Phase locked loop with limited tracking range

Claim 1

In a phase locked loop, a phase comparator for receiving an input signal and providing an output, filtering means coupled to the output of the phase comparator and providing a filtered output serving as a frequency control signal, and a voltage controlled oscillator coupled between the output of said filter means and the phase comparator said voltage controlled oscillator having a free running frequency of oscillation, said voltage controlled oscillator being responsive to the magnitude of said frequency control signal for producing an output signal having a frequency related to such magnitude of said frequency control signal, said voltage controlled oscillator including an input terminal and an output terminal, a multivibrator coupled between said input terminal and said output terminal for respectively receiving said frequency control currents, and producing said output signal, said multivibrator including first and second transistors, load means coupled to said first and second transistors, cross coupling means coupling said transistors to provide regenerative feedback between said transistors, said cross coupling means including a timing capacitor, means for charging the timing capacitor including a constant current source for supplying a constant current, I5, to said timing capacitor to substantially determine said free running frequency of oscillation, fo, and a variable current source for supplying a variable current, I2, to said timing capacitor, the sum of said current, I2 + I5, determining said frequency of said output signal, said summed currents being alternately switched by said first and second transistors, means for controlling said variable current from said variable current source in accordance with said frequency control signal whereby the frequency deviation, Delta f, of said output signal of said multivibrator is substantially independent of the free running frequency of oscillation of the multivibrator and means coupled to said variable current source for limiting, I2, to a maximum value, I2(MAX), the maximum frequency deviation of said output signal being determined by ( Delta f/fo MAX (I2(MAX)/I2(MAX) + I5).