Tuning system for a microwave local oscillator

Apparatus for generating a signal at a selected frequency, comprising:

• a. a voltage controlled oscillator having, at each frequency in a range of interest, a rate of change df/dv of frequency with respect to a change in control voltage that is constant despite changes in environmental conditions,
• b. means for generating a primary reference frequency signal,
• c. means for generating a varying control voltage to sweep the tuning of said oscillator through a first frequency range which includes a secondary reference frequency that differs from said primary reference frequency by a specified difference frequency,
• d. means for applying said varying control voltage to the control port of the oscillator,
• e. means for comparing the primary reference signal with the oscillator signal to produce an output pulse when the two differ by said specified difference frequency and said oscillator is at said secondary reference frequency,
• f. means responsive to said output pulse to store the instantaneous value of said varying voltage,
• g. means for producing a dc offset voltage equal to the difference in the control voltage at said secondary reference frequency and at said selected frequency,
• h. a voltage summing network to add the stored voltage and the offset voltage to produce a sum voltage, and
• i. means for substituting said sum voltage for said varying voltage at the control port of the oscillator.