SDR Magic

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In preparation for his new column, Andrew Barron, the author of a widely-noticed book on the subject, provides an introduction

 

In preparation for his new column, Andrew Barron, the author of a widely-noticed book on the subject, provides an introduction to the background, technology, characteristics, and uses of Software Defined Radio (SDR) receivers.

 

 

Everyone is talking about Software Defined Radio (SDR), it seems. But is it right for you? This article and my future columns will seek to provide information that will help you to decide.

The good news is that, although software defined radios can be used for transmitting, the biggest advantages are obtained when using them for receiving signals. I will explain the features offered by SDR receivers and a few disadvantages of using them as well.

In future issues I will develop the theme further, delving deeper into the way SDR receivers work and explaining some of the neat things that you can do with them.

May readers of this magazine will be interested in receiving all kinds of signals over a vast frequency range. Some are receiving signals from satellites, others are into TV reception or broadcast radio. Some receive secret ‘number’ stations or enjoy ferreting around on the short wave bands – the variety is endless.

I believe that using a software defined radio receiver can improve your enjoyment of the hobby. It certainly has for me. I never thought I would ever graduate to a radio with no knobs.

I believed in the maxim of “the more knobs the better.”

 

Initial Experiments

I started experimenting with SDR because I wanted one of those fancy band scopes, which were appearing on the latest high-end amateur radio transceivers. I purchased a single band Softrock kit on 9MHz and connected it to the 9MHz IF (receiver intermediate frequency stage) on my old Yaesu FT-301 transceiver. It worked very well. I was amazed to be able to see signals on the panadapter display as well as hearing them.

Since then, I have become ‘hooked’ on the SDR panadapter (band scope) display and convinced of the remarkable performance and the ease of use of SDR transceivers. Now, when I operate a ‘conventional’ radio I feel deprived. It feels like I am operating blind (Fig. 1).

Software Defined Radio has been around for a while, especially in cellular radio base stations and military radio applications. However, in the hobby market relating to radio listeners, amateur radio operators and ‘hackers’, SDR is a more recent phenomenon.

 

Tayloe Detector

The Tayloe detector, which is the basis of all Generation-One SDRs, was patented in 2001, and the first commercially-built ham radio SDR transceiver, the FlexRadio SDR-1000 was released in 2006. The same company produced the Flex-6700 HF / VHF radio, which can support up to eight receivers on up to eight bands (Fig. 2).

Initially, this new technology was seen as ‘experimental’ and a bit ‘fringe’: Interesting toys for computer geeks, hackers, and electronics freaks. Since then, the technology has slowly gained acceptance, and more and more people have found that there are real benefits.

Magazine articles started to feature screenshots from SDR receiver panadapters. It seemed that the people who were interested in the development of new digital modes, propagation, short wave signals, weather satellites, amateur radio satellites, contesting, and interference detection, also found that software defined radios were very useful for their particular interests.

However, SDR technology is constantly evolving, and it has now matured to the point where the big radio manufacturers are starting to release SDR-based radios. Icom and Yaesu have released SDR-based HF transceivers and Icom has released the IC R8600 SDR Wideband Receiver, which covers 10 kHz to 3 GHz.

 

In the Mainstream

I think it is fair to say that SDR has now reached the ‘mainstream.’ I believe we will continue to see the release of SDR-based radios, and that the days of new commercial superheterodyne receivers arriving on the market are numbered. This is partly because of performance gains and features that these radios provide and partly because SDR radios are much cheaper to make.

The inside of an SDR receiver is quite different to a superheterodyne receiver. A few large integrated circuits replace all of the RF and IF tuned circuits. There are no little ‘cans’ to adjust, so each radio does not need to be carefully ‘aligned’ or ‘tuned up’ at the factory, prior to shipping.

There are at least four generations of software defined radio receivers and transceivers, so you ought to be careful when you buy one. Make sure that your choice meets your needs.

There are USB ‘dongle-type’ receivers like the RTL, Colibri Nano, and the FUNcube dongles. The RTL dongles are extremely cheap, starting at around US$10. However, don’t expect a $10 radio to perform as well as a conventional receiver costing a thousand dollars or more. SDR is amazing, but not that amazing!

The next stage is ‘small box’ receivers like the SDRplay and Perseus receivers. There are bare boards, including the SoftRock and HermesLite boards. You can find SDRs with knobs, like the Elad FDM-Duo and the CommRadio CR1a. And there are ‘hacker’s radios’ with extreme frequency ranges, such as the USRP, HackRF One, and Blade RF, and SDR transceivers.

Overall, there are literally hundreds of SDR receivers and transceivers on the market right now, each with varying capabilities and performance specifications.

The image in Fig. 3 shows one of the radios I am using, the ANAN-100 from Apache Labs.

 

Class Distinctions

As well as different technical generations there are several distinct classes. Some software defined radios have the usual knobs and buttons on the front panel. They don’t need to be connected to a computer.

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Radios that do not have front panel controls, require connection to a computer running suitable SDR software. The underlying hardware technology is the same in both types and it is radically different to conventional radios. Some radios have dedicated software, specifically written to match the hardware; others can be used with a variety of different software applications. This is one of the advantages of SDR technology. You can completely change how your radio looks and operates, simply by using a different software package.

Different software may also offer different modes. For example, one program may include DRM digital broadcast mode, and another might include FM and SSB or D-Star digital voice. Using an SDR that has front panel controls is much the same as using a receiver based on the conventional superheterodyne architecture. Using an SDR connected to a PC is no more difficult than using any other computer program.

 

Technical Variety

Radios for the LF (low frequency), HF (high frequency), and in some cases VHF (very high frequency) bands are typically ‘direct sampling,’ meaning that the RF (radio frequencies) arriving at the receiver input are sampled and converted into a digital signal using an ADC (analog to digital converter).

The signals are then processed using ‘digital signal processing’ (DSP) software, running on a programmable device such as an FPGA (field programmable gate array) or possibly on dedicated DSP chips. Receivers that cover higher UHF and microwave frequencies use a conventional oscillator and mixer arrangement to shift a band of radio frequencies down to a frequency that the SDR part of the receiver can sample. The SDR section is often a dedicated receiver or tuner chip, but it can be an ADC-based radio as described above.

The big advantages of SDR for radio listeners are the panadapter display (Fig. 4), fully adjustable receiver bandwidth, great noise and interference suppression, and excellent receiver performance. The panadapter display lets you see signals across the band of interest. This means that you can ‘jump’ to stations directly, rather than carefully tuning across the band straining to hear that elusive weak signal.

A panadapter is different to the band scope found on some conventional superheterodyne receivers. A band scope displays signals above and below the frequency that the receiver is tuned to. A panadapter displays a much wider range of frequencies and you can place one or more receivers at any point on the display.

Most panadapters also include a waterfall display. A waterfall display indicates signals over a period of time. It can show you signals that have a short duration, such as air traffic voice channel traffic, land-mobile radio, or burst transmissions. You can see the traces of signals that could easily be missed if you were tuning across a band using a conventional receiver.

SDR receivers feature a completely adjustable receiver bandwidth. This means that users can tune the receiver to exactly match the bandwidth of the signal they are listening to. On AM and SSB, this means operators are not listening to noise that is outside of the wanted signal range. You can set the receiver to a wide bandwidth for broadcast AM or FM signals and switch to a narrow bandwidth for narrowband signals. Adjustable bandwidth is a real advantage if you want to receive weather images from the NOAA weather satellites on 137MHz. A conventional FM receiver is too narrow. You can decode any received images much better by using an SDR receiver.

 

Signal Processing

In SDR receivers, you will get very good noise blankers and filters because these are created using DSP software code. The filter parameters are often user-adjustable, and some extremely sharp filter responses can be achieved without the 'ringing' artifacts typical of sharp hardware filters. It is true that very aggressive DSP filters can make the audio sound ‘weird’ and ‘fluttery’. However, this may well be preferable to listening to a very noisy signal.

What is more, in most SDR software, the filter taps can be adjusted for the best compromise. Some software supports tracking notch filters which stay on a selected frequency. When you tune the receiver, the notch stays over the interfering signal; if you return to the frequency, later on, the notch filter will still be engaged.

Furthermore, SDR receivers tend to have excellent receiver performance. Because of the different way that these radios work, they do very well in the receiver performance tests published in magazines and on websites.

Generally, there is no receiver blocking on strong signals until the incoming signal reaches the clipping point of the ADC. At that stage, the receiver performance will crash. But this only occurs with extremely strong signals, and it can easily be cured by switching in some front-end attenuation.

SDR receivers do not suffer from ‘AGC pumping,’ which is the effect where a strong signal within the receiver’s passband causes the receiver AGC (automatic gain control) to operate, desensitizing the receiver and causing the weak signal that you want to receive to disappear. SDR receivers are unaffected. They can, in fact, be technically improved, by the presence of nearby signals.

Many people comment that SDR receivers ‘sound better’ or ‘quieter’ that their old conventional radios. This is because SDR receivers don’t have the multiple oscillators, mixers, and IF amplifiers found inside a standard (double or triple conversion) receiver. Each amplifier and oscillator in a conventional receiver contributes noise to the output audio signal, and the mixing processes contribute intermodulation distortion products and image signals – those ‘birdies’ you hear as you tune across the band.

Direct conversion SDR receivers sample the signal close to the antenna, usually following a preamplifier, step attenuator, and bandpass filter. They don’t have hardware mixers, oscillators and IF amplifiers, so you don’t get the noise and intermodulation that these can produce. The result is much cleaner, less ‘hissy’, audio. This fundamental difference in architecture creates one of the major benefits of software defined radios over conventional radios, and it is the reason that a low-cost SDR can sound better than an expensive double or triple conversion receiver.

 

SDR Downsides

I promised to discuss some disadvantages too.

First, SDR receivers do not make good scanning receivers. It is certainly possible to program SDR software, for the receiver to operate ‘like’ a scanning receiver, but I am not aware of any software that has that functionality.

The reason for this is that the SDR panadapter display lets you see channels as they are activated. You can click on the panadapter to hear the activity on that channel. To do that successfully on the high VHF and the UHF bands, you will need a radio that supports a wide panadapter display, of at least 10MHz in scope.

The problem here is that you end up looking at a lot of bandwidth that does not contain the channels you want to hear. You can, of course, program memories and scan through them. But this is not the same as using a scanning receiver. Fundamentally, the objectives are different. A scanning receiver ‘sweeps’ a band, stopping on active channels. An SDR panadapter lets you see which channels are active, but you have to click the panadapter or waterfall display in order to hear the signal.

Second, SDR receivers are not well suited to mobile operation. They rely heavily on the ability to display signals on a panadapter or waterfall display. You click on the signals that you want to hear. Of course, this is not practical when you are driving a vehicle. You need to keep your eyes on the road, not on the screen of your radio!

 

Preview

Over the next few months, I plan to answer questions such as: What is different about SDR receivers? How do they work? Are they really better?  And do you need to be a ‘computer-whiz’ or a ‘technology-boffin’ to use one?

I also welcome your questions, but please don’t tell me that you don’t like software defined radios or offer reasons why they don’t work. I am perfectly happy for you to use whatever radio brings the biggest smile to your face – and they do work!

[Andrew Barron is the author of ‘SDR Software Defined Radio for Amateur Radio Operators and Shortwave Listeners,’ available from the RadioUser website, Amazon, Kindle, and the RSGB – Ed].

 

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