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Sorting the Fabs from the Fakes


SDR has become a new ‘buzzword’. Many magazines and internet forums are full of advertisements and articles about SDR.


SDR has become a new ‘buzzword’. Many magazines and internet forums are full of advertisements and articles about SDR. But what is it, and how do you select a radio that is right for you?   


A Matter of Definition

It seems like the whole world is talking about software defined radios. The August issue of Radio User contained three articles that mentioned SDRs and advertisements for twelve different SDR receivers.

However, the problem is that there is no clear definition of what exactly an SDR receiver is. There are some ‘traps’ to be avoided as equipment manufacturers try to ‘ride the wave’ of SDR popularity.

In the adverts, you sometimes see terms like ‘+SDR,’ ‘SDR capability’ or ‘SDR technology.’ These terms usually mean that the radio is based on a conventional superheterodyne architecture with the addition of a band-scope display or an IQ output for connection to a PC soundcard.

There is nothing wrong with these conventional superheterodyne receivers but the ‘SDR’ performance and the maximum panadapter bandwidth of these ‘SDR-as-an-add-on’ radios are often disappointing. They usually only support a single receiver on a 48kHz wide panadapter.

Some non-SDR receivers feature an output connector, specifically to allow connection to an external SDR receiver. These usually provide a tap off the receiver’s IF (intermediate frequency) or directly from the antenna, via a coupler. This is actually quite useful, as you can connect a ‘black box’ or dongle (1) type SDR (Figs. 1 and 2) and get the advantages of a second receiver and a panadapter band display.

Some software, such as HDSDR, for example, can be set to an ‘IF mode’ so that the SDR receives the IF frequency from your radio but displays the incoming RF frequencies on the screen. CAT or CI-V control via Omni-Rig software tunes your main receiver as you click on the SDR receiver’s panadapter display, thus integrating the operation of the SDR and the conventional receiver.

If your SDR is connected to your receiver’s IF output, the bandwidth that you can display on the SDR panadapter will be restricted to the IF bandwidth of the receiver. If the SDR is connected to a port that shares the main receiver’s antenna, you will probably be able to tune across the full bandwidth of the antenna.

However, this depends on where the output port is, in the main receiver chain. It may follow preselecting filters, in which case they will restrict the possible bandwidth, but they will also offer protection from strong out-of-band signals.

The American Federal Communications Commission (FCC) defines a software defined radio as being, “a radio that includes a transmitter in which the operating parameters of frequency range, modulation type or maximum output power (either radiated or conducted), or the circumstances under which the transmitter operates in accordance with Commission rules, can be altered by making a change in software without making any changes to hardware components that affect the radio frequency emissions” (2).

Unfortunately, this definition completely ignores the possibility of an SDR receiver, so it is not very helpful.

A better definition is that a “Software-defined radio (SDR) is a radio communication system, where components that have been typically implemented in hardware (such as mixers, filters, amplifiers, modulators/demodulators, detectors and so on) are instead implemented by means of software on a personal computer or embedded system” (3).

While definitions vary, it is accurate to state, in general, terms, that – if the filtering and demodulation in a receiver or the filtering and modulation in a transmitter are done using software which is re-configurable without replacing chips in the radio – then the radio is ‘Software Defined’.

In other words, if the signal is converted to a digital signal and processed using reconfigurable software or firmware running on a PC, embedded computer processor, programmable DSP chip, or FPGA (field programmable gate array) it is a software defined radio.

By contrast, if the signal is converted to a digital signal and processed in a dedicated (not reconfigurable in the radio) DSP chip it is not a software defined radio.


Blurred Lines

Even this latter definition is not really adequate any longer because the lines have become blurred. Many recent, conventional, radios can have firmware updates, making them at least partially field-upgradable. Often, the firmware running on DSP chips can be updated without removing the chips from the radio. Therefore, at least in some respects, those radios can be considered as ‘software defined’. Nevertheless, most folks don’t refer to those as software defined radios.

On the ‘flip side’, radios like the Elecraft KX3 and the FlexRadio Signature 6000 series are considered to be SDRs, even though they use dedicated DSP chips and a microcomputer inside the radio to perform the modulation, demodulation, and other DSP functions.

Many manufacturers are claiming that their radios are SDRs or partly SDRs and we can’t really say what is an SDR and what is not. What this means is that you should base your buying decisions on published performance specifications and, more importantly, choose according to on the features you want, rather than worrying about the exact form of the technology. In a word, if it works well, use it.


What to look for in an SDR

People often ask me “why should I buy an SDR.” My answer is always “Buy the radio that will make you smile the most”. I am a big believer that you should buy a radio that will make you happy, whether or not it is an SDR.

That said, if you do want to buy an SDR receiver, there are some things to consider. Firstly, if you opt for a dongle receiver (Figs. 1 and 2) or for any kind of SDR that needs a computer for digital signal processing, control, and the panadapter display, you will need a computer and some pertinent computer skills.

You will have to load a new computer program. With the cheaper dongle receivers, you may also need to download and install some driver software. It makes the radio less portable because you would need to take the PC as well. If you buy one of the new SDR or ‘conventional +SDR’ radios, that have all of the usual knobs and controls, you can use it straight out of the box, exactly the same as you would use any other new radio.

You may be able to add a computer monitor to enhance the spectrum display or connect the radio to a PC and run SDR software, but this is not essential. Many of the new breed of software defined radios have built-in panadapter displays, but some, like the Elad receivers, do not.

Look at the technology and compare the specifications of different radios. Most SDRs for the HF bands use direct digital sampling. An ADC near the antenna input converts the RF signals into a digital signal. Look for a 14-bit or 16-bit ADC, preselecting filters and a maximum panadapter bandwidth of at least 1MHz. SDRs that cover bands above 50MHz use a tuner or heterodyne frequency shifter to move a wide bandwidth of signals down to a lower range of frequencies, which the ADC can then sample. They usually support a panadapter bandwidth greater than 1MHz, although the FUNcube Pro+ dongle only supports a 192kHz panadapter.

These radios will be able to tune the VHF and UHF bands and often work up to 1.8GHz or higher. However, they might not be able to tune to HF frequencies. Even if they can, they will not have the same technical performance at HF as a direct sampling HF receiver will.

Some older SDR designs may use a QSD (quadrature sampling detector). This is not necessarily a bad thing, and these may offer excellent performance. However, they will have a much smaller panadapter bandwidth, and they will only be able to support a single receiver on a single panadapter. If you see a receiver that advertises a maximum bandwidth of 192, 96 or 48kHz, it will probably be a QSD design.

If you see a reference to having an internal soundcard or requiring connection to a PC soundcard, it is a QSD design. The Genesis G59, Elecraft KX3, McHF, and Recent RS-918 radios, for instance, all have QSD-based receivers.

I find that, for HF, a 192kHz wide panadapter is a good size. You can zoom in so that SSB signals are an easy to click size. This makes ‘jumping’ from station to station easier. You can also zoom out and see a good part of the band.

For broadcast signals, which have a much wider bandwidth, a wider panadapter is better. For VHF and UHF reception a wider panadapter is good because the working channels are spaced further apart, and you are normally receiving FM, AM, or digitally modulated signals. The band may be divided into 12.5 or even 6.25kHz spacing, but the channels actually in use in your area may be separated by several unused channels. Multiple narrow panadapters may be of more use than one wide panadapter.


Go-Kart to Limousine

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What you buy is, naturally, up to you. The choices are a bit like buying a car. An RTL dongle (Fig. 1) is like a ‘Go-kart.’ It is ‘technically’ a ‘car’. However, it lacks some of the capability and features that you might prefer. My concern is that you might, all too soon, be disappointed with the performance of one of these little radios and that this might put you off software defined radios in general.

The older, QSD-based, receivers are like a ‘Model T.’ Again, they have many of the features of a modern car but nowhere near the performance available from a newer model. Of course, many people love the cheap price of the Go-kart or they only need the capabilities of the Model T and that is fine. Owning either of them can be loads of fun.

As with everything that you choose to buy for your hobby, the purchase of an SDR comes down to a careful analysis of what you want to achieve, balanced by an equally careful analysis of your available funds.



You can get a sense of what it is like to operate an SDR, by logging on to one of the online web-SDR receivers at these URLs

These allow you to control and listen to an SDR located in another area or country. It can also enable you to listen to HF signals when the bands are closed at your place, and you can see band activity on the panadapter. Several listeners can use the same SDR at the same time.


The Elephant in the Room!

Thousands of people have seen the incredible value of buying one of a growing range of ‘dongle-type’ receivers (Fig. 1). For example, the RTL brand is very popular (Fig. 1).

These give you a taste of the ‘SDR-experience’, at an extremely affordable price. I am on record as not being a big fan of these radios, but it can’t be denied that these little devices are pretty amazing. They have a huge frequency range, usually, from about 24MHz up to around 1.8GHz.

You don’t need a separate power supply because these radios are powered over the USB port, and you can carry one around in your top pocket. RTL dongles (Fig. 1) are much cheaper than wideband alternatives like the dreaded Yaesu FRG-9600, Icom and WinRadio receivers, and scanners like the Bearcats. Probably the biggest disadvantage, apart from their poor dynamic range and lack of front-end band-pass pre-selector filters, is that they don’t cover the HF bands.

However, you can buy HF up-converters at about the same cheap price as the radio. These convert the HF spectrum up into a range above 24MHz so that the RTL dongle (Fig. 1) can receive HF signals.

RTL dongles (Fig. 1) were originally designed as a cheap way to receive DVB-T digital television stations on your notebook or tablet PC. Some online sources are crediting Eric Fry, Antti Palosaari and a company called Osmocom, with working out how they work and how they can be used as general purpose SDR receivers.

You can buy them from a wide variety of online sources for less than £50, but you must make sure to buy one with the Realtek RTL2382U, R820T, or R820T2 chipset; preferably, one advertised as an ‘SDR Receiver’.

Other models, advertised as ‘TV Receivers’, may not be compatible with the ‘Zadig’ driver, in which case they won’t work as a general purpose SDR receiver.

Some people find that getting the RTL dongles (Fig. 1) to work can be a challenge. This is usually a problem with the Zadig driver, used to configure the computer’s USB port. There are instructions and videos online, but the process can be rather fiddly.

The more sophisticated dongle and black box receivers (Fig. 2) are generally easier to get going, with most only needing the installation of an SDR program. I like SDR# (SDR Sharp) from AirSpy and HDSDR, but there are many others including SDR apps for Linux and Mac.

Many small SDR receivers are designed to work with particular SDR programs.

For example, the SDRplay RSP receivers work with the SDRuno software, supplied by the manufacturer as a free download (Figs. 3 and 4).

The ColibriDDC receiver from SunSDR works with ExpertSDR2.

One of the key benefits of the SDR experience is that you can also use these radios with other software like SDR# and HDSDR. To do that you have to place a special file called ExtIO.dll into the SDR program’s program directory. For HDSDR, you do not have to rename the file as you will be asked to choose the ExtIO file appropriate to the radio you are using when you start HDSDR. This means you can have several different versions of ExtIO.dll in the HDSDR program directory, each for a different radio.

I have found that most dongles (Figs. 1 and 2) will work with the same ExtIO file. I believe that SDR# can only manage a single version of the ExtIO.dll file and that it must be renamed to ExtIO.dll. You may have to modify one of the SDR# config files as well, but instructions will be on your SDR manufacture’s website.

Next month I plan to discuss some of the things that are different about SDR receivers. For example, alias rejection and why they work better when the band is full of signals.

[Editor’s reading tip: Barron, A. (2018) SDR Radio: -Ed.]





  1. A ‘dongle’ (Figs. 1 and 2) is a small device that plugs into something. In the case of a dongle receiver, the radio has a small form- factor similar to a USD memory stick, with a USB Type A male connector on one end and an antenna connector at, or near, the other end.
  2. Spectrum Use Employing Cognitive Radio Technologies, ET Docket No. 03-108, Report and Order, 20 FCC Rcd 5486 (2005).
  3. Software Defined Radio: Architectures, Systems, and Functions (Markus Dillinger, Kambiz Madani, Nancy Alonistioti). See:


This article was featured in the October 2018 issue of Radio User