In an automatic frequency control device for tuning the frequency of an intermediate frequency (IF) signal to a desired or target frequency, a frequency mixer subtracts the frequency of a received signal coming in through an antenna from the frequency of a local oscillation signal output from...http://www.google.de/patents/US5706315?utm_source=gb-gplus-sharePatent US5706315 - Automatic frequency control device for tuning an intermediate frequency signal to a target frequency 
