RF tuning circuit which provides image frequency rejection

An improved radio frequency (RF) tuning circuit which suppresses image frequencies of the frequency band to which it is tuned and which comprises:

• an RF amplifier having an input for RF signals supplied thereto, a feedback signal input, and an output;
• such output being connected to the input of an N-path filter which includes a local oscillator;
• and a feedback path connecting the output of said filter to the feedback signal input of said RF amplifier;
• such improvement being characterized in that said N-path filter comprises:
• first and second quadrature related signal paths connecting the output of said RF amplifier to first and second inputs, respectively, of a summing amplifier, said summing amplifier having an output at which it produces output RF signals corresponding to the supplied RF signals, the output of said summing amplifier being connected to said feedback path to supply such output RF signals thereto as feedback signals;
• said first signal path including in series a first mixer, a first filter and a second mixer, the output of said second mixer being coupled to the first input of said summing amplifier;
• said second signal path including in series a third mixer, a second filter and a fourth mixer, the output of said fourth mixer being coupled to the second input of said summing amplifier;
• and phase shift means coupling the output of said local oscillator in quadrature to said first and third mixers, respectively, and in quadrature to said second and fourth mixers, respectively;
• such feedback path causing the center frequency of the transfer characteristic of said filter to be displaced from the frequency of said local oscillator therein by an amount determined by the relative gains of said feedback path and said RF amplifier;
• whereby substantially only a single sideband of the signals produced at the outputs of said third and fourth mixers is included in the output RF signals produced by said summing amplifier.