How are RF signals processed by a software defined radio different from a traditional receiver?
A software defined radio differs from a traditional radio in several ways. The biggest difference is in how RF is detected and demodulated. A SDR uses a quadrature sampling detector (QSD) that divides the incoming waveform into an in-phase or (I) signals and quadrature (Q) signal. The in-phase signal is the first 90º of the RF sine wave and the quadrature signal is the second 90º segment of the RF sine wave. Some simple math allows one to determine or recover the instantaneous phase and amplitude of the original signal. So, measuring the instantaneous values of I and Q, we would know everything we needed to know about the RF signal at a given moment in time. In order for a sound card (the A/D converter) to digitize the incoming I and Q signal, the two signals are mixed with a local oscillator so that the resulting frequency is within the audio frequency (AF) range of the sound card known as the baseband signal. The resulting baseband signal is passed through a low pass filter to remove
