Abstract: A tunable selective heterodyne receiver which receives an input signal and is capable of supplying such input signal through one or two receiving channels and wherein in the first channel a first mixer receives a tunable input from an injection oscillator and supplies it to a second mixer which receives an input from a fixed frequency oscillator and the output of the second mixer comprises a fixed IF and wherein the second receiving channel comprises a third mixer which receives the input signal and also receives an output from a frequency divider which receives the output of the tunable injection oscillator and which has a dividing factor which is determined by the center frequency of a first band pass filter at the output of the first mixer divided by the center frequency of the fixed IF, thus, allowing a receiver tunable over a very broad frequency range.

Abstract: A calibratable heterodyne receiver can be adjusted to a calibration mark or similar entity of an indicating device connected to the output of the receiver during feeding of a calibration alternating current with the help of adjusting means which change the gain of the amplifier. The calibration alternating current frequency is adapted during each calibration to the tuning frequency and is obtained by means of mixing the output voltage of the heterodyne oscillator with an auxiliary voltage which corresponds to the intermediate frequency. The auxiliary voltage is tapped from an intermediate frequency stage of the receiver and is derived by way of a limiting circuit which is constructed and dimensioned in such a way that a calibration voltage amplitude and calibration voltage phase results which causes self oscillation around the circuit.