Introduction to SDR

Written by Dilusha Samarasekara – Associate Member, RSSL

A bit of a History

Radio has come a long way since 1890. They have helped people to communicate and broadcast information and for some people, it has become an interesting hobby. So amateur radio was born soon after and people started making contact all over the world, getting to know new people, sharing knowledge, and having fun. On some occasions, it has proven to be more than a hobby and helped save many lives.

What is an SDR?

The term SDR (Software Defined Radio) has been around for more than 30 years. Software Defined Radio is a radio communication system that employs reconfigurable software-based components for the processing and conversion of digital signals. Simply put, where conventional radios use fixed hardware components to control their intended functions, SDR uses software that runs on a computer to define and manipulate functions implemented on it. They have an “analog front end” to tune and receive radio signals, an “analog to digital converter” to convert those signals to digital form, and a “digital backend” to interface with a computer. Then a computer is used to further demodulate and decode the intended signal to output either sounds, images, or data.

Why should I care?

It was around 2010 when SDRs became popular among amateurs. Back then a cheap USB TV stick called “DVB-T+FM+DAB” appeared in the market which allowed people to watch terrestrial TV with their computer. Sometime after that, someone figured out, by modifying the driver, the full frequency range supported by the dongle can be interpreted by a computer, thus allowing it to be used as an SDR. Those dongles were extremely cheap and had a considerably wide frequency range. Needless to say, they became widespread among amateurs and radio enthusiasts soon enough. As a result of that, many companion software was developed to support them and add more and more functionality over time. Today we have mature open-source software and stable hardware cheap enough that anyone can afford it. So unlike in the past, anyone can get into amateur radio with a cheap SDR coupled with a simple antenna without needing expensive hardware.

What can I do with an SDR?

Since SDRs use software to define their functions, we can use different software to achieve our intended purpose. Because of this, possibilities are virtually endless with SDRs, unlike conventional radios or transceivers where only a fixed set of functions are present. Nowadays there are entire communities dedicated to a different aspect of what you can do with SDRs. You can listen to FM radio and shortwave radio, listen to other HAM radio operators, decode HAM digital modes such as RTTY/FT8/WSPR, track airplanes and ships, track satellites, receive real-time satellite images, receive analog TV, listen to international space stations and decode SSTV images when there is an event, track weather balloons, and much more.

Understanding Sample Rate and Bandwidth

There are two key concepts to understand about SDRs in general before we do anything with them. Which are “Sample Rate” and “Bandwidth”. It will help to have a good grasp of them moving forward.

Sample rate refers to the number of samples of the radio signal that are converted to digital form per second by the ADC on the SDR. It is typically measured in Mega samples per second (MSPS). The sample rate determines the amount of information that can be captured from the radio signal. A higher sample rate means more information can be captured, but it also requires more processing power.

Bandwidth, on the other hand, refers to the range of frequencies that can be captured by the SDR at any given time. It is typically measured in kilohertz (kHz) or megahertz (MHz). A wider bandwidth means a more frequency range can be captured, but it also requires a higher sample rate and more processing power.

In general, the sample rate and bandwidth of an SDR are related. A wider bandwidth requires a higher sample rate to capture all of the information in the signal. However, it is important to note that the sample rate and bandwidth are different. It is possible to have a high sample rate and a narrow bandwidth, or a low sample rate and a wide bandwidth, depending on the specific SDR hardware and software being used.

It is important to choose the appropriate sample rate and bandwidth for the specific application you are using the SDR for. For example, if you are trying to capture a wide range of frequencies, you will need a wider bandwidth and a higher sample rate. However, if you are only interested in a narrow range of frequencies, you can use a narrower bandwidth and a lower sample rate to save processing power.