SDR Radio

Software Defined Radio by Justin Johnson, G0KSC.

Software Defined Radio equipment first established itself in amateur radio with the introduction of Howard Long’s Funcube Dongle. Subsequently, lower cost, imported TV dongles have been hacked and used with freeware, to monitor the radio spectrum from 25MHz to 1900MHz. Frequency convertors and further hacks increased the spectrum cover down to 100kHz. This device and software combination produce very graphic results, with both Fast Fourier Transform (FFT) spectrum and Waterfall displays, allowing visualisation of up to 2MHz bandwidth. To listen to a station one only has to click on a signal in either display to tune to it. Software choice of demodulator and bandwidth enhance the listening experience.

The problem with these tiny devices is that there is no physical room for the bandpass filters, required to prevent Inter Modulation Distortion (IMD) from strong out of band signals and harmonics. The new Watson W-SDRX1 Communications Receiver has been designed with separate filtered sections for HF and VHF/UHF to provide better dynamic range and strong signal handling. However it is limited compared with higher end equipment that this presentation is about.


The higher end SDR equipment extends from receivers to transceivers able to transmit from a few watts to 100w. Early versions relied heavily on the PC processor and software to work at their full potential, but the PCs of the day were not always able to meet demands, especially if other applications were also in use.

SDRs have since evolved into very powerful devices. The latest 4th Generation use the very latest Field Programmable Gate Arrays (FPGA) with embedded micro-processors. Compared with their predecessors these are extremely quiet devices. Reciprocal mixing, phase noise and distortion has been reduced to previously unattainable levels, making these receivers and transceivers the best currently available.


Until recently SDRs were supplied as boxes with antenna sockets and various interfaces, with the control switches and knobs of traditional transceivers placed in software. As traditional amateurs have been be reluctant to changeover to this style of radio in 2011 the USA based Elecraft produced the KX3. It is a compact hybrid transceiver combining the technology, power and performance of SDR but with additional knobs and switches. The KX3 is unique in that it can either be used as a “normal” radio, SDR radio, or both simultaneously.


SDR 3After a tea break Justin continued by demonstrating open source “Flex Power SDR” software, the legacy software originally designed for Flex 1500 and Flex 3000. The display includes a spectrum scope and waterfall display together options to control bandwidth. It is also used for others such as the latest ANAN-100DE.



The Flex 1000 was limited by the slowness of its parallel data port. With the Flex 5000 the manufacturer introduced FireWire (IEEE 1394) but it was not compatible with many PC’s unless expansion cards were added to them. A second hand Flex 5000 can now be purchased for about £1000 and the Flex 3000 for £600.


With the Flex 5000 it possible to run multiple receivers which provide very useful options including removing interference signals, such as broadband plasma noise, by feeding signals into the demodulator 180 degrees out of phase.

Birdies can also be removed by using notch filters. Once set up the software remembers the frequencies and remains active in the future. Bandwidth filters can also be custom made to suit individual modulation types. E.g. CW, RTTY, PSK. Performance of these DSP Filters are dependent on buffer size. The more memory dedicated to a filter the more selective it will be. Audio filters can also be individually set for both TX and RX.



Flex Power SDR allows Virtual Audio Cables to be set up linking to Ham Radio Deluxe, SSTV etc thereby removing the necessity of interconnecting cables between PC and radio.







A CW Skimmer can be set. This places callsigns tags against signals on the FTT Spectrum display aiding identification of DX signals. Recently Justin heard the K1N DXCC station calling CQ and stating that they were listening up frequency. The cluster of callers was clearly shown on the waterfall but K1N replied to a station he could identify away from the main group. As the CW Skimmer tagged that station G0KSC was able to reply on that frequency as he knew that was where K1N would start listening for the next QSO. This resulted in a successful QSO without joining in the main cluster of stations.

Menus include other adjustments such as Power Output, RF Gain, AF gain and a band selective Pre-amp, etc. Another window allows adjustment of the software and receiver to suit PC capability. This may include Buffer size and the Video frame speed to suit the PC or how much detail is required by the user. Noise blanker thresholds can be adjusted to remove noise. The CW keyer allows storage and auto transmission of CQ s etc.

Justin related how he has connected both vertical and horizontally polarised antennas for 10M band to the Flex 5000 and run dual receivers in Flex Power SDR, with the left earpiece of his headset on one receiver and the right on the other. Both receivers were set to track the same frequency. As the ionosphere caused rotation in propagation of the signal the audio strength in each ear varied in sympathy.

Justin advises that those seriously interested in taking up SDR should consider 2nd or 3rd Generation units such as the Flex 3000 or Flex 5000. For him the decision to changeover completely to SDR was when they became capable of transmitting at 100watts.

Asked what he thought the future holds Justin’s opinion was that for now SDR hardware will limit receiver performance but further improvement in processor speed, filtering and pre-distortion technology will allow increased monitoring bandwidth and multiplexing many virtual radios in software using a single SDR. Needless to say, SDRs readily suited to remote operation and links to them can be found on the internet.

Clearly SDR has set new, much higher benchmarks, in radio communication and will become the dominant technology. The next year or two will see many more affordable SDR on the market as the 4 major equipment manufacturers follow suite. It will not be long before the KX3 has serious contenders.

Further Reading

How SDR Works



G7CNF Videos:

Sherwood Engineering:

SDR Products

Electraft KX3:

KX3 review:




RF Space:


SDR Kits


SDR Cube:


SDR Software

High Definition Software Defined Radio:

Flex Software:

M0KGK SDR Decoder:


Write up by Norman, M0FZW.


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