Linear frequency modulation having a large time-bandwidth product and an remely linear frequency versus time waveform, is provided by a feedback technique utilizing a linear ramp voltage generator whose output is summed with a correction voltage to drive a voltage controlled oscillator to output the LFM waveform. Part of the LFM waveform is fed back through a digital discriminator for error signal generation and comparison with a preset value associated with the output slope linearity, thus yielding the basis for generating the correction voltage.