After a brief introduction to SDR, this article introduces a low-cost USB-based SDR module from Adafruit Industries that can receive and demodulate a wide range of signals, from simple continuous- wave (CW) Morse code to the most complex digital modulation forms. It will show how users can use the module and associated software to add radio reception, radio frequency spectrum, and spectrogram analysis to computers. What is SDR? SDR uses digital techniques to replace traditional radio hardware like mixers, modulators, demodulators, and related analog circuits. By digitizing the radio signals directly using an appropriate analog-to-digital converter (ADC), an SDR can
implement all these functions in software so that the same
From military and aerospace to hobbyists, the promise of software- defined radio (SDR) is that with one piece of hardware, users can capture, demodulate, and access RF signals across a wide swath of radio frequencies. How wide a swath depends upon the hardware’s RF front end, while the number and types of signals that can be accessed depends upon the software and underlying processing capabilities. Both of these are a function of the application requirements and the associated cost and power budgets. For military and aerospace, the cost can run into the tens of thousands. For short wave listeners, amateur radio enthusiasts, and do-it- yourselfers (DIYers), what’s needed is a simple, low-cost means of accessing radio waves using readily available desktop computers or laptops.
hardware is used for multiple radio modes, whether AM, FM, CW, single sideband (SSB), or double sideband (DSB). The result is an extremely flexible radio that can be quickly reconfigured to handle different signaling technologies (Figure 1). Traditional radios like the superheterodyne receiver (Figure 1, top) are hardware based and implemented with analog components. The SDR receiver uses an RF tuner to down convert the frequency band of interest to an intermediate frequency (IF) within the range of the ADC. From that point on, all the circuits are digital. The digital down converter translates the signal frequency to baseband, performing a low- pass filtering function. The digital signal processor (DSP) performs demodulation, decoding, and related tasks. These circuits are generally based on application- specific ICs (ASICs), field- programmable gate arrays (FPGAs) and programmable DSP devices. With the appropriate software, these digital circuits provide a very flexible radio capable of receiving a wide range of modulation types. Low-cost SDR hardware The Adafruit Industries 1497 is a low-cost SDR receiver covering a frequency range of 24 megahertz (MHz) to 1.85 gigahertz (GHz) and is based on a Digital Video
Figure 1: Comparing a traditional analog receiver (top) with an SDR-based receiver (bottom). All functions in the SDR receiver after the ADC are implemented using programmable digital circuits, which allows programmable changes and updates. Image source: Digi-Key Electronics
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