Phase locked loop frequency modulator using data modulated digital synthesizer as reference

Claim 1

In a frequency-modulated frequency generator of the type in which a phase-controlled oscillator is operated in response to an input frequency signal to alternately produce first and second frequency signals respectively above and below a reference frequency, a digital synthesizer including a memory storing interpolation nodes of a full sine wave period, cyclically operating adder means connected to the memory for incrementally reading a full sine wave period from the memory during one adding cycle, and a digital-to-analog converter connected to the memory for producing the reference frequency signal, the improvement therein comprising: modulating means connected to the adder means for modulating the operation of the digital synthesizer in response to an input data signal having first and second levels respectively representing the first and second frequency signals, said modulating means including an increment register storing first data representing the first frequency, second data representing the second frequency and increment data representing individual frequencies adjacent the first and second frequencies; frequency setting means connected to said increment register for reading the first and second data into the adder means to set the frequency of the input frequency signal; and overshoot correction means connected to receive said input frequency signal and connected to said register means and operable in response to each half cycle of the input data signal to read the increment data into said adder means as overshoot correction data ending with the respective first and second data.