Introduction to Baseband Processing

Today, much RF digital signal processing is done at baseband, defined here as a signal with frequencies centered around DC or 0 Hz. Baseband processing is somewhat analogous to using an intermediate frequency (IF), as done in a superheterodyne receiver with typical IF frequencies of 455 kHz (AM) or 10.7 MHz (FM), but where the IF is at 0 Hz and where the resulting baseband signal contains both positive and negative frequencies. Just as intermediate frequencies of superheterodyne receivers gave advantages in analog receivers, baseband processing gives certain advantages in digital receivers. Furthermore, most modern Software Defined Radios output in-phase / quadrature (IQ) data streams, containing both amplitude and phase information and centered at baseband. This includes the IQ output of the popular “ka9q-radio” software environment used in HamSCI development of the Personal Space Weather Station system. A good understanding of baseband processing and its typical applications is thus useful. In this tutorial, we will introduce a common definition of baseband processing and explore its uses. The presentation will include a reminder of Fourier / frequency domain and complex signal representations. We will discuss the advantages of processing of IQ signals at baseband and will give examples of their use in common tasks. These include signal demodulation and filtering, including asymmetric filtering around 0 Hz. A brief discussion on creating a baseband signal from a non-baseband signal and converting a baseband signal to a non-baseband signal will also be presented. Interactive demonstrations will be provided as well for active learning, easy modification, and experimentation.