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Learn the Fundamentals of Software-Defined Radio

By Art Pini

Contributed By Digi-Key's North American Editors

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.

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 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).

Diagram of comparing a traditional analog receiver (top) with an SDR-based receiver (bottom)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)

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 Broadcasting – Terrestrial (DVB-T) coded orthogonal frequency-division multiplexing (COFDM) demodulator with a separate tuner IC.

The DVB consortium is a European-based standards organization for the broadcast transmission of digital terrestrial television. This system uses an MPEG transport stream to transmit compressed digital audio, digital video, and other data, using COFDM or OFDM modulation. These devices can be repurposed by programming to other applications and are ideal for hobbyists and DIYers wanting to listen to and investigate VHF, UHF, and low-microwave-frequency radio signals.

For all the signal processing power in the Adafruit SDR, it has an exceedingly small physical size at only 22.24 millimeters (mm) x 23.1 mm x 9.9 mm (Figure 2). It interfaces with the host computer via a USB port, and off-the-shelf SDR software provides the user interface on the computer/laptop. The manufacturer recommends Airspy’s SDR Sharp (SDR#) in their getting started guide. Software installation takes less than five minutes and is well documented.

Image of Adafruit 1497 low-cost SDR receiverFigure 2: The 1497 is a low-cost SDR receiver that fits in a package the size of a quarter and comes with an accessory antenna and remote control. This receiver tunes from 24 MHz to 1.85 GHz, interfacing with a host computer via USB. (Image source: Adafruit Industries)

The antenna connection on the receiver is through an MCX connector. The MCX jack on the receiver accepts the plug mounted on the antenna cable, or the supplied antenna can be replaced with a user-supplied custom antenna.

If the user decides to replace the supplied antenna with a different one, it can be connected using an MCX plug. Coaxial adaptors can be used to mate the MCX input connector on the SDR with either SMA or BNC connectors, which are more commonly used. Amphenol RF offers both an MCX plug to SMA jack (242127) or a BNC jack to MCX plug (242204), providing the more common connector interfaces.

SDR support software

The SDR# software connects with the receiver and provides the user interface and visual display (Figure 3).

Image of Airspy SDR# user interface (click to enlarge)Figure 3: The Airspy SDR# user interface controls the SDR receiver from the drop-down menus on the left. The spectrum analyzer display is shown in the top grid while the spectrum history is below it. (Image source: Digi-Key Electronics)

The SDR# default user interface has three major elements:

  • The column on the left contains controls for the SDR device. There are fourteen pull down menus controlling all aspects of the SDR receiver. The principal controls are for the radio, audio, and display.
  • The top grid contains the spectrum analyzer display. This plots frequency on the horizontal axis and signal power vertically using a logarithmic scale calibrated in decibels. Spectrum analyzers are the primary test tool used by RF engineers to measure and analyze RF devices. The numeric readout on the top of the screen displays and controls the center frequency of the spectrum analyzer. The maximum displayed frequency range is the bandwidth of the receiver which is about 2 MHz. There is a horizontal zoom slider control to the right of the display. Zoom permits a horizontal expansion of the display about the center frequency.
  • Beneath the spectrum analyzer display is a spectrum history display sometimes called a spectrogram, which shows the time history of the spectrum. The horizontal axis is frequency as in the spectrum analyzer display; the vertical scale is time. In the figure there are time markers showing the date and time. The third dimension is the signal power, which is indicated by the color. The default color scale runs from black, as the minimum power level, to red as the maximum power level. There are a variety of styles and color mappings available under display controls.

The signal displayed in Figure 3 is that of an FM broadcast station at 105.1 MHz. This is a wideband FM signal that has a bandwidth of 200 kilohertz (kHz). This is one of eight demodulators available in the SDR receiver. The other demodulators support narrowband FM, AM, upper and lower SSB, DSB, CW, and raw in-phase and quadrature signal components. The selections are in the radio controls in the upper left of the display.

The signal spectrum consists of the analog signal about the center frequency. This carries the analog radio program. Outside of that are dual sub-bands that contain other program material and digital information. The program information content is decoded and appears immediately above the spectrum analyzer display. As well as the spectrum display, the radio station’s audio components are available through the host computer for listening.

Wideband FM has a large bandwidth because it is expected to carry high fidelity stereophonic music. A radio service such as the National Weather Service carries only voice and uses narrowband FM (Figure 4).

Image of National Weather Service weather broadcast at 162.471 MHz (click to enlarge)Figure 4: Tuning in a National Weather Service weather broadcast at 162.471 MHz. This station uses narrowband FM. (Image source: Digi-Key Electronics)

The National Weather Service station is received using a bandwidth of only 11.2 kHz because the program content is only voice. Again, the audio program material is available as well as the spectrum displays. The SDR receiver adds all of these services to the host computer.

The spectrum history or spectrogram display is useful for seeing changes in the received signals spectrum over time. A simple example is to see that of a continuous wave (CW) Morse code signal (Figure 5).

Image of spectrogram view of a CW Morse code signal (click to enlarge)Figure 5: The spectrogram view of a CW Morse code signal. (Image source: Digi-Key Electronics)

CW signals encode data by turning an RF carrier on and off (on-off keying). On the spectrogram display the periods when the key is down—and the carrier is being transmitted—are indicated by the light blue-grey track on the display. The Morse character “V” (di di di dah) indicating testing can be seen in the signal track. Note that the software makes provision for receiving CW signals by supplying a beat frequency oscillator (BFO) labeled “CW shift” to provide a user-controlled audio tone to hear the code transmission. Since CW transmissions are narrowband, the receiver reduces the bandwidth to 300 hertz (Hz) as seen in the radio control pull-down menu. Keeping the receiver bandwidth to the minimum value needed for the mode being received minimizes the noise level in the channel.

Some measurement applications for an SDR receiver

In an increasingly linked world, there are many RF sources that need to be checked and serviced. An example is the verification of the update period of a remote weather station transmitter module (Figure 6).

The spectrogram shows two RF bursts at the remote transmitter’s 433.93 MHz carrier frequency. The time scale on the spectrogram indicates that the FM bursts occur roughly 50 seconds apart.

Image of spectrogram of a remote weather station transmitter at 433.92 MHz (click to enlarge)Figure 6: The spectrogram of a remote weather station transmitter at 433.92 MHz that sends data in bursts. The spectrogram captures and displays transmitted bursts roughly 50 seconds apart. (Image source: Digi-Key Electronics)

Automotive remote keyless entry (RKE) systems operate at either at 315 or 433 MHz, depending on where the vehicle is being used and the governing regulations. In this case, the user just needs to hold the key fob near the antenna and push one of the buttons to see the type of modulation used (Figure 7).

The spectrum of the RKE key fob shows dual peaks at about 433.9 MHz. Data encoding for this device uses frequency shift keying (FSK) where the carrier is shifted between two frequencies to indicate a digital one or zero. Other RKE fobs use amplitude shift keying (ASK) where the amplitude of a carrier is shifted between two levels, not too different from the CW signal.

Image of spectrum of a remote keyless entry device uses FSK of a 433.9 MHz carrier (click to enlarge)Figure 7: The spectrum of a remote keyless entry device uses FSK of a 433.9 MHz carrier to encode digital data to control entry into a vehicle. (Image source: Digi-Key Electronics)

Conclusion

The Adafruit 1497 SDR receiver opens up the whole world of VHF, UHF, and low microwave frequency bands to investigation hobbyists and professionals alike. It enables users to use a computer to tune into FM, TV, amateur radio, citizens band, weather, and short-wave broadcasts. It also can be used as a spectrum analyzer to verify the operation of a wide range of portable RF devices. The 1497 has also been used to create interferometers for radio astronomy—all at low cost.

Disclaimer: The opinions, beliefs, and viewpoints expressed by the various authors and/or forum participants on this website do not necessarily reflect the opinions, beliefs, and viewpoints of Digi-Key Electronics or official policies of Digi-Key Electronics.

About this author

Art Pini

Arthur (Art) Pini is a contributing author at Digi-Key Electronics. He has a Bachelor of Electrical Engineering degree from City College of New York and a Master of Electrical Engineering degree from the City University of New York. He has over 50 years experience in electronics and has worked in key engineering and marketing roles at Teledyne LeCroy, Summation, Wavetek, and Nicolet Scientific. He has interests in measurement technology and extensive experience with oscilloscopes, spectrum analyzers, arbitrary waveform generators, digitizers, and power meters.

About this publisher

Digi-Key's North American Editors