer provides all of the team members with independent virtual transcievers, defined by software and limited only by the available computing power. The team could operate in any part of the spectrum, in any mode, using any bandwidth, applying any noise reduction, and so forth. If everyone connects via true broadband, it is possible for hard processing tasks to be distributed and done in real time. Multiple radio operaor teams could form the ultimate monitoring or emergency communications network. Single operator positions aew simple as a tablet or Ipad computer with proper software and a wideband internet connection... Read between the lines! It is already possible to digitize and record huge chunks of spectrum and retroactively do all manners of monitoring signal analysis. The NSA uses SDR technology to analyze phone and wi-fi traffic recorded days, months, and years in the past. Why not do the same for tracking intruders or distress calls on HF?

Amateur radio is already seeing quite a lot of innovation as shown in projects like the Quicksilver QS1R and Softrock SDRs. News organizations such as the BBC are moving to SDRs for their monitoring. Even military SDR applications are being implemented (i.e. PAVE PAWS and Sea Based X-Band radar systems). For a glimpse of how high end, commercial QSD type SDR transceivers are evolving, look at the Flex 5000.

There are two special things possible with software defined radio that are simply impossible in the analog domain: "tracking RF notch filters" and "enhanced signal clarity". Flexradio has demonstrated both in its latest versions of PowerSDR:

• Flexradio's tracking notch filter (TNF) works on the RF signal, and when a signal is nulled - it stays nulled. When the user tunes up or down, the offending signal stays nulled. Several notches may be implemented, so the user may null out plenty of birdies and RF junk across multiple bands.
• Enhanced signal clarity (ESC) uses digitally manipulated signals from separate antennas to null out interfering signals and enhance desired signals. ESC can make a strong jammer disappear into the background noise, and bring weak DX up to a readable level!

Here at AB9IL, I plan to post more SDR information as hardware and software are put into use here. I intend to use a Linux based system using a simple quadrature sampling detector and Linrad for starters. Linrad is developing into a prime program for weak-signal work. It is flexible and based on some truly slick programming from the community of software defined radio operators. It looks good too:

Stay tuned for the latest in software defined radio developments!