In a double superheterodyne tuner for receiving TV channel signals, a frequency in a range defined between 2520 and 2700 MHz is selected as the first intermediate frequency, while the first intermediate frequency is obtained by subtracting a desired channel frequency from a first local oscillator frequency....http://www.google.de/patents/US4340975?utm_source=gb-gplus-sharePatent US4340975 - Microwave mixing circuit and a VHF-UHF tuner having the mixing circuit
