General: Software Defined Radio

Delving into the crease where radio and computers converge, this week’s General class exam question is……

What is meant by the term “software defined radio” (SDR)?

A. A radio in which most major signal processing functions are performed by software
B. A radio that provides computer interface for automatic logging of band and frequency
C. A radio that uses crystal filters designed using software
D. A computer model that can simulate performance of a radio to aid in the design process

Rather than delve into an explanation of all possible answers, we’ll just come right out and tell you that the correct answer is A. A radio in which most major signal processing functions are performed by software.

But what does that mean, really? Typically a software defined radio, or SDR, takes the RF signal, mixes it into an IF signal, or intermediate frequency, and samples that electrical signal directly. This is different from, say, simply piping your radios output into a sound card, where it then looks at the AF, or audio frequency output.

The big benefit of doing this is that processing, usually filters, can be applied directly to the radio signal itself, vs. simply the audio.  These filters are limited only by the capabilities of the software and computer hardware, instead of being physical circuits.

You can buy complete software defined radio transceivers, the Flex series comes to mind.  A lot of modern transceivers are what I would consider to be SDR hybrids, such as my IC-7100.  The filters are definitely programmable, but there are limits on the bandwidth of the RF it samples, and further limitations.

The image above is from a computer program called SDR Sharp.  In conjunction with an inexpensive RTL-SDR chip (~$20 from Amazon) and this free software you can start experimenting with SDR right away, without a license.

I’m currently developing a course on using the RTL-SDR chip, so stay tuned for that!


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