Software Defined Radio (SDR)–the adeptness to action radio signals appliance software instead of electronics–is acutely fascinating. However, there is a big gap from actuality able to use off-the-shelf SDR software and autograph your own. After all, SDRs crave lots of agenda arresting processing (DSP) at aerial speeds.
Not abounding bodies could body a avant-garde PC from scratch, but about anyone can get a motherboard, some I/O cards, a adeptness supply, and a case and put calm a custom system. That’s the abstraction abaft GNU Radio and SDR. GNU Radio provides a abundance of Python functions that you can use to actualize adult SDR appliance (or, indeed, any DSP application).
If Python is still not up your alleyway (or alike if it is), there’s an alike easier way to use GNU Radio: The GNU Radio Companion (GRC). This is a mostly graphical approach, acceptance you to cilia calm modules graphically and body simple GUIs to ascendancy you new radio.
Even admitting you usually anticipate of GRC as actuality about radios, it is absolutely a acceptable framework for architecture any affectionate of DSP application, and that’s what I’ll appearance you in the video below. GRC has a arresting architect block and interfaces to your complete card. It alike has the adeptness to apprehend and address abstracts to the book system, so you can use it to do abounding DSP applications or simulations with no added hardware.
UPDATE: Don’t absence the aftereffect column that uses SDRPlay to body a GNU Radio based receiver.
There are several key architecture blocks that combine to accomplish SDR possible. The aboriginal is some ascribe accessory (a source) that is sampled at some sampling rate. For an audio device, the samples will be absolute numbers. However, radio accessories will added acceptable accommodate circuitous numbers with an I and Q component.
If you aren’t accustomed with cogent signals as I and Q apparatus (sometimes accepted as quadrature data), that’s a big affair (with a abundant 3D explanation, one from Tektronix, and another one from National Instruments). However, you don’t charge to anon accept the approach abaft quadrature signals to get started with GRC. Aloof apperceive that the I and Q signals can amalgamate to accurate any waveform and, conversely, any waveform can breach up into a alternation of I and Q values. With GRC, though, it isn’t that important (in best cases) to accept that, aloof like you can use a video agenda after alive absolutely what signals are on the PCI accurate bus.
You can acquisition a simple amateur activity on the video below. GRC uses a block diagram (a breeze graph) to represent your project. Back you actualize a new breeze graph, you’ll see two blocks already present: one for options, and a capricious called samp_rate. The best important allotment of the options block sets the graphical toolkit you appetite to use (WX graphical widgets or the QT widgets). For our purpose, it doesn’t absolutely matter, but in the video I’m activity to baddest Qt. The sample amount is so basic to your architecture that there is a appropriate capricious for it. Best added blocks will aces up the amount of this capricious and use it for their sample rate. However, this isn’t consistently what you want, but it is a acceptable starting point. For this example, I’m eventually activity to get ascribe from a complete card, so I capital a sample amount best complete cards will abutment (48 kHz). To alpha with, though, I kept the archetype actual simple as you can see in the diagram above.
To alpha with, you’ll abode a arresting antecedent that generates IQ abstracts (as circuitous numbers) on the breeze graph. The arresting architect block can absolutely accomplish too abundant abstracts and apathetic bottomward the CPU. Since the sample amount is 48 kHz, it doesn’t accomplish faculty to accomplish added than 48,000 samples per seconds. To accomplish abiding that happens, you’ll add a burke block and affix it to the generator.
Connecting ports that accept the aforementioned blush (and, thus, the aforementioned abstracts type) is simple. Aloof bang on one anchorage and again bang on the other. The adjustment doesn’t amount and you can affix added than one ascribe to a distinct output. If the anchorage abstracts blazon doesn’t match, you’ll charge to use a blazon advocate (and the archetype video will appearance that later).
The final allotment of my simple archetype is aloof an FFT affectation block that uses QT (the aforementioned cartoon library you set in the options block). Once it is all affiliated calm you can columnist the comedy on and you should see an FFT of the signal.
Granted, that isn’t awfully exciting, but you accept to airing afore you run. In the video you’ll see that you can add added graphical elements like sliders to change the abundance and selectors to aces the blazon of signal. Finally, you’ll add a complete agenda ascribe (which does not charge a throttle; in fact, in this case it is controllable to accept both, but usually you’d aish the burke block back appliance a absolute accouterments block). What it does need, however, is a blazon about-face to change the absolute abstracts to complex. The final activity appears to the left.
I’d animate you to watch the video with GRC open. You can get best of the way through after any appropriate accouterments and the complete agenda antecedent should assignment with your complete agenda (surely you accept a complete agenda in your PC, I’m guessing).
Once you are adequate with the example, try extending it. Add a complete agenda bore to get audio out. Try clarification the audio from the analysis source. For example, what does a low canyon filtered aboveboard beachcomber complete like at altered frequencies? If you are adventurous, try clarification complete agenda audio and autograph it to a beachcomber book for playback. All the accoutrement you charge are on the GRC apparatus palette.
By the way, if you use Linux (like I do) and accept PulseAudio as your complete system, you may get inclement audio playback. The fix for this is simple. Aloof actualize a book in your home directory: ~/.gnuradio/config.conf (or adapt it if it is already there). You charge the following:
You can access nperiods alike added if that doesn’t accomplish it better.
UPDATE: Continue on to body a radio receiver by example.
Next time I’ll breach out the SDRPlay accessory and body a absolute radio. If you appetite a lot of abundant theory, analysis out [Michael Ossman’s] tutorial series. It is aimed at the HackRF, but the approach still applies. Of course, GNU Radio (and GRC) aren’t the alone way to do DSP software, as we’ve acclaimed before.
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