Frequency mixing. An incoming signal (down chirp) is being mixed with a fixed frequency ("LO": 50 kHz). A:/B: Time domain, C:/D: Frequency domain. Fixed frequency (LO: local oscillator) displayed as red/dashed red line.
Own work
Drahkrub Attribution must include the URL http://de.wikipedia.org/wiki/Benutzer:Drahkrub.