Frequency synthesizer with frequency modulated output

Claim 1

Means to generate an output signal having a desired center frequency, and to frequency modulate said output signal, comprising a voltage-controlled oscillator providing said output signal, a frequency divider coupled to the output of said oscillator, a frequency-to-current converter connected to the output of said frequency divider and producing an actual-frequency-representing current corresponding with the frequency of said output signal, a digital-to-analog converter converting an input digital signal representing a desired output signal frequency to a corresponding desired-frequency-representing current, means to produce an error current representing the difference between said actual-frequency-representing current and said desired-frequency-representing current, a current-controlled voltage source translating said error current to a corresponding error voltage, a low-pass filter, and an integrator, coupling said error voltage to the control voltage input of said voltage-controlled oscillator, to produce an output at the desired center frequency, and means to apply an audio-frequency modulating voltage also to the input of said integrator, to frequency modulate the output signal from said voltage-controlled oscillator.