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SDR Receiver Fundamental Mysteries

sdr receiver fundamental mysteries

Consider the SDR receiver fundamental mysteries. Namely, how they are designed, work and perform. This series will do just that. 

Most hams and shortwave listeners have at least one software defined radio or SDR. Audio DSP showed up in the late 1980’s, followed by HF gear with a DSP IF chain a decade later. Today, we have advanced to the point of basically connecting an analog-to-digital converter to the antenna and just treating signals as data.

Digital signal processing got its start during the 1960’s. At first, scientists and engineers focused on using computers to simulate analog hardware. This effort got a big boost from the discovery of the Fast Fourier Transform in 1965, as well as methods for simulating filters in both analog and frequency domains.

As computers became more powerful, some bright engineer realized that computers could actually do signal processing, rather than just simulating how it worked. By the 1990’s, all of the math and computing power was in place to make DSP and its cousin, Software Defined Radio, a reality.

DSP and SDR eliminate most of the imperfections and limitations of analog electronics. Analog components suffer from non-linearity and distortion, aging, temperature variations. Digital systems are more accurate, perfectly reproducible and much lower cost. No more noise, images or spurs from mixers. No more phase noise or reciprocal mixing from oscillators. And so on. Maintenance and calibration is more about debugging software.

In addition, SDR provides the opportunity to re-design and re-configure a receiver just through software. The hardware becomes almost generic.

SDR Receiver Fundamental Mysteries in Three Parts

This series will focus on our understanding of what I call the SDR receiver fundamental mysteries. The first is about architecture or design. Just how and where exactly does a radio signal get converted into data? Do we really need front-end filters and analog amplifiers? Does it really matter where the signal processing is done?

The second mystery is the math. Even if we don’t understand the implementation details, we should at least have a layman’s grasp of concepts like complex numbers and quadrature sampling. Today’s ham should be able to explain how SDR works in the same way as we once explained superheterodyne receivers, amplifiers and oscillators.

Lastly, we should be able to understand and talk about receiver performance. How do the new SDR designs effect sensitivity, selectivity and, especially dynamic range? How do we need to configure our SDR to avoid spurious responses and distortion?

In the next two articles we examine the hardest parts first: complex numbers and quadrature sampling.

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