Software Defined Radio - how does it work? Part 2

    Hello, Habr.

    In the first part , the main types of SDR devices were described and their characteristics were briefly described. In the second part I will tell you more about the pros and cons of SDR, examples of using this technology will also be given.

    Continuation under the cut (carefully, traffic).

    If someone missed the first part , it is advisable to start with it in order to better understand what it is about.

    Benefits of SDR

    First, let's talk about the benefits of SDR, about what they give the user. It’s clear that everyone’s priorities are different, so different items are mixed up. Which of them is “more important” is difficult to say, but probably impossible, everyone will decide for himself.

    Panoramic broadcast review

    First and most obvious, the SDR shows the panorama of the airwaves “as is”.

    This is quite convenient, both in terms of visibility and in terms of searching for new signals. On the screen, you can immediately see, for example, that a station is working on the right 100KHz higher in frequency, various interference is visible, new and especially short signals, etc. Signals with a rapidly changing frequency, for example, are simply not visible on an ordinary “classic” receiver or transceiver, and the person will not even guess about their presence.

    Anyone can see how it looks and listen to what's happening on the air by opening a web-panorama on the Dutch SDR. As an example of a picture from there, the operation of the frequency hopping system is clearly visible (but this is not accurate).

    Adjustable digital filters and sound effects

    Hobbyists with experience probably remember the times when narrow-band quartz filters were bought separately for radio stations, and they cost not so little money. The situation is even worse in cheap household receivers - the filters are simpler there, and generally configured in any way with a Chinese random, as luck would have it. Yes, and the settings in such receivers are usually only two - Wide and Narrow. In SDR, all filters are performed mathematically, so any filter width can simply be selected with the "mouse" or set in the settings. The rectangularity of the filters can also be almost perfect - mathematically, you can select any order of filters, everything is limited only by the algorithm.

    Similarly, noise reduction settings, AGC coefficients, etc. can be set.

    By the way, some audiophiles believe that “ordinary” analog filters soundWarmer and better than lamp , it's hard for me to say. Perhaps the form of digital filters affects the perception of sound, there is also a field for experimentation.

    Measurement capability

    SDR is a pretty accurate meter. Starting from the banal display of the levels of different stations in decibels, to the assessment of interference, signal quality, etc. All the shortcomings of your own or someone else's signal are clearly visible on the spectrum.

    For example, the picture shows the signal of FM stations. You can immediately see who is broadcasting where, with what level, which stations interfere with each other.

    You can also use SDR as a spectrum analyzer.

    Broadband Processing

    Reception of a signal immediately with a wide band of several megahertz opens up amazing opportunities for signal processing. Fans of Morse code and radio competitions, for example, can decode callsigns immediately in a wide band (although, in my opinion, this is unsportsmanlike, but this is another question).

    You can also create an unlimited number of "virtual receivers" within the bandwidth. For example, the decoding of amateur radio signals WSPR on two bands at once from one physical receiver is shown.

    With the help of SDR and Virtual Audio Cable, users with a split personality can listen to two stations at once, one in the right ear and the other in the left;)

    We can assume that in prof. Broadband processing offers great opportunities for search, classification, detection and suppressionvarious signals. Probably, the technology is actively used in radars and other devices.

    Reception and transmission of almost any type of modulation

    Signal processing is done on a PC, so the computing capabilities are limited only by the presence of the necessary decoders. AM, FM, WFM, DRM, DAB +, TETRA and many other scary words - for almost all modern open protocols you can find a decoder.

    So far, the SDR is “too tough”, it’s probably GSM and WiFi, although there are some advances here ( Open BTS , WiFi Analyzer ).

    As for the transfer, the owners of URRP or LimeSDR can, for example, experiment with DAB + .

    Broadband Recording and Playback

    Because in SDR, all data is written initially in digital form; there is no problem recording the entire band at once in order to listen or analyze later. One recording can contain several radio stations at once, which can be listened to in the same way as from a real receiver. This is somewhat similar to a RAW file from the camera, where post-processing (white balance, etc.) can be done after shooting.

    The link gives an example of a recording with a bandwidth of 760 KHz, the panorama of which looks like this:

    In the recording you can find a lot of things at once - an exact time beacon, telemetry, drive beacons, broadcast stations. You can open it in Matlab free program HDSDR. There is only one drawback here, it is a large size. One minute of recording at this bitrate takes 350 MB. But of course, with real recordings, it makes no sense to write everything, you can choose the bandwidth depending on the tasks.


    Another actively developing area. Some SDR receivers and transceivers have the ability to work remotely - as Since the receiver is accessible by IP address, there is no fundamental difference, it stands next to the table or 100 km in the country (of course, the transmission bitrate must be set in accordance with the width of the Internet channel). Given that there are more and more interference in cities, this can become very relevant.

    Engineering and research tasks

    Of course, with broadband recording and playback, there are great opportunities for debugging various signals, protocols, testing DSP methods, etc. Specialized devices for research purposes (USRP) allow full duplex operation, can also have multiple inputs and the ability to synchronize, which allows testing algorithms such as direction finding.

    With GNU Radio, you can create and test complex signal processing systems using a large number of off-the-shelf units.

    I hope the advantages are clear, let's talk about the disadvantages.

    SDR disadvantages

    Stationary work only

    The main drawback for most users - SDR is actually a stationary desktop device, taking it with you is very inconvenient and uncomfortable. In cities, alas, the level of interference is often off the charts, while there are practically no portable receivers with SDR and IQ recording.

    In recent years, receivers and transceivers made using SDR technology began to appear, but a compact and lightweight device that could simply be taken with you, while there is still no market. Of course, if you wish, you can take a laptop, SDR, antenna, wires, adapters, powerbank with you, but all this is quite cumbersome and inconvenient. The market for portable SDR devices does not exist de facto. Of the exceptions, except that RDR-Pocket, but its price is about 1500 Euro, and it is produced only on order. Modern technologies make it possible to make such devices, but there is simply no solvent demand for them.


    As is obvious from the description, a good SDR is a fairly expensive device. Ultrafast ADCs and FPGAs, low-noise input stages, multilayer printed circuit boards, high-quality filters, etc. - the price of a good DDC SDR starts at $ 500 and higher, and you can’t do this cheaply. Professional receivers (USRP, Winradio) cost from $ 1,500 and above.

    However, there are also shifts. Quite good SDRPlay receivers with a range from 10KHz to 2GHz sell for $ 150, and although this is not a DDC and an ADC of only 12 bits, it is enough for most reception tasks. "Whistles" RTL-SDR for $ 30 probably already have every radio amateur. For fans of VHF and signal processing, there are inexpensive LimeSDR, which at a price of $ 200-300 cover the range up to 6 GHz.

    The complex algorithmic part

    It is clear that serious simulations of digital signal processing “hide” behind the external simplicity and convenience. Over the past 10 years, a large number of various software for SDR has been created, and most likely the average user will not have to add anything. But if you want to fix something or create your own, it’s not so easy to do, the “threshold of entry” is quite large.

    Power Consumption and Processor Requirements

    It is also obvious that the current consumption is quite high, as are the requirements for the CPU and video card. The usual "classic" radio can work for a month on batteries, the DDC SDR will consume at least 1-2A during operation.


    In the previous part, some wondered why this is necessary at all, I hope partly managed to answer this question.

    All that was planned, again did not fit into one article. In the next part, we will look at the software interface to the SDR receiver in Python, and perhaps a bit of work with GNU Radio.

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