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Abstract

A multi-marker microwave sweep linearization system comprising a voltage controlled microwave oscillator utilizing an adaptive sweep control circuit is presented. The oscillator control voltage is to be adjusted as necessary to maintain a constant spacing between markers. To produce the markers, the output of the oscillator is mixed with the output at a comb generator having harmonics. The lower side bands generated as the oscillator sweeps past the comb frequencies generates a series of `chirps`, centered about each of the comb harmonics. It will be noted, that the frequency of the chirp passes through zero when the oscillator frequency is exactly equal to one of the comb harmonics. The marker is to be associated with a higher frequency in the chirp envelope to avoid phase uncertainties. This can be accomplished with a frequency-detector circuit which may comprise a retriggerable monostable multi-vibrator having a time constant equal to the period of the frequency to be detected. The multi-vibrator is preceded by a high speed comparator which squares up the chirp signal. At the onset of the chirp, the signal amplitude is too low to be detected by the comparer. The comparer threshold is set with hysteresis to allow triggering when the chirp frequency is higher than the desired marker detection frequency. Further, by averaging several sweeps it is possible to get very precise marker position measurements. Any long term change in the oscillator frequency versus voltage characteristics which degrades sweep linearity will be manifested as a shift in the position of one of the markers, and the sweep voltage can be readily adjusted to compensate for it.

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