Linear sweep frequency modulator for FM/CW radio altimeter

摘要

A linear sweep frequency modulator for a varactor tuned transistor microwave oscillator used in an FM/CW radio altimeter where the oscillator-frequency v. varactor bias characteristic is non-linear. A linear triangular wave is converted by means of a function generator to the non-linear wave required for varactor bias to cause linear frequency modulation of the oscillator. Linearity of modulation is tested by measuring variations in the period of a calibration signal derived from a delay line. Variations in the period of the calibration signal are measured by determining the difference between the average and the instantaneous calibration period and sampling the difference during each cycle of the triangular wave, once during the upswing thereof when the oscillator is at low frequency and once during the down swing thereof when the oscillator is at high frequency and separately integrating the up swing and down swing samples. By applying both results of integration to the function generator it thereby has its biases controlling the low frequency and the high frequency operation of the oscillator separately adjusted.