Multiple conversion tuner

摘要

A multiple conversion tuner comprises a plurality of cascade-connected frequency changers, each of which comprises a mixer and a local oscillator. The tuner also comprises a local oscillator frequency selecting circuit which controls the frequencies of the local oscillators. The frequencies are controlled so that the final mixer converts a desired signal to the final intermediate frequency and so that the frequency band occupied by the desired signal at the output of each mixer is within the passband of the following intermediate frequency part of the tuner. The local oscillator frequencies are also chosen so that there is no signal at a frequency Fi in the input spectrum to the ith mixer such that:where IF1 is the ith intermediate frequency to which the desired signal is converted, Bwi is the bandwidth of the desired signal at the output of the ith frequency changer and Fs is the frequency of any component signal which may be generated by non-linear mixing of any combination of the signals from the local oscillators.