Software Defined Radio

I became interested in Software Defined Radio in early 2010 when I was deciding if and how to upgrade my short wave listening setup. The idea of being able to view a large chunk of spectrum at one time and use the computer to perform DSP (digital signal processing) on the output of my SW receiver was very appealing. My JRC NRD535D is a great radio, but the audio and noise reduction capabilities are very limited. The audio was fixed by having the KIWA modifications done, but the other limitations were still there.

A possible solution was the addition of a SDR to an IF output, so this was tried and works well, within bounds. The next step was to get a full SDR with band switching and computer control. The solution was a Softrock RX Ensemble that covers 1.8MHZ to 30MHZ in four overlapping bands. The Softrock radios have been available as kits for several years and thousands have been built by amateurs and SWL enthusiasts. It works great, as will be described later.

So, what is SDR?

  • Software-defined not software-controlled radio
  • Most of the complex signal handling using DSP
  • RF Spectrum and waterfall displays using FFT
  • User interface through computer
  • Usually some form of direct conversion
    • RF to baseband with no IF
    • RF band pass filtering is required to avoid images and birdies
  • IF band pass filtering equivalent, demodulation, amplification, detectors, noise reduction, noise blanking, etc. are all done in software
  • Usually requires a PC to run software and control the “receiver/transmitter”
SDR Receiver

Softrock RX Ensemble


Softrock is SDR made affordable thanks to a dedicated group of radio amateurs, Tony Parks KB9YIG, Robby WB5RVZ, Bill KD5TFD and others. The radios are available as well designed kits with full documentation. It required about 15hrs. to construct the kit and takes a bit of patience to work with the surface mount components, but it all very straight forward and very well documented. There are RX only and RXTX versions available. The kits are in great demand and in short supply. Check and for information and availability.

Ready-built SDR radios are available from a variety of sources ranging in price up to several thousand dollars. Obviously performance improves as the price increases, but as a second radio or as a pan-adapter, softrock with a good sound card works very well.

Typical SDR Display

A typical SDR screen using WinradHD with the Softrock Ensemble is shown below, with RF Spectrum and Waterfall displays, audio spectrum and full radio controls. Several other SDR programs have varying UI and features, but the best feature is that they are all free! All that is need to make it work is a PC with a good sound card. My PC has an on-board Realtek card that gives acceptable performance even at 192KHz sample rates.

The display shows 19oKHz bandwidth centered at 7.268MHz. Each of the RF spectrum peaks is a station, most of which are 3KHz wide SSB transmissions. The Waterfall display at the top shows time vs frequency vs amplitude (as color intensity). See the demo below for SDR in action.



Presentations and Demos

1. SDR PowerPoint Presentation

The previous images were taken from a PowerPoint I presented to the Milford Amateur Radio Club MARC in Aug 2010. All of the material was derived from various sources listed at the end of the article.

What is SDR Presentation? - PDF

2. Demonstration of Softrock SDR in Action

To view a Flash Demo of SRD in operation using WinradHD with Softrock click on the the link below (Flash must be installed to view):

SDR Demo - Flash

A windows MP4 file version is also available (does not require Flash):

SDR Demo - MP4

3. Comparison of Analog Radio vs SDR

A comparison of the audio output using the same signal between a Yaesu FT-950 and a Softrock Ensemble is available in the files below. The Yaesu is on the left channel and Softrock ensemble is on the right channel. There are several clips from 7-19-2010 at about 7:30PM on the 20m band, separated by silence. One of the transmissions originated in Mexico City working DX. Use the balance controls for your sound card to select one channel or the other. The same antenna was used through a splitter to feed the radios. Other than to equalize levels and compensate for time delays, no other modifications were made to the files.

SOftrock vs FT950

Audio Comparison of Yaesu FT-950 (left) and SDR (right)

Audio from Yaesu only - left channel

Audio from Softrock only - right channel

How well does SDR work?

SDR has some refinement to go before it will replace conventional analog receivers. The waterfall and RF spectrum displays greatly enhance finding signals rapidly, especially when the band are crowded and the conversations are short, such as during contests. The current top-of-the-line receivers from Yaesu and ICOM, for instance have many of the features of SDR already, and the dedicated high-end SDR receivers are beginning to approach the specs of mid-range analog or hybrid receivers.

Simple, very affordable SDR receivers like Softrock make experimenting with this new technology easy and fun. This is a great way to start for a modest investment in time and money. Experimenting with SDR has been lots of fun!


All about Softrocks

All about Softrocks 2

Links to articles on SDR & FlexRadio

Tutorial on SDR

Tutorial on SDR 2

Software-defined Radio for the Masses, Part 1

Software-defined Radio for the Masses, Part 2

Software-defined Radio for the Masses, Part 3

Software-defined Radio for the Masses, Part 4


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