Converter circuit for television signals

A RF signal converter circuit positioned externally to a superheterodyne receiver, said converter circuit having a signal input and a signal output said output being connected to the aerial RF input of said superheterodyne receiver, said converter circuit comprising:

• a cascade arrangement of a RF signal selector which provides a selected and modified RF signal and a modulator connected between the signal input and the signal output of said converter,
• said converter also comprising a directional coupler connected to said RF aerial signal input and said converter signal output for coupling oscillator radiation present at the aerial input of said RF superheterodyne receiver,
• a frequency search circuit connected to said directional coupler for receiving said oscillator radiation, a first output of said frequency search circuit being coupled to said RF signal selector to control selections of a RF signal from RF signals applied to a signal input of said signal selector, a second output thereof being coupled to said modulator to control modulation of the selected and modified RF signal onto a carrier frequency,
• said frequency search circuit comprising a first mixer stage, a control circuit, a first filter element and a controllable oscillator, an input of said controllable oscillator is connected to said control circuit and an output of said controllable oscillator is coupled to a first input of said first mixer stage, said first mixer stage also having a second input to which said directional coupler is connected, and also having an output which is coupled to said control circuit through said first filter element,
• said modulator comprising a second filter element and a second mixer stage an input of which is connected to a fixed frequency oscillator, and
• a third mixer stage an input of which is connected to said second output of said frequency search circuit, said second and third mixer stages being arranged in cascade between the RF signal selector and the second filter element, an output of which is coupled to the signal output of said converter, said second filter element being a low-pass filter and the frequency of said fixed frequency oscillator being equal to the intermediate frequency of said RF superheterodyne receiver coupled to said converter signal output.