Selectively tunable heterodyne receiver

Claim 1

A tunable heterodyne receiver receiving an input signal and having at least two conversion stages which utilizes separate coarse and fine tuning and which can be set to a desired received frequency by first and second adjustable variable frequency signals including first and second mixers, a first adjustable injection oscillator developing said first signal which can be adjusted and locked to the harmonics of a fundamental frequency derived from a first crystal, said first mixer receiving said input signal and an input from said first injection oscillator to produce an output signal in a first intermediate frequency band, a second injection oscillator having a frequency determining circuit and developing said second signal which is continuously adjustable within a frequency range corresponding to the frequency spacing between adjacent harmonics of said first crystal, a second crystal in said frequency determining circuit for stabilizing its frequency to a stable frequency, an adjustment means connected to said second crystal oscillator for shifting its stable frequency over a continuously adjustable range which is very small in comparison to its nominal frequency, the frequency spacing between adjacent harmonics of the output of said first crystal oscillator fundamental frequency which control said first injection oscillator selected to be small enough that such spacing does not exceed the width of the adjustment range of said stable frequency of said second crystal oscillator, said second mixer receiving said first mixer output signal and said second variable frequency signal and a frequency counting means deriving the selected received frequency from the frequencies of said first and second injection oscillators, and wherein said adjustment means consists of a voltage controlled capacitive diode connected in the frequency-determining circuit of said second injection oscillator, and an adjustable voltage source connected to said diode, wherein the output of said second mixer comprises a second intermediate frequency band, the output of one of said first and second injection oscillators is supplied to said frequency counting means, said frequency counting means including a gate circuit which is opened by a signal from the other of said first and second injection oscillators whereby a counted output is obtained which corresponds to the frequency of the input signal and wherein said frequency counting means includes a preadjusted counter enabling indication of said selected received frequency.