Linear frequency modulation apparatus and technique

Abstract

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.

What is claimed is: 
     
       1. An apparatus for generating selectable linear frequency versus time waveforms comprising, in combination: an analog forward path having input and output means;   an error signal generator receiving the output of said analog forward path and having an output that is proportional to the rate of change in frequency of the output of said analog forward path; and   a feedback path receiving the output of said error signal generator, connected to an input of said forward path for supplying a feedback signal, and having means for providing a sample period determined by the slope selected for said waveform, and digitally comparing during said period said error signal to a value determined by said slope.   
     
     
       2. The apparatus of claim 1 wherein said analog forward path comprises: means for generating a linear voltage ramp having an output;   means for summing said voltage generating means output with said feedback signal; and   means for varying frequency of a signal over time in an extremely linear manner in accordance with a voltage signal input thereto from said summing means.   
     
     
       3. The apparatus of claim 1 wherein said error signal generator comprises: a surface acoustic wave delay line receiving an input from said analog forward path and having an output derived from the output thereof;   means for deriving a signal proportionate to the rate of change in frequency of the output of said analog forward path, having inputs from said surface acoustic wave delay line and said analog forward path.   
     
     
       4. The apparatus of claim 1 wherein said feedback path comprises: means for converting the output of said error signal generator into a digital waveform;   means for comparing said digital waveform to a predetermined value associated with the linearity of said frequency versus time waveform, outputting a digital signal related thereto; and   means for converting said digital signal into an analog voltage for input into said analog forward path to provide feedback loop stability to the analog forward path output.   
     
     
       5. Apparatus for producing a linear frequency ramp, comprising: a voltage controlled oscillator providing a relatively linear frequency versus time waveform at its output responsive to the voltage of a signal at its input;   a first waveform generator producing a relatively linear ramp signal of voltage versus time at its output, wherein the slope of said ramp is selectable from a predetermined set of voltage versus time rates;   a second waveform generator providing the error signal of a feedback circuit at its output to correct the slope of the output signal of said voltage controlled oscillator;   a summing circuit coupled to said outputs of said first and said second waveform generators for combining said error signal with said relatively linear ramp signal of voltage versus time, and coupled to said input of said voltage controlled oscillator;   means in said feedback circuit coupled to an output of said voltage controlled oscillator for providing a frequency signal that is proportional to the rate of the frequency change in the output of said voltage controlled oscillator;   means in said feedback circuit coupled to said frequency signal providing means for converting said proportional signal into a square wave digital signal;   means in said feedback circuit coupled to said converting means for providing a sample period determined by the slope selected for the ramp of said first waveform generator, and comparing during said period said square wave digital signal to a value determined by said slope; and   means in said feedback circuit coupled to said comparing means for converting the results of said comparison to a digital signal and coupling said digital signal to said second waveform generator.