A Technical Introduction to Software-defined Radio or SDR

Updated January 2021

Software-defined radio (SDR) is a radio system where traditional analog components are replaced with digital components and software technologies. In traditional radio systems, a radio was designed entirely in analog and for a specific application such as HF signal reception for instance. By contrast an SDR can be used to process a wide variety of signals from HF to Bluetooth thanks to the flexibility of software. One SDR to process any signal. This is what makes SDR so compelling.

Conventional Radio Receiver

The picture below shows a conventional radio receiver comprised entirely of analog building blocks such as amplifiers and mixers.

A Conventional Analog Radio Receiver

Ideal SDR

In an ideal SDR receiver, the radio frequency (RF) signal received by an antenna is first digitized using an analog-to-digital converter (ADC) and the output is processed by a Field Programmable Gate Array (FPGA) in software. The digital signal is transferred across a serial or parallel interface into a host processor such as a PC. This signal can then be further processed in software. Similarly, in an ideal SDR transmitter, the RF signal to be transmitted is converted from digital to RF in one step using a digital-to-analog converter (DAC). With the advent of sophisticated data converters, an ideal SDR can be realized today as shown in the picture below.

The Ideal SDR

However, the cost of such products is very high — often in the many thousands of dollars.

Hybrid SDR

Luckily there are many hybrid SDR products on the market today that include a mix of analog and digital technologies. These products include traditional building blocks such as low noise amplifiers and mixers. However, they also allow for software control of these analog blocks in addition to software processing of digitized data samples. This enables users to process a wide variety of waveforms.  The block diagram below shows such a radio system and it is the typical architecture of most SDRs on the market today.

Hybrid SDR

Software-defined radio has many applications in Cellular communication systems, Aerospace, Defense and Satellite Communication. As well, one of the most common and growing use cases for SDR is in the area of Education where it is used for experimentation and developing an understanding of Wireless Communication Systems and Signal Processing.

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